docs: block: convert to ReST

Rename the block documentation files to ReST, add an
index for them and adjust in order to produce a nice html
output via the Sphinx build system.

At its new index.rst, let's add a :orphan: while this is not linked to
the main index.rst file, in order to avoid build warnings.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

1 files changed, 1168 insertions, 0 deletions

diff --git a/Documentation/block/biodoc.rst b/Documentation/block/biodoc.rst
new file mode 100644
index 000000000000..d6e30b680405
--- /dev/null
+++ b/Documentation/block/biodoc.rst
@@ -0,0 +1,1168 @@
+=====================================================
+Notes on the Generic Block Layer Rewrite in Linux 2.5
+=====================================================
+
+.. note::
+
+	It seems that there are lot of outdated stuff here. This seems
+	to be written somewhat as a task list. Yet, eventually, something
+	here might still be useful.
+
+Notes Written on Jan 15, 2002:
+	- Jens Axboe <jens.axboe@oracle.com>
+	- Suparna Bhattacharya <suparna@in.ibm.com>
+
+Last Updated May 2, 2002
+
+September 2003: Updated I/O Scheduler portions
+	- Nick Piggin <npiggin@kernel.dk>
+
+Introduction
+============
+
+These are some notes describing some aspects of the 2.5 block layer in the
+context of the bio rewrite. The idea is to bring out some of the key
+changes and a glimpse of the rationale behind those changes.
+
+Please mail corrections & suggestions to suparna@in.ibm.com.
+
+Credits
+=======
+
+2.5 bio rewrite:
+	- Jens Axboe <jens.axboe@oracle.com>
+
+Many aspects of the generic block layer redesign were driven by and evolved
+over discussions, prior patches and the collective experience of several
+people. See sections 8 and 9 for a list of some related references.
+
+The following people helped with review comments and inputs for this
+document:
+
+	- Christoph Hellwig <hch@infradead.org>
+	- Arjan van de Ven <arjanv@redhat.com>
+	- Randy Dunlap <rdunlap@xenotime.net>
+	- Andre Hedrick <andre@linux-ide.org>
+
+The following people helped with fixes/contributions to the bio patches
+while it was still work-in-progress:
+
+	- David S. Miller <davem@redhat.com>
+
+
+.. Description of Contents:
+
+   1. Scope for tuning of logic to various needs
+     1.1 Tuning based on device or low level driver capabilities
+	- Per-queue parameters
+	- Highmem I/O support
+	- I/O scheduler modularization
+     1.2 Tuning based on high level requirements/capabilities
+	1.2.1 Request Priority/Latency
+     1.3 Direct access/bypass to lower layers for diagnostics and special
+	 device operations
+	1.3.1 Pre-built commands
+   2. New flexible and generic but minimalist i/o structure or descriptor
+      (instead of using buffer heads at the i/o layer)
+     2.1 Requirements/Goals addressed
+     2.2 The bio struct in detail (multi-page io unit)
+     2.3 Changes in the request structure
+   3. Using bios
+     3.1 Setup/teardown (allocation, splitting)
+     3.2 Generic bio helper routines
+       3.2.1 Traversing segments and completion units in a request
+       3.2.2 Setting up DMA scatterlists
+       3.2.3 I/O completion
+       3.2.4 Implications for drivers that do not interpret bios (don't handle
+	  multiple segments)
+     3.3 I/O submission
+   4. The I/O scheduler
+   5. Scalability related changes
+     5.1 Granular locking: Removal of io_request_lock
+     5.2 Prepare for transition to 64 bit sector_t
+   6. Other Changes/Implications
+     6.1 Partition re-mapping handled by the generic block layer
+   7. A few tips on migration of older drivers
+   8. A list of prior/related/impacted patches/ideas
+   9. Other References/Discussion Threads
+
+
+Bio Notes
+=========
+
+Let us discuss the changes in the context of how some overall goals for the
+block layer are addressed.
+
+1. Scope for tuning the generic logic to satisfy various requirements
+=====================================================================
+
+The block layer design supports adaptable abstractions to handle common
+processing with the ability to tune the logic to an appropriate extent
+depending on the nature of the device and the requirements of the caller.
+One of the objectives of the rewrite was to increase the degree of tunability
+and to enable higher level code to utilize underlying device/driver
+capabilities to the maximum extent for better i/o performance. This is
+important especially in the light of ever improving hardware capabilities
+and application/middleware software designed to take advantage of these
+capabilities.
+
+1.1 Tuning based on low level device / driver capabilities
+----------------------------------------------------------
+
+Sophisticated devices with large built-in caches, intelligent i/o scheduling
+optimizations, high memory DMA support, etc may find some of the
+generic processing an overhead, while for less capable devices the
+generic functionality is essential for performance or correctness reasons.
+Knowledge of some of the capabilities or parameters of the device should be
+used at the generic block layer to take the right decisions on
+behalf of the driver.
+
+How is this achieved ?
+
+Tuning at a per-queue level:
+
+i. Per-queue limits/values exported to the generic layer by the driver
+
+Various parameters that the generic i/o scheduler logic uses are set at
+a per-queue level (e.g maximum request size, maximum number of segments in
+a scatter-gather list, logical block size)
+
+Some parameters that were earlier available as global arrays indexed by
+major/minor are now directly associated with the queue. Some of these may
+move into the block device structure in the future. Some characteristics
+have been incorporated into a queue flags field rather than separate fields
+in themselves.  There are blk_queue_xxx functions to set the parameters,
+rather than update the fields directly
+
+Some new queue property settings:
+
+	blk_queue_bounce_limit(q, u64 dma_address)
+		Enable I/O to highmem pages, dma_address being the
+		limit. No highmem default.
+
+	blk_queue_max_sectors(q, max_sectors)
+		Sets two variables that limit the size of the request.
+
+		- The request queue's max_sectors, which is a soft size in
+		  units of 512 byte sectors, and could be dynamically varied
+		  by the core kernel.
+
+		- The request queue's max_hw_sectors, which is a hard limit
+		  and reflects the maximum size request a driver can handle
+		  in units of 512 byte sectors.
