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Howlett" X-Rspam-User: X-Stat-Signature: eguwoe371kx5mzjapjags1oyw57n79f7 X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 237FC40005 X-HE-Tag: 1739486856-699664 X-HE-Meta: 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 uiuUf22P 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Change the documentation to reflect that vm_lock is integrated into vma and replaced with vm_refcnt. Document newly introduced vma_start_read_locked{_nested} functions. Signed-off-by: Suren Baghdasaryan Reviewed-by: Liam R. Howlett Reviewed-by: Lorenzo Stoakes --- Changes since v9 [1]: - Updated documenation, per Lorenzo Stoakes - Add Reviewed-by, per Lorenzo Stoakes [1] https://lore.kernel.org/all/20250111042604.3230628-18-surenb@google.com/ Documentation/mm/process_addrs.rst | 44 ++++++++++++++++++------------ 1 file changed, 26 insertions(+), 18 deletions(-) diff --git a/Documentation/mm/process_addrs.rst b/Documentation/mm/process_addrs.rst index 81417fa2ed20..e6756e78b476 100644 --- a/Documentation/mm/process_addrs.rst +++ b/Documentation/mm/process_addrs.rst @@ -716,9 +716,14 @@ 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 +731,9 @@ 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 +743,7 @@ 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 +784,31 @@ 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.