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Howlett" X-Stat-Signature: 8xuobbnsysoncexbnop4zwiydd5ixjg6 X-Rspam-User: X-Rspamd-Queue-Id: E333BA0009 X-Rspamd-Server: rspam08 X-HE-Tag: 1736569604-978697 X-HE-Meta: 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 8KaFE5Kk 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000205, 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 --- 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..f573de936b5d 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, 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.