+
+		The default for both max_sectors and max_hw_sectors is
+		255. The upper limit of max_sectors is 1024.
+
+	blk_queue_max_phys_segments(q, max_segments)
+		Maximum physical segments you can handle in a request. 128
+		default (driver limit). (See 3.2.2)
+
+	blk_queue_max_hw_segments(q, max_segments)
+		Maximum dma segments the hardware can handle in a request. 128
+		default (host adapter limit, after dma remapping).
+		(See 3.2.2)
+
+	blk_queue_max_segment_size(q, max_seg_size)
+		Maximum size of a clustered segment, 64kB default.
+
+	blk_queue_logical_block_size(q, logical_block_size)
+		Lowest possible sector size that the hardware can operate
+		on, 512 bytes default.
+
+New queue flags:
+
+	QUEUE_FLAG_CLUSTER (see 3.2.2)
+	QUEUE_FLAG_QUEUED (see 3.2.4)
+
+
+ii. High-mem i/o capabilities are now considered the default
+
+The generic bounce buffer logic, present in 2.4, where the block layer would
+by default copyin/out i/o requests on high-memory buffers to low-memory buffers
+assuming that the driver wouldn't be able to handle it directly, has been
+changed in 2.5. The bounce logic is now applied only for memory ranges
+for which the device cannot handle i/o. A driver can specify this by
+setting the queue bounce limit for the request queue for the device
+(blk_queue_bounce_limit()). This avoids the inefficiencies of the copyin/out
+where a device is capable of handling high memory i/o.
+
+In order to enable high-memory i/o where the device is capable of supporting
+it, the pci dma mapping routines and associated data structures have now been
+modified to accomplish a direct page -> bus translation, without requiring
+a virtual address mapping (unlike the earlier scheme of virtual address
+-> bus translation). So this works uniformly for high-memory pages (which
+do not have a corresponding kernel virtual address space mapping) and
+low-memory pages.
+
+Note: Please refer to Documentation/DMA-API-HOWTO.txt for a discussion
+on PCI high mem DMA aspects and mapping of scatter gather lists, and support
+for 64 bit PCI.
+
+Special handling is required only for cases where i/o needs to happen on
+pages at physical memory addresses beyond what the device can support. In these
+cases, a bounce bio representing a buffer from the supported memory range
+is used for performing the i/o with copyin/copyout as needed depending on
+the type of the operation.  For example, in case of a read operation, the
+data read has to be copied to the original buffer on i/o completion, so a
+callback routine is set up to do this, while for write, the data is copied
+from the original buffer to the bounce buffer prior to issuing the
+operation. Since an original buffer may be in a high memory area that's not
+mapped in kernel virtual addr, a kmap operation may be required for
+performing the copy, and special care may be needed in the completion path
+as it may not be in irq context. Special care is also required (by way of
+GFP flags) when allocating bounce buffers, to avoid certain highmem
+deadlock possibilities.
+
+It is also possible that a bounce buffer may be allocated from high-memory
+area that's not mapped in kernel virtual addr, but within the range that the
+device can use directly; so the bounce page may need to be kmapped during
+copy operations. [Note: This does not hold in the current implementation,
+though]
+
+There are some situations when pages from high memory may need to
+be kmapped, even if bounce buffers are not necessary. For example a device
+may need to abort DMA operations and revert to PIO for the transfer, in
+which case a virtual mapping of the page is required. For SCSI it is also
+done in some scenarios where the low level driver cannot be trusted to
+handle a single sg entry correctly. The driver is expected to perform the
+kmaps as needed on such occasions as appropriate. A driver could also use
+the blk_queue_bounce() routine on its own to bounce highmem i/o to low
+memory for specific requests if so desired.
+
+iii. The i/o scheduler algorithm itself can be replaced/set as appropriate
+
+As in 2.4, it is possible to plugin a brand new i/o scheduler for a particular
+queue or pick from (copy) existing generic schedulers and replace/override
+certain portions of it. The 2.5 rewrite provides improved modularization
+of the i/o scheduler. There are more pluggable callbacks, e.g for init,
+add request, extract request, which makes it possible to abstract specific
+i/o scheduling algorithm aspects and details outside of the generic loop.
+It also makes it possible to completely hide the implementation details of
+the i/o scheduler from block drivers.
+
+I/O scheduler wrappers are to be used instead of accessing the queue directly.
+See section 4. The I/O scheduler for details.
+
+1.2 Tuning Based on High level code capabilities
+------------------------------------------------
+
+i. Application capabilities for raw i/o
+
+This comes from some of the high-performance database/middleware
+requirements where an application prefers to make its own i/o scheduling
+decisions based on an understanding of the access patterns and i/o
+characteristics
+
+ii. High performance filesystems or other higher level kernel code's
+capabilities
+
+Kernel components like filesystems could also take their own i/o scheduling
+decisions for optimizing performance. Journalling filesystems may need
+some control over i/o ordering.
+
+What kind of support exists at the generic block layer for this ?
+
+The flags and rw fields in the bio structure can be used for some tuning
+from above e.g indicating that an i/o is just a readahead request, or priority
+settings (currently unused). As far as user applications are concerned they
+would need an additional mechanism either via open flags or ioctls, or some
+other upper level mechanism to communicate such settings to block.
+
+1.2.1 Request Priority/Latency
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Todo/Under discussion::
+
+  Arjan's proposed request priority scheme allows higher levels some broad
+  control (high/med/low) over the priority  of an i/o request vs other pending
+  requests in the queue. For example it allows reads for bringing in an
+  executable page on demand to be given a higher priority over pending write
+  requests which haven't aged too much on the queue. Potentially this priority
+  could even be exposed to applications in some manner, providing higher level
+  tunability. Time based aging avoids starvation of lower priority
+  requests. Some bits in the bi_opf flags field in the bio structure are
+  intended to be used for this priority information.
+
+
+1.3 Direct Access to Low level Device/Driver Capabilities (Bypass mode)
+-----------------------------------------------------------------------
+
+(e.g Diagnostics, Systems Management)
+
+There are situations where high-level code needs to have direct access to
+the low level device capabilities or requires the ability to issue commands
+to the device bypassing some of the intermediate i/o layers.
