btrfs: change timing for qgroup reserved space for ordered extents to fix reserved space leak

[BUG]
The following simple workload from fsstress can lead to qgroup reserved
data space leak:
  0/0: creat f0 x:0 0 0
  0/0: creat add id=0,parent=-1
  0/1: write f0[259 1 0 0 0 0] [600030,27288] 0
  0/4: dwrite - xfsctl(XFS_IOC_DIOINFO) f0[259 1 0 0 64 627318] return 25, fallback to stat()
  0/4: dwrite f0[259 1 0 0 64 627318] [610304,106496] 0

This would cause btrfs qgroup to leak 20480 bytes for data reserved
space.  If btrfs qgroup limit is enabled, such leak can lead to
unexpected early EDQUOT and unusable space.

[CAUSE]
When doing direct IO, kernel will try to writeback existing buffered
page cache, then invalidate them:
  generic_file_direct_write()
  |- filemap_write_and_wait_range();
  |- invalidate_inode_pages2_range();

However for btrfs, the bi_end_io hook doesn't finish all its heavy work
right after bio ends.  In fact, it delays its work further:

  submit_extent_page(end_io_func=end_bio_extent_writepage);
  end_bio_extent_writepage()
  |- btrfs_writepage_endio_finish_ordered()
     |- btrfs_init_work(finish_ordered_fn);

  <<< Work queue execution >>>
  finish_ordered_fn()
  |- btrfs_finish_ordered_io();
     |- Clear qgroup bits

This means, when filemap_write_and_wait_range() returns,
btrfs_finish_ordered_io() is not guaranteed to be executed, thus the
qgroup bits for related range are not cleared.

Now into how the leak happens, this will only focus on the overlapping
part of buffered and direct IO part.

1. After buffered write
   The inode had the following range with QGROUP_RESERVED bit:
   	596		616K
	|///////////////|
   Qgroup reserved data space: 20K

2. Writeback part for range [596K, 616K)
   Write back finished, but btrfs_finish_ordered_io() not get called
   yet.
   So we still have:
   	596K		616K
	|///////////////|
   Qgroup reserved data space: 20K

3. Pages for range [596K, 616K) get released
   This will clear all qgroup bits, but don't update the reserved data
   space.
   So we have:
   	596K		616K
	|		|
   Qgroup reserved data space: 20K
   That number doesn't match the qgroup bit range anymore.

4. Dio prepare space for range [596K, 700K)
   Qgroup reserved data space for that range, we got:
   	596K		616K			700K
	|///////////////|///////////////////////|
   Qgroup reserved data space: 20K + 104K = 124K

5. btrfs_finish_ordered_range() gets executed for range [596K, 616K)
   Qgroup free reserved space for that range, we got:
   	596K		616K			700K
	|		|///////////////////////|
   We need to free that range of reserved space.
   Qgroup reserved data space: 124K - 20K = 104K

6. btrfs_finish_ordered_range() gets executed for range [596K, 700K)
   However qgroup bit for range [596K, 616K) is already cleared in
   previous step, so we only free 84K for qgroup reserved space.
   	596K		616K			700K
	|		|			|
   We need to free that range of reserved space.
   Qgroup reserved data space: 104K - 84K = 20K

   Now there is no way to release that 20K unless disabling qgroup or
   unmounting the fs.

[FIX]
This patch will change the timing of btrfs_qgroup_release/free_data()
call.  Here it uses buffered COW write as an example.

	The new timing			|	The old timing
----------------------------------------+---------------------------------------
 btrfs_buffered_write()			| btrfs_buffered_write()
 |- btrfs_qgroup_reserve_data() 	| |- btrfs_qgroup_reserve_data()
					|
 btrfs_run_delalloc_range()		| btrfs_run_delalloc_range()
 |- btrfs_add_ordered_extent()  	|
    |- btrfs_qgroup_release_data()	|
       The reserved is passed into	|
       btrfs_ordered_extent structure	|
					|
 btrfs_finish_ordered_io()		| btrfs_finish_ordered_io()
 |- The reserved space is passed to 	| |- btrfs_qgroup_release_data()
    btrfs_qgroup_record			|    The resereved space is passed
					|    to btrfs_qgroup_recrod
					|
 btrfs_qgroup_account_extents()		| btrfs_qgroup_account_extents()
 |- btrfs_qgroup_free_refroot()		| |- btrfs_qgroup_free_refroot()

The point of such change is to ensure, when ordered extents are
submitted, the qgroup reserved space is already released, to keep the
timing aligned with file_write_and_wait_range().

So that qgroup data reserved space is all bound to btrfs_ordered_extent
and solve the timing mismatch.

Fixes: f695fdcef83a ("btrfs: qgroup: Introduce functions to release/free qgroup reserve data space")
Suggested-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 21 insertions, 1 deletions

diff --git a/fs/btrfs/ordered-data.c b/fs/btrfs/ordered-data.c
index e13b3d28c063..c8bd7a4e67bb 100644
--- a/fs/btrfs/ordered-data.c
+++ b/fs/btrfs/ordered-data.c
@@ -15,6 +15,7 @@
 #include "disk-io.h"
 #include "compression.h"
 #include "delalloc-space.h"
+#include "qgroup.h"
 
 static struct kmem_cache *btrfs_ordered_extent_cache;
 
@@ -152,7 +153,8 @@ static inline struct rb_node *tree_search(struct btrfs_ordered_inode_tree *tree,
 	return ret;
 }
 
-/* allocate and add a new ordered_extent into the per-inode tree.
+/*
+ * Allocate and add a new ordered_extent into the per-inode tree.
  *
  * The tree is given a single reference on the ordered extent that was
  * inserted.
@@ -167,7 +169,24 @@ static int __btrfs_add_ordered_extent(struct inode *inode, u64 file_offset,
 	struct btrfs_ordered_inode_tree *tree;
 	struct rb_node *node;
 	struct btrfs_ordered_extent *entry;
+	int ret;
 
+	if (type == BTRFS_ORDERED_NOCOW || type == BTRFS_ORDERED_PREALLOC) {
+		/* For nocow write, we can release the qgroup rsv right now */
+		ret = btrfs_qgroup_free_data(inode, NULL, file_offset,
+					     num_bytes);
+		if (ret < 0)
+			return ret;
+		ret = 0;
+	} else {
+		/*
+		 * The ordered extent has reserved qgroup space, release now
+		 * and pass the reserved number for qgroup_record to free.
+		 */
+		ret = btrfs_qgroup_release_data(inode, file_offset, num_bytes);
+		if (ret < 0)
+			return ret;
+	}
 	tree = &BTRFS_I(inode)->ordered_tree;
 	entry = kmem_cache_zalloc(btrfs_ordered_extent_cache, GFP_NOFS);
 	if (!entry)
@@ -181,6 +200,7 @@ static int __btrfs_add_ordered_extent(struct inode *inode, u64 file_offset,
 	entry->inode = igrab(inode);
 	entry->compress_type = compress_type;
 	entry->truncated_len = (u64)-1;
+	entry->qgroup_rsv = ret;
 	if (type != BTRFS_ORDERED_IO_DONE && type != BTRFS_ORDERED_COMPLETE)
 		set_bit(type, &entry->flags);