docs/mm: document latest changes to vm_lock

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.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Suren Baghdasaryan <[e-mail]>
Reviewed-by: Liam R. Howlett <[e-mail]>
Reviewed-by: Lorenzo Stoakes <[e-mail]>
Tested-by: Shivank Garg <[e-mail]>
Link: https://lkml.kernel.org/r/[e-mail]
Reviewed-by: Vlastimil Babka <[e-mail]>
Cc: Christian Brauner <[e-mail]>
Cc: David Hildenbrand <[e-mail]>
Cc: David Howells <[e-mail]>
Cc: Davidlohr Bueso <[e-mail]>
Cc: Hugh Dickins <[e-mail]>
Cc: Jann Horn <[e-mail]>
Cc: Johannes Weiner <[e-mail]>
Cc: Jonathan Corbet <[e-mail]>
Cc: Klara Modin <[e-mail]>
Cc: Lokesh Gidra <[e-mail]>
Cc: Mateusz Guzik <[e-mail]>
Cc: Matthew Wilcox <[e-mail]>
Cc: Mel Gorman <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Cc: Minchan Kim <[e-mail]>
Cc: Oleg Nesterov <[e-mail]>
Cc: Pasha Tatashin <[e-mail]>
Cc: "Paul E . McKenney" <[e-mail]>
Cc: Peter Xu <[e-mail]>
Cc: Peter Zijlstra (Intel) <[e-mail]>
Cc: Shakeel Butt <[e-mail]>
Cc: Sourav Panda <[e-mail]>
Cc: Suren Baghdasaryan <[e-mail]>
Cc: Wei Yang <[e-mail]>
Cc: Will Deacon <[e-mail]>
Cc: Heiko Carstens <[e-mail]>
Cc: Stephen Rothwell <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

Documentation/mm/process_addrs.rst

···
 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
···
 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.
···
 
 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
···
 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.