+These could, for example, be special control commands issued through ioctl
+interfaces, or could be raw read/write commands that stress the drive's
+capabilities for certain kinds of fitness tests. Having direct interfaces at
+multiple levels without having to pass through upper layers makes
+it possible to perform bottom up validation of the i/o path, layer by
+layer, starting from the media.
+
+The normal i/o submission interfaces, e.g submit_bio, could be bypassed
+for specially crafted requests which such ioctl or diagnostics
+interfaces would typically use, and the elevator add_request routine
+can instead be used to directly insert such requests in the queue or preferably
+the blk_do_rq routine can be used to place the request on the queue and
+wait for completion. Alternatively, sometimes the caller might just
+invoke a lower level driver specific interface with the request as a
+parameter.
+
+If the request is a means for passing on special information associated with
+the command, then such information is associated with the request->special
+field (rather than misuse the request->buffer field which is meant for the
+request data buffer's virtual mapping).
+
+For passing request data, the caller must build up a bio descriptor
+representing the concerned memory buffer if the underlying driver interprets
+bio segments or uses the block layer end*request* functions for i/o
+completion. Alternatively one could directly use the request->buffer field to
+specify the virtual address of the buffer, if the driver expects buffer
+addresses passed in this way and ignores bio entries for the request type
+involved. In the latter case, the driver would modify and manage the
+request->buffer, request->sector and request->nr_sectors or
+request->current_nr_sectors fields itself rather than using the block layer
+end_request or end_that_request_first completion interfaces.
+(See 2.3 or Documentation/block/request.rst for a brief explanation of
+the request structure fields)
+
+::
+
+  [TBD: end_that_request_last should be usable even in this case;
+  Perhaps an end_that_direct_request_first routine could be implemented to make
+  handling direct requests easier for such drivers; Also for drivers that
+  expect bios, a helper function could be provided for setting up a bio
+  corresponding to a data buffer]
+
+  <JENS: I dont understand the above, why is end_that_request_first() not
+  usable? Or _last for that matter. I must be missing something>
+
+  <SUP: What I meant here was that if the request doesn't have a bio, then
+   end_that_request_first doesn't modify nr_sectors or current_nr_sectors,
+   and hence can't be used for advancing request state settings on the
+   completion of partial transfers. The driver has to modify these fields
+   directly by hand.
+   This is because end_that_request_first only iterates over the bio list,
+   and always returns 0 if there are none associated with the request.
+   _last works OK in this case, and is not a problem, as I mentioned earlier
+  >
+
+1.3.1 Pre-built Commands
+^^^^^^^^^^^^^^^^^^^^^^^^
+
+A request can be created with a pre-built custom command  to be sent directly
+to the device. The cmd block in the request structure has room for filling
+in the command bytes. (i.e rq->cmd is now 16 bytes in size, and meant for
+command pre-building, and the type of the request is now indicated
+through rq->flags instead of via rq->cmd)
+
+The request structure flags can be set up to indicate the type of request
+in such cases (REQ_PC: direct packet command passed to driver, REQ_BLOCK_PC:
+packet command issued via blk_do_rq, REQ_SPECIAL: special request).
+
+It can help to pre-build device commands for requests in advance.
+Drivers can now specify a request prepare function (q->prep_rq_fn) that the
+block layer would invoke to pre-build device commands for a given request,
+or perform other preparatory processing for the request. This is routine is
+called by elv_next_request(), i.e. typically just before servicing a request.
+(The prepare function would not be called for requests that have RQF_DONTPREP
+enabled)
+
+Aside:
+  Pre-building could possibly even be done early, i.e before placing the
+  request on the queue, rather than construct the command on the fly in the
+  driver while servicing the request queue when it may affect latencies in
+  interrupt context or responsiveness in general. One way to add early
+  pre-building would be to do it whenever we fail to merge on a request.
+  Now REQ_NOMERGE is set in the request flags to skip this one in the future,
+  which means that it will not change before we feed it to the device. So
+  the pre-builder hook can be invoked there.
+
+
+2. Flexible and generic but minimalist i/o structure/descriptor
+===============================================================
+
+2.1 Reason for a new structure and requirements addressed
+---------------------------------------------------------
+
+Prior to 2.5, buffer heads were used as the unit of i/o at the generic block
+layer, and the low level request structure was associated with a chain of
+buffer heads for a contiguous i/o request. This led to certain inefficiencies
+when it came to large i/o requests and readv/writev style operations, as it
+forced such requests to be broken up into small chunks before being passed
+on to the generic block layer, only to be merged by the i/o scheduler
+when the underlying device was capable of handling the i/o in one shot.
+Also, using the buffer head as an i/o structure for i/os that didn't originate
+from the buffer cache unnecessarily added to the weight of the descriptors
+which were generated for each such chunk.
+
+The following were some of the goals and expectations considered in the
+redesign of the block i/o data structure in 2.5.
+
+1.  Should be appropriate as a descriptor for both raw and buffered i/o  -
+    avoid cache related fields which are irrelevant in the direct/page i/o path,
+    or filesystem block size alignment restrictions which may not be relevant
+    for raw i/o.
+2.  Ability to represent high-memory buffers (which do not have a virtual
+    address mapping in kernel address space).
+3.  Ability to represent large i/os w/o unnecessarily breaking them up (i.e
+    greater than PAGE_SIZE chunks in one shot)
+4.  At the same time, ability to retain independent identity of i/os from
+    different sources or i/o units requiring individual completion (e.g. for
+    latency reasons)
+5.  Ability to represent an i/o involving multiple physical memory segments
+    (including non-page aligned page fragments, as specified via readv/writev)
+    without unnecessarily breaking it up, if the underlying device is capable of
+    handling it.
+6.  Preferably should be based on a memory descriptor structure that can be
+    passed around different types of subsystems or layers, maybe even
+    networking, without duplication or extra copies of data/descriptor fields
+    themselves in the process
+7.  Ability to handle the possibility of splits/merges as the structure passes
+    through layered drivers (lvm, md, evms), with minimal overhead.
