Re: [PATCH v7 17/17] docs/mm: document latest changes to vm_lock

Liam R. Howlett
2025-01-08T15:46:50Z

* Suren Baghdasaryan <surenb@google.com> [241226 12:08]:
> 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 <surenb@google.com>

Reviewed-by: Liam R. Howlett <Liam.Howlett@Oracle.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..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.
>
> --
> 2.47.1.613.gc27f4b7a9f-goog
>