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

Suren Baghdasaryan
2024-12-26T17:07:58Z

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>
---
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