+
+The solution was to define a new structure (bio)  for the block layer,
+instead of using the buffer head structure (bh) directly, the idea being
+avoidance of some associated baggage and limitations. The bio structure
+is uniformly used for all i/o at the block layer ; it forms a part of the
+bh structure for buffered i/o, and in the case of raw/direct i/o kiobufs are
+mapped to bio structures.
+
+2.2 The bio struct
+------------------
+
+The bio structure uses a vector representation pointing to an array of tuples
+of <page, offset, len> to describe the i/o buffer, and has various other
+fields describing i/o parameters and state that needs to be maintained for
+performing the i/o.
+
+Notice that this representation means that a bio has no virtual address
+mapping at all (unlike buffer heads).
+
+::
+
+  struct bio_vec {
+       struct page     *bv_page;
+       unsigned short  bv_len;
+       unsigned short  bv_offset;
+  };
+
+  /*
+   * main unit of I/O for the block layer and lower layers (ie drivers)
+   */
+  struct bio {
+       struct bio          *bi_next;    /* request queue link */
+       struct block_device *bi_bdev;	/* target device */
+       unsigned long       bi_flags;    /* status, command, etc */
+       unsigned long       bi_opf;       /* low bits: r/w, high: priority */
+
+       unsigned int	bi_vcnt;     /* how may bio_vec's */
+       struct bvec_iter	bi_iter;	/* current index into bio_vec array */
+
+       unsigned int	bi_size;     /* total size in bytes */
+       unsigned short	bi_hw_segments; /* segments after DMA remapping */
+       unsigned int	bi_max;	     /* max bio_vecs we can hold
+                                        used as index into pool */
+       struct bio_vec   *bi_io_vec;  /* the actual vec list */
+       bio_end_io_t	*bi_end_io;  /* bi_end_io (bio) */
+       atomic_t		bi_cnt;	     /* pin count: free when it hits zero */
+       void             *bi_private;
+  };
+
+With this multipage bio design:
+
+- Large i/os can be sent down in one go using a bio_vec list consisting
+  of an array of <page, offset, len> fragments (similar to the way fragments
+  are represented in the zero-copy network code)
+- Splitting of an i/o request across multiple devices (as in the case of
+  lvm or raid) is achieved by cloning the bio (where the clone points to
+  the same bi_io_vec array, but with the index and size accordingly modified)
+- A linked list of bios is used as before for unrelated merges [*]_ - this
+  avoids reallocs and makes independent completions easier to handle.
+- Code that traverses the req list can find all the segments of a bio
+  by using rq_for_each_segment.  This handles the fact that a request
+  has multiple bios, each of which can have multiple segments.
+- Drivers which can't process a large bio in one shot can use the bi_iter
+  field to keep track of the next bio_vec entry to process.
+  (e.g a 1MB bio_vec needs to be handled in max 128kB chunks for IDE)
+  [TBD: Should preferably also have a bi_voffset and bi_vlen to avoid modifying
+  bi_offset an len fields]
+
+.. [*]
+
+	unrelated merges -- a request ends up containing two or more bios that
+	didn't originate from the same place.
+
+bi_end_io() i/o callback gets called on i/o completion of the entire bio.
+
+At a lower level, drivers build a scatter gather list from the merged bios.
+The scatter gather list is in the form of an array of <page, offset, len>
+entries with their corresponding dma address mappings filled in at the
+appropriate time. As an optimization, contiguous physical pages can be
+covered by a single entry where <page> refers to the first page and <len>
+covers the range of pages (up to 16 contiguous pages could be covered this
+way). There is a helper routine (blk_rq_map_sg) which drivers can use to build
+the sg list.
+
+Note: Right now the only user of bios with more than one page is ll_rw_kio,
+which in turn means that only raw I/O uses it (direct i/o may not work
+right now). The intent however is to enable clustering of pages etc to
+become possible. The pagebuf abstraction layer from SGI also uses multi-page
+bios, but that is currently not included in the stock development kernels.
+The same is true of Andrew Morton's work-in-progress multipage bio writeout
+and readahead patches.
+
+2.3 Changes in the Request Structure
+------------------------------------
+
+The request structure is the structure that gets passed down to low level
+drivers. The block layer make_request function builds up a request structure,
+places it on the queue and invokes the drivers request_fn. The driver makes
+use of block layer helper routine elv_next_request to pull the next request
+off the queue. Control or diagnostic functions might bypass block and directly
+invoke underlying driver entry points passing in a specially constructed
+request structure.
+
+Only some relevant fields (mainly those which changed or may be referred
+to in some of the discussion here) are listed below, not necessarily in
+the order in which they occur in the structure (see include/linux/blkdev.h)
+Refer to Documentation/block/request.rst for details about all the request
+structure fields and a quick reference about the layers which are
+supposed to use or modify those fields::
+
+  struct request {
+	struct list_head queuelist;  /* Not meant to be directly accessed by
+					the driver.
+					Used by q->elv_next_request_fn
+					rq->queue is gone
+					*/
+	.
+	.
+	unsigned char cmd[16]; /* prebuilt command data block */
+	unsigned long flags;   /* also includes earlier rq->cmd settings */
+	.
+	.
+	sector_t sector; /* this field is now of type sector_t instead of int
+			    preparation for 64 bit sectors */
+	.
+	.
+
+	/* Number of scatter-gather DMA addr+len pairs after
+	 * physical address coalescing is performed.
+	 */
+	unsigned short nr_phys_segments;
+
+	/* Number of scatter-gather addr+len pairs after
+	 * physical and DMA remapping hardware coalescing is performed.
+	 * This is the number of scatter-gather entries the driver
+	 * will actually have to deal with after DMA mapping is done.
+	 */
+	unsigned short nr_hw_segments;
+
+	/* Various sector counts */
+	unsigned long nr_sectors;  /* no. of sectors left: driver modifiable */
+	unsigned long hard_nr_sectors;  /* block internal copy of above */
+	unsigned int current_nr_sectors; /* no. of sectors left in the
+					   current segment:driver modifiable */
+	unsigned long hard_cur_sectors; /* block internal copy of the above */
+	.
+	.
+	int tag;	/* command tag associated with request */
+	void *special;  /* same as before */
+	char *buffer;   /* valid only for low memory buffers up to
+			 current_nr_sectors */
+	.
