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Diffstat (limited to 'Documentation/mm/process_addrs.rst')
| -rw-r--r-- | Documentation/mm/process_addrs.rst | 98 |
1 files changed, 74 insertions, 24 deletions
diff --git a/Documentation/mm/process_addrs.rst b/Documentation/mm/process_addrs.rst index 81417fa2ed20..be49e2a269e4 100644 --- a/Documentation/mm/process_addrs.rst +++ b/Documentation/mm/process_addrs.rst @@ -303,7 +303,9 @@ There are four key operations typically performed on page tables: 1. **Traversing** page tables - Simply reading page tables in order to traverse them. This only requires that the VMA is kept stable, so a lock which establishes this suffices for traversal (there are also lockless variants - which eliminate even this requirement, such as :c:func:`!gup_fast`). + which eliminate even this requirement, such as :c:func:`!gup_fast`). There is + also a special case of page table traversal for non-VMA regions which we + consider separately below. 2. **Installing** page table mappings - Whether creating a new mapping or modifying an existing one in such a way as to change its identity. This requires that the VMA is kept stable via an mmap or VMA lock (explicitly not @@ -335,15 +337,13 @@ ahead and perform these operations on page tables (though internally, kernel operations that perform writes also acquire internal page table locks to serialise - see the page table implementation detail section for more details). +.. note:: We free empty PTE tables on zap under the RCU lock - this does not + change the aforementioned locking requirements around zapping. + When **installing** page table entries, the mmap or VMA lock must be held to keep the VMA stable. We explore why this is in the page table locking details section below. -.. warning:: Page tables are normally only traversed in regions covered by VMAs. - If you want to traverse page tables in areas that might not be - covered by VMAs, heavier locking is required. - See :c:func:`!walk_page_range_novma` for details. - **Freeing** page tables is an entirely internal memory management operation and has special requirements (see the page freeing section below for more details). @@ -355,6 +355,44 @@ has special requirements (see the page freeing section below for more details). from the reverse mappings, but no other VMAs can be permitted to be accessible and span the specified range. +Traversing non-VMA page tables +------------------------------ + +We've focused above on traversal of page tables belonging to VMAs. It is also +possible to traverse page tables which are not represented by VMAs. + +Kernel page table mappings themselves are generally managed but whatever part of +the kernel established them and the aforementioned locking rules do not apply - +for instance vmalloc has its own set of locks which are utilised for +establishing and tearing down page its page tables. + +However, for convenience we provide the :c:func:`!walk_kernel_page_table_range` +function which is synchronised via the mmap lock on the :c:macro:`!init_mm` +kernel instantiation of the :c:struct:`!struct mm_struct` metadata object. + +If an operation requires exclusive access, a write lock is used, but if not, a +read lock suffices - we assert only that at least a read lock has been acquired. + +Since, aside from vmalloc and memory hot plug, kernel page tables are not torn +down all that often - this usually suffices, however any caller of this +functionality must ensure that any additionally required locks are acquired in +advance. + +We also permit a truly unusual case is the traversal of non-VMA ranges in +**userland** ranges, as provided for by :c:func:`!walk_page_range_debug`. + +This has only one user - the general page table dumping logic (implemented in +:c:macro:`!mm/ptdump.c`) - which seeks to expose all mappings for debug purposes +even if they are highly unusual (possibly architecture-specific) and are not +backed by a VMA. + +We must take great care in this case, as the :c:func:`!munmap` implementation +detaches VMAs under an mmap write lock before tearing down page tables under a +downgraded mmap read lock. + +This means such an operation could race with this, and thus an mmap **write** +lock is required. + Lock ordering ------------- @@ -461,6 +499,10 @@ Locking Implementation Details Page table locking details -------------------------- +.. note:: This section explores page table locking requirements for page tables + encompassed by a VMA. See the above section on non-VMA page table + traversal for details on how we handle that case. + In addition to the locks described in the terminology section above, we have additional locks dedicated to page tables: @@ -716,9 +758,14 @@ calls :c:func:`!rcu_read_lock` to ensure that the VMA is looked up in an RCU critical section, then attempts to VMA lock it via :c:func:`!vma_start_read`, before releasing the RCU lock via :c:func:`!rcu_read_unlock`. -VMA read locks hold the read lock on the :c:member:`!vma->vm_lock` semaphore for -their duration and the caller of :c:func:`!lock_vma_under_rcu` must release it -via :c:func:`!vma_end_read`. +In cases when the user already holds mmap read lock, :c:func:`!vma_start_read_locked` +and :c:func:`!vma_start_read_locked_nested` can be used. These functions do not +fail due to lock contention but the caller should still check their return values +in case they fail for other reasons. + +VMA read locks increment :c:member:`!vma.vm_refcnt` reference counter for their +duration and the caller of :c:func:`!lock_vma_under_rcu` must drop it via +:c:func:`!vma_end_read`. VMA **write** locks are acquired via :c:func:`!vma_start_write` in instances where a VMA is about to be modified, unlike :c:func:`!vma_start_read` the lock is always @@ -726,9 +773,9 @@ acquired. An mmap write lock **must** be held for the duration of the VMA write lock, releasing or downgrading the mmap write lock also releases the VMA write lock so there is no :c:func:`!vma_end_write` function. -Note that a semaphore write lock is not held across a VMA lock. Rather, a -sequence number is used for serialisation, and the write semaphore is only -acquired at the point of write lock to update this. +Note that when write-locking a VMA lock, the :c:member:`!vma.vm_refcnt` is temporarily +modified so that readers can detect the presense of a writer. The reference counter is +restored once the vma sequence number used for serialisation is updated. This ensures the semantics we require - VMA write locks provide exclusive write access to the VMA. @@ -738,7 +785,7 @@ Implementation details The VMA lock mechanism is designed to be a lightweight means of avoiding the use of the heavily contended mmap lock. It is implemented using a combination of a -read/write semaphore and sequence numbers belonging to the containing +reference counter and sequence numbers belonging to the containing :c:struct:`!struct mm_struct` and the VMA. Read locks are acquired via :c:func:`!vma_start_read`, which is an optimistic @@ -779,28 +826,31 @@ release of any VMA locks on its release makes sense, as you would never want to keep VMAs locked across entirely separate write operations. It also maintains correct lock ordering. -Each time a VMA read lock is acquired, we acquire a read lock on the -:c:member:`!vma->vm_lock` read/write semaphore and hold it, while checking that -the sequence count of the VMA does not match that of the mm. +Each time a VMA read lock is acquired, we increment :c:member:`!vma.vm_refcnt` +reference counter and check that the sequence count of the VMA does not match +that of the mm. -If it does, the read lock fails. If it does not, we hold the lock, excluding -writers, but permitting other readers, who will also obtain this lock under RCU. +If it does, the read lock fails and :c:member:`!vma.vm_refcnt` is dropped. +If it does not, we keep the reference counter raised, excluding writers, but +permitting other readers, who can also obtain this lock under RCU. Importantly, maple tree operations performed in :c:func:`!lock_vma_under_rcu` are also RCU safe, so the whole read lock operation is guaranteed to function correctly. -On the write side, we acquire a write lock on the :c:member:`!vma->vm_lock` -read/write semaphore, before setting the VMA's sequence number under this lock, -also simultaneously holding the mmap write lock. +On the write side, we set a bit in :c:member:`!vma.vm_refcnt` which can't be +modified by readers and wait for all readers to drop their reference count. +Once there are no readers, the VMA's sequence number is set to match that of +the mm. During this entire operation mmap write lock is held. This way, if any read locks are in effect, :c:func:`!vma_start_write` will sleep until these are finished and mutual exclusion is achieved. -After setting the VMA's sequence number, the lock is released, avoiding -complexity with a long-term held write lock. +After setting the VMA's sequence number, the bit in :c:member:`!vma.vm_refcnt` +indicating a writer is cleared. From this point on, VMA's sequence number will +indicate VMA's write-locked state until mmap write lock is dropped or downgraded. -This clever combination of a read/write semaphore and sequence count allows for +This clever combination of a reference counter and sequence count allows for fast RCU-based per-VMA lock acquisition (especially on page fault, though utilised elsewhere) with minimal complexity around lock ordering. |