The patch titled
Subject: docs/mm: document latest changes to vm_lock
has been added to the -mm mm-unstable branch. Its filename is
docs-mm-document-latest-changes-to-vm_lock.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/docs-mm-document-latest-changes-to-vm_lock.patch
This patch will later appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Suren Baghdasaryan <surenb@xxxxxxxxxx>
Subject: docs/mm: document latest changes to vm_lock
Date: Thu, 26 Dec 2024 09:07:09 -0800
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/20241226170710.1159679-18-surenb@xxxxxxxxxx
Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
Cc: Christian Brauner <brauner@xxxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: David Howells <dhowells@xxxxxxxxxx>
Cc: Davidlohr Bueso <dave@xxxxxxxxxxxx>
Cc: Hillf Danton <hdanton@xxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Jann Horn <jannh@xxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Jonathan Corbet <corbet@xxxxxxx>
Cc: kernel test robot <oliver.sang@xxxxxxxxx>
Cc: Klara Modin <klarasmodin@xxxxxxxxx>
Cc: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
Cc: Lokesh Gidra <lokeshgidra@xxxxxxxxxx>
Cc: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
Cc: Mateusz Guzik <mjguzik@xxxxxxxxx>
Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Minchan Kim <minchan@xxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
Cc: Peter Xu <peterx@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Shakeel Butt <shakeel.butt@xxxxxxxxx>
Cc: Sourav Panda <souravpanda@xxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
Documentation/mm/process_addrs.rst | 46 +++++++++++++++------------
1 file changed, 27 insertions(+), 19 deletions(-)
--- a/Documentation/mm/process_addrs.rst~docs-mm-document-latest-changes-to-vm_lock
+++ a/Documentation/mm/process_addrs.rst
@@ -716,9 +716,14 @@ calls :c:func:`!rcu_read_lock` to ensure
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
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
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.
-
-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.
+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 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.
_
Patches currently in -mm which might be from surenb@xxxxxxxxxx are
seqlock-add-raw_seqcount_try_begin.patch
mm-convert-mm_lock_seq-to-a-proper-seqcount.patch
mm-introduce-mmap_lock_speculate_try_beginretry.patch
mm-introduce-vma_start_read_locked_nested-helpers.patch
mm-move-per-vma-lock-into-vm_area_struct.patch
mm-mark-vma-as-detached-until-its-added-into-vma-tree.patch
mm-modify-vma_iter_store_gfp-to-indicate-if-its-storing-a-new-vma.patch
mm-mark-vmas-detached-upon-exit.patch
mm-nommu-fix-the-last-places-where-vma-is-not-locked-before-being-attached.patch
types-move-struct-rcuwait-into-typesh.patch
mm-allow-vma_start_read_locked-vma_start_read_locked_nested-to-fail.patch
mm-move-mmap_init_lock-out-of-the-header-file.patch
mm-uninline-the-main-body-of-vma_start_write.patch
refcount-introduce-__refcount_addinc_not_zero_limited.patch
mm-replace-vm_lock-and-detached-flag-with-a-reference-count.patch
mm-debug-print-vm_refcnt-state-when-dumping-the-vma.patch
mm-debug-print-vm_refcnt-state-when-dumping-the-vma-fix.patch
mm-remove-extra-vma_numab_state_init-call.patch
mm-prepare-lock_vma_under_rcu-for-vma-reuse-possibility.patch
mm-make-vma-cache-slab_typesafe_by_rcu.patch
docs-mm-document-latest-changes-to-vm_lock.patch