+	.
+	struct bio *bio, *biotail;  /* bio list instead of bh */
+	struct request_list *rl;
+  }
+
+See the req_ops and req_flag_bits definitions for an explanation of the various
+flags available. Some bits are used by the block layer or i/o scheduler.
+
+The behaviour of the various sector counts are almost the same as before,
+except that since we have multi-segment bios, current_nr_sectors refers
+to the numbers of sectors in the current segment being processed which could
+be one of the many segments in the current bio (i.e i/o completion unit).
+The nr_sectors value refers to the total number of sectors in the whole
+request that remain to be transferred (no change). The purpose of the
+hard_xxx values is for block to remember these counts every time it hands
+over the request to the driver. These values are updated by block on
+end_that_request_first, i.e. every time the driver completes a part of the
+transfer and invokes block end*request helpers to mark this. The
+driver should not modify these values. The block layer sets up the
+nr_sectors and current_nr_sectors fields (based on the corresponding
+hard_xxx values and the number of bytes transferred) and updates it on
+every transfer that invokes end_that_request_first. It does the same for the
+buffer, bio, bio->bi_iter fields too.
+
+The buffer field is just a virtual address mapping of the current segment
+of the i/o buffer in cases where the buffer resides in low-memory. For high
+memory i/o, this field is not valid and must not be used by drivers.
+
+Code that sets up its own request structures and passes them down to
+a driver needs to be careful about interoperation with the block layer helper
+functions which the driver uses. (Section 1.3)
+
+3. Using bios
+=============
+
+3.1 Setup/Teardown
+------------------
+
+There are routines for managing the allocation, and reference counting, and
+freeing of bios (bio_alloc, bio_get, bio_put).
+
+This makes use of Ingo Molnar's mempool implementation, which enables
+subsystems like bio to maintain their own reserve memory pools for guaranteed
+deadlock-free allocations during extreme VM load. For example, the VM
+subsystem makes use of the block layer to writeout dirty pages in order to be
+able to free up memory space, a case which needs careful handling. The
+allocation logic draws from the preallocated emergency reserve in situations
+where it cannot allocate through normal means. If the pool is empty and it
+can wait, then it would trigger action that would help free up memory or
+replenish the pool (without deadlocking) and wait for availability in the pool.
+If it is in IRQ context, and hence not in a position to do this, allocation
+could fail if the pool is empty. In general mempool always first tries to
+perform allocation without having to wait, even if it means digging into the
+pool as long it is not less that 50% full.
+
+On a free, memory is released to the pool or directly freed depending on
+the current availability in the pool. The mempool interface lets the
+subsystem specify the routines to be used for normal alloc and free. In the
+case of bio, these routines make use of the standard slab allocator.
+
+The caller of bio_alloc is expected to taken certain steps to avoid
+deadlocks, e.g. avoid trying to allocate more memory from the pool while
+already holding memory obtained from the pool.
+
+::
+
+  [TBD: This is a potential issue, though a rare possibility
+   in the bounce bio allocation that happens in the current code, since
+   it ends up allocating a second bio from the same pool while
+   holding the original bio ]
+
+Memory allocated from the pool should be released back within a limited
+amount of time (in the case of bio, that would be after the i/o is completed).
+This ensures that if part of the pool has been used up, some work (in this
+case i/o) must already be in progress and memory would be available when it
+is over. If allocating from multiple pools in the same code path, the order
+or hierarchy of allocation needs to be consistent, just the way one deals
+with multiple locks.
+
+The bio_alloc routine also needs to allocate the bio_vec_list (bvec_alloc())
+for a non-clone bio. There are the 6 pools setup for different size biovecs,
+so bio_alloc(gfp_mask, nr_iovecs) will allocate a vec_list of the
+given size from these slabs.
+
+The bio_get() routine may be used to hold an extra reference on a bio prior
+to i/o submission, if the bio fields are likely to be accessed after the
+i/o is issued (since the bio may otherwise get freed in case i/o completion
+happens in the meantime).
+
+The bio_clone_fast() routine may be used to duplicate a bio, where the clone
+shares the bio_vec_list with the original bio (i.e. both point to the
+same bio_vec_list). This would typically be used for splitting i/o requests
+in lvm or md.
+
+3.2 Generic bio helper Routines
+-------------------------------
+
+3.2.1 Traversing segments and completion units in a request
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The macro rq_for_each_segment() should be used for traversing the bios
+in the request list (drivers should avoid directly trying to do it
+themselves). Using these helpers should also make it easier to cope
+with block changes in the future.
+
+::
+
+	struct req_iterator iter;
+	rq_for_each_segment(bio_vec, rq, iter)
+		/* bio_vec is now current segment */
+
+I/O completion callbacks are per-bio rather than per-segment, so drivers
+that traverse bio chains on completion need to keep that in mind. Drivers
+which don't make a distinction between segments and completion units would
+need to be reorganized to support multi-segment bios.
+
+3.2.2 Setting up DMA scatterlists
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The blk_rq_map_sg() helper routine would be used for setting up scatter
+gather lists from a request, so a driver need not do it on its own.
+
+	nr_segments = blk_rq_map_sg(q, rq, scatterlist);
+
+The helper routine provides a level of abstraction which makes it easier
+to modify the internals of request to scatterlist conversion down the line
+without breaking drivers. The blk_rq_map_sg routine takes care of several
+things like collapsing physically contiguous segments (if QUEUE_FLAG_CLUSTER
+is set) and correct segment accounting to avoid exceeding the limits which
+the i/o hardware can handle, based on various queue properties.
+
+- Prevents a clustered segment from crossing a 4GB mem boundary
+- Avoids building segments that would exceed the number of physical
+  memory segments that the driver can handle (phys_segments) and the
+  number that the underlying hardware can handle at once, accounting for
+  DMA remapping (hw_segments)  (i.e. IOMMU aware limits).
+
+Routines which the low level driver can use to set up the segment limits:
+
+blk_queue_max_hw_segments() : Sets an upper limit of the maximum number of
+hw data segments in a request (i.e. the maximum number of address/length
+pairs the host adapter can actually hand to the device at once)
+
+blk_queue_max_phys_segments() : Sets an upper limit on the maximum number
+of physical data segments in a request (i.e. the largest sized scatter list
+a driver could handle)
+
+3.2.3 I/O completion
+^^^^^^^^^^^^^^^^^^^^
+
+The existing generic block layer helper routines end_request,
+end_that_request_first and end_that_request_last can be used for i/o
+completion (and setting things up so the rest of the i/o or the next
+request can be kicked of) as before. With the introduction of multi-page
+bio support, end_that_request_first requires an additional argument indicating
+the number of sectors completed.
+
+3.2.4 Implications for drivers that do not interpret bios
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+(don't handle multiple segments)
+
+Drivers that do not interpret bios e.g those which do not handle multiple
+segments and do not support i/o into high memory addresses (require bounce
+buffers) and expect only virtually mapped buffers, can access the rq->buffer
+field. As before the driver should use current_nr_sectors to determine the
+size of remaining data in the current segment (that is the maximum it can
+transfer in one go unless it interprets segments), and rely on the block layer
+end_request, or end_that_request_first/last to take care of all accounting
+and transparent mapping of the next bio segment when a segment boundary
+is crossed on completion of a transfer. (The end*request* functions should
+be used if only if the request has come down from block/bio path, not for
+direct access requests which only specify rq->buffer without a valid rq->bio)
+
+3.3 I/O Submission
+------------------
+
+The routine submit_bio() is used to submit a single io. Higher level i/o
+routines make use of this:
+
+(a) Buffered i/o:
+
+The routine submit_bh() invokes submit_bio() on a bio corresponding to the
+bh, allocating the bio if required. ll_rw_block() uses submit_bh() as before.
+
+(b) Kiobuf i/o (for raw/direct i/o):
+
+The ll_rw_kio() routine breaks up the kiobuf into page sized chunks and
+maps the array to one or more multi-page bios, issuing submit_bio() to
+perform the i/o on each of these.
+
+The embedded bh array in the kiobuf structure has been removed and no
+preallocation of bios is done for kiobufs. [The intent is to remove the
+blocks array as well, but it's currently in there to kludge around direct i/o.]
+Thus kiobuf allocation has switched back to using kmalloc rather than vmalloc.
+
+Todo/Observation:
+
+ A single kiobuf structure is assumed to correspond to a contiguous range
+ of data, so brw_kiovec() invokes ll_rw_kio for each kiobuf in a kiovec.
+ So right now it wouldn't work for direct i/o on non-contiguous blocks.
+ This is to be resolved.  The eventual direction is to replace kiobuf
+ by kvec's.
+
+ Badari Pulavarty has a patch to implement direct i/o correctly using
+ bio and kvec.
+
+
+(c) Page i/o:
+
+Todo/Under discussion:
+
+ Andrew Morton's multi-page bio patches attempt to issue multi-page
+ writeouts (and reads) from the page cache, by directly building up
+ large bios for submission completely bypassing the usage of buffer
+ heads. This work is still in progress.
+
+ Christoph Hellwig had some code that uses bios for page-io (rather than
+ bh). This isn't included in bio as yet. Christoph was also working on a
+ design for representing virtual/real extents as an entity and modifying
+ some of the address space ops interfaces to utilize this abstraction rather
+ than buffer_heads. (This is somewhat along the lines of the SGI XFS pagebuf
+ abstraction, but intended to be as lightweight as possible).
+
+(d) Direct access i/o:
+
+Direct access requests that do not contain bios would be submitted differently
+as discussed earlier in section 1.3.
+
+Aside:
+
+  Kvec i/o:
+
+  Ben LaHaise's aio code uses a slightly different structure instead
+  of kiobufs, called a kvec_cb. This contains an array of <page, offset, len>
+  tuples (very much like the networking code), together with a callback function
+  and data pointer. This is embedded into a brw_cb structure when passed
+  to brw_kvec_async().
+
+  Now it should be possible to directly map these kvecs to a bio. Just as while
+  cloning, in this case rather than PRE_BUILT bio_vecs, we set the bi_io_vec
+  array pointer to point to the veclet array in kvecs.
+
+  TBD: In order for this to work, some changes are needed in the way multi-page
+  bios are handled today. The values of the tuples in such a vector passed in
+  from higher level code should not be modified by the block layer in the course
+  of its request processing, since that would make it hard for the higher layer
+  to continue to use the vector descriptor (kvec) after i/o completes. Instead,
+  all such transient state should either be maintained in the request structure,
+  and passed on in some way to the endio completion routine.
+
+
+4. The I/O scheduler
+====================
+
+I/O scheduler, a.k.a. elevator, is implemented in two layers.  Generic dispatch
+queue and specific I/O schedulers.  Unless stated otherwise, elevator is used
+to refer to both parts and I/O scheduler to specific I/O schedulers.
+
+Block layer implements generic dispatch queue in `block/*.c`.
+The generic dispatch queue is responsible for requeueing, handling non-fs
+requests and all other subtleties.
+
+Specific I/O schedulers are responsible for ordering normal filesystem
+requests.  They can also choose to delay certain requests to improve
+throughput or whatever purpose.  As the plural form indicates, there are
+multiple I/O schedulers.  They can be built as modules but at least one should
+be built inside the kernel.  Each queue can choose different one and can also
+change to another one dynamically.
+
+A block layer call to the i/o scheduler follows the convention elv_xxx(). This
+calls elevator_xxx_fn in the elevator switch (block/elevator.c). Oh, xxx
+and xxx might not match exactly, but use your imagination. If an elevator
+doesn't implement a function, the switch does nothing or some minimal house
+keeping work.
+
+4.1. I/O scheduler API
+----------------------
+
+The functions an elevator may implement are: (* are mandatory)
+
+=============================== ================================================
+elevator_merge_fn		called to query requests for merge with a bio
+
+elevator_merge_req_fn		called when two requests get merged. the one
+				which gets merged into the other one will be
+				never seen by I/O scheduler again. IOW, after
+				being merged, the request is gone.
+
+elevator_merged_fn		called when a request in the scheduler has been
+				involved in a merge. It is used in the deadline
+				scheduler for example, to reposition the request
+				if its sorting order has changed.
+
+elevator_allow_merge_fn		called whenever the block layer determines
+				that a bio can be merged into an existing
+				request safely. The io scheduler may still
+				want to stop a merge at this point if it
+				results in some sort of conflict internally,
+				this hook allows it to do that. Note however
+				that two *requests* can still be merged at later
+				time. Currently the io scheduler has no way to
+				prevent that. It can only learn about the fact
+				from elevator_merge_req_fn callback.
+
+elevator_dispatch_fn*		fills the dispatch queue with ready requests.
+				I/O schedulers are free to postpone requests by
+				not filling the dispatch queue unless @force
+				is non-zero.  Once dispatched, I/O schedulers
+				are not allowed to manipulate the requests -
+				they belong to generic dispatch queue.
+
+elevator_add_req_fn*		called to add a new request into the scheduler
+
+elevator_former_req_fn
+elevator_latter_req_fn		These return the request before or after the
+				one specified in disk sort order. Used by the
+				block layer to find merge possibilities.
+
+elevator_completed_req_fn	called when a request is completed.
+
+elevator_may_queue_fn		returns true if the scheduler wants to allow the
+				current context to queue a new request even if
+				it is over the queue limit. This must be used
+				very carefully!!
+
+elevator_set_req_fn
+elevator_put_req_fn		Must be used to allocate and free any elevator
+				specific storage for a request.
+
+elevator_activate_req_fn	Called when device driver first sees a request.
+				I/O schedulers can use this callback to
+				determine when actual execution of a request
+				starts.
+elevator_deactivate_req_fn	Called when device driver decides to delay
+				a request by requeueing it.
+
+elevator_init_fn*
+elevator_exit_fn		Allocate and free any elevator specific storage
+				for a queue.
+=============================== ================================================
+
+4.2 Request flows seen by I/O schedulers
+----------------------------------------
+
+All requests seen by I/O schedulers strictly follow one of the following three
+flows.
+
+ set_req_fn ->
+
+ i.   add_req_fn -> (merged_fn ->)* -> dispatch_fn -> activate_req_fn ->
+      (deactivate_req_fn -> activate_req_fn ->)* -> completed_req_fn
+ ii.  add_req_fn -> (merged_fn ->)* -> merge_req_fn
+ iii. [none]
+
+ -> put_req_fn
+
+4.3 I/O scheduler implementation
+--------------------------------
+
+The generic i/o scheduler algorithm attempts to sort/merge/batch requests for
+optimal disk scan and request servicing performance (based on generic
+principles and device capabilities), optimized for:
+
+i.   improved throughput
+ii.  improved latency
+iii. better utilization of h/w & CPU time
+
+Characteristics:
+
+i. Binary tree
+AS and deadline i/o schedulers use red black binary trees for disk position
+sorting and searching, and a fifo linked list for time-based searching. This
+gives good scalability and good availability of information. Requests are
+almost always dispatched in disk sort order, so a cache is kept of the next
+request in sort order to prevent binary tree lookups.
+
+This arrangement is not a generic block layer characteristic however, so
+elevators may implement queues as they please.
+
+ii. Merge hash
+AS and deadline use a hash table indexed by the last sector of a request. This
+enables merging code to quickly look up "back merge" candidates, even when
+multiple I/O streams are being performed at once on one disk.
+
+"Front merges", a new request being merged at the front of an existing request,
+are far less common than "back merges" due to the nature of most I/O patterns.
+Front merges are handled by the binary trees in AS and deadline schedulers.
+
+iii. Plugging the queue to batch requests in anticipation of opportunities for
+     merge/sort optimizations
+
+Plugging is an approach that the current i/o scheduling algorithm resorts to so
+that it collects up enough requests in the queue to be able to take
+advantage of the sorting/merging logic in the elevator. If the
+queue is empty when a request comes in, then it plugs the request queue
+(sort of like plugging the bath tub of a vessel to get fluid to build up)
+till it fills up with a few more requests, before starting to service
+the requests. This provides an opportunity to merge/sort the requests before
+passing them down to the device. There are various conditions when the queue is
+unplugged (to open up the flow again), either through a scheduled task or
+could be on demand. For example wait_on_buffer sets the unplugging going
+through sync_buffer() running blk_run_address_space(mapping). Or the caller
+can do it explicity through blk_unplug(bdev). So in the read case,
+the queue gets explicitly unplugged as part of waiting for completion on that
+buffer.
+
+Aside:
+  This is kind of controversial territory, as it's not clear if plugging is
+  always the right thing to do. Devices typically have their own queues,
+  and allowing a big queue to build up in software, while letting the device be
+  idle for a while may not always make sense. The trick is to handle the fine
+  balance between when to plug and when to open up. Also now that we have
+  multi-page bios being queued in one shot, we may not need to wait to merge
+  a big request from the broken up pieces coming by.
+
+4.4 I/O contexts
+----------------
+
+I/O contexts provide a dynamically allocated per process data area. They may
+be used in I/O schedulers, and in the block layer (could be used for IO statis,
+priorities for example). See `*io_context` in block/ll_rw_blk.c, and as-iosched.c
+for an example of usage in an i/o scheduler.
+
+
+5. Scalability related changes
+==============================
+
+5.1 Granular Locking: io_request_lock replaced by a per-queue lock
+------------------------------------------------------------------
+
+The global io_request_lock has been removed as of 2.5, to avoid
+the scalability bottleneck it was causing, and has been replaced by more
+granular locking. The request queue structure has a pointer to the
+lock to be used for that queue. As a result, locking can now be
+per-queue, with a provision for sharing a lock across queues if
+necessary (e.g the scsi layer sets the queue lock pointers to the
+corresponding adapter lock, which results in a per host locking
+granularity). The locking semantics are the same, i.e. locking is
+still imposed by the block layer, grabbing the lock before
+request_fn execution which it means that lots of older drivers
+should still be SMP safe. Drivers are free to drop the queue
+lock themselves, if required. Drivers that explicitly used the
+io_request_lock for serialization need to be modified accordingly.
+Usually it's as easy as adding a global lock::
+
+	static DEFINE_SPINLOCK(my_driver_lock);
+
+and passing the address to that lock to blk_init_queue().
+
+5.2 64 bit sector numbers (sector_t prepares for 64 bit support)
+----------------------------------------------------------------
+
+The sector number used in the bio structure has been changed to sector_t,
+which could be defined as 64 bit in preparation for 64 bit sector support.
+
+6. Other Changes/Implications
+=============================
+
+6.1 Partition re-mapping handled by the generic block layer
+-----------------------------------------------------------
+
+In 2.5 some of the gendisk/partition related code has been reorganized.
+Now the generic block layer performs partition-remapping early and thus
+provides drivers with a sector number relative to whole device, rather than
+having to take partition number into account in order to arrive at the true
+sector number. The routine blk_partition_remap() is invoked by
+generic_make_request even before invoking the queue specific make_request_fn,
+so the i/o scheduler also gets to operate on whole disk sector numbers. This
+should typically not require changes to block drivers, it just never gets
+to invoke its own partition sector offset calculations since all bios
+sent are offset from the beginning of the device.
+
+
+7. A Few Tips on Migration of older drivers
+===========================================
+
+Old-style drivers that just use CURRENT and ignores clustered requests,
+may not need much change.  The generic layer will automatically handle
+clustered requests, multi-page bios, etc for the driver.
+
+For a low performance driver or hardware that is PIO driven or just doesn't
+support scatter-gather changes should be minimal too.
+
+The following are some points to keep in mind when converting old drivers
+to bio.
+
+Drivers should use elv_next_request to pick up requests and are no longer
+supposed to handle looping directly over the request list.
+(struct request->queue has been removed)
+
+Now end_that_request_first takes an additional number_of_sectors argument.
+It used to handle always just the first buffer_head in a request, now
+it will loop and handle as many sectors (on a bio-segment granularity)
+as specified.
+
+Now bh->b_end_io is replaced by bio->bi_end_io, but most of the time the
+right thing to use is bio_endio(bio) instead.
+
+If the driver is dropping the io_request_lock from its request_fn strategy,
+then it just needs to replace that with q->queue_lock instead.
+
+As described in Sec 1.1, drivers can set max sector size, max segment size
+etc per queue now. Drivers that used to define their own merge functions i
+to handle things like this can now just use the blk_queue_* functions at
+blk_init_queue time.
+
+Drivers no longer have to map a {partition, sector offset} into the
+correct absolute location anymore, this is done by the block layer, so
+where a driver received a request ala this before::
+
+	rq->rq_dev = mk_kdev(3, 5);	/* /dev/hda5 */
+	rq->sector = 0;			/* first sector on hda5 */
+
+it will now see::
+
+	rq->rq_dev = mk_kdev(3, 0);	/* /dev/hda */
+	rq->sector = 123128;		/* offset from start of disk */
+
+As mentioned, there is no virtual mapping of a bio. For DMA, this is
+not a problem as the driver probably never will need a virtual mapping.
+Instead it needs a bus mapping (dma_map_page for a single segment or
+use dma_map_sg for scatter gather) to be able to ship it to the driver. For
+PIO drivers (or drivers that need to revert to PIO transfer once in a
+while (IDE for example)), where the CPU is doing the actual data
+transfer a virtual mapping is needed. If the driver supports highmem I/O,
+(Sec 1.1, (ii) ) it needs to use kmap_atomic or similar to temporarily map
+a bio into the virtual address space.
+
+
+8. Prior/Related/Impacted patches
+=================================
+
+8.1. Earlier kiobuf patches (sct/axboe/chait/hch/mkp)
+-----------------------------------------------------
+
+- orig kiobuf & raw i/o patches (now in 2.4 tree)
+- direct kiobuf based i/o to devices (no intermediate bh's)
+- page i/o using kiobuf
+- kiobuf splitting for lvm (mkp)
+- elevator support for kiobuf request merging (axboe)
+
+8.2. Zero-copy networking (Dave Miller)
+---------------------------------------
+
+8.3. SGI XFS - pagebuf patches - use of kiobufs
+-----------------------------------------------
+8.4. Multi-page pioent patch for bio (Christoph Hellwig)
+--------------------------------------------------------
+8.5. Direct i/o implementation (Andrea Arcangeli) since 2.4.10-pre11
+--------------------------------------------------------------------
+8.6. Async i/o implementation patch (Ben LaHaise)
+-------------------------------------------------
+8.7. EVMS layering design (IBM EVMS team)
+-----------------------------------------
+8.8. Larger page cache size patch (Ben LaHaise) and Large page size (Daniel Phillips)
+-------------------------------------------------------------------------------------
+
+    => larger contiguous physical memory buffers
+
+8.9. VM reservations patch (Ben LaHaise)
+----------------------------------------
+8.10. Write clustering patches ? (Marcelo/Quintela/Riel ?)
+----------------------------------------------------------
+8.11. Block device in page cache patch (Andrea Archangeli) - now in 2.4.10+
+---------------------------------------------------------------------------
+8.12. Multiple block-size transfers for faster raw i/o (Shailabh Nagar, Badari)
+-------------------------------------------------------------------------------
+8.13  Priority based i/o scheduler - prepatches (Arjan van de Ven)
+------------------------------------------------------------------
+8.14  IDE Taskfile i/o patch (Andre Hedrick)
+--------------------------------------------
+8.15  Multi-page writeout and readahead patches (Andrew Morton)
+---------------------------------------------------------------
+8.16  Direct i/o patches for 2.5 using kvec and bio (Badari Pulavarthy)
+-----------------------------------------------------------------------
+
+9. Other References
+===================
+
+9.1 The Splice I/O Model
+------------------------
+
+Larry McVoy (and subsequent discussions on lkml, and Linus' comments - Jan 2001
+
+9.2 Discussions about kiobuf and bh design
+------------------------------------------
+
+On lkml between sct, linus, alan et al - Feb-March 2001 (many of the
+initial thoughts that led to bio were brought up in this discussion thread)
+
+9.3 Discussions on mempool on lkml - Dec 2001.
+----------------------------------------------