linux-kernel - [PATCH 1/1] mm: remove extra check for VMA_LOCK

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Message-ID: <20250107030415.721474-1-surenb@google.com>
Date: Mon,  6 Jan 2025 19:04:15 -0800
From: Suren Baghdasaryan <surenb@...gle.com>
To: akpm@...ux-foundation.org
Cc: richard.weiyang@...il.com, peterz@...radead.org, willy@...radead.org, 
	liam.howlett@...cle.com, lorenzo.stoakes@...cle.com, mhocko@...e.com, 
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	linux-mm@...ck.org, linux-kernel@...r.kernel.org, kernel-team@...roid.com, 
	surenb@...gle.com
Subject: [PATCH 1/1] mm: remove extra check for VMA_LOCK_OFFSET when
 read-locking a vma

Since we limit vm_refcnt at VMA_REF_LIMIT and it's smaller than
VMA_LOCK_OFFSET, there is no need to check again if VMA_LOCK_OFFSET bit
is set. Remove the extra check and add a clarifying comment.

Fixes: e8f32ff00a66 ("mm: replace vm_lock and detached flag with a reference count")
Suggested-by: Wei Yang <richard.weiyang@...il.com>
Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
---
Applies over mm-unstable

 include/linux/mm.h | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 486638d22fc6..b5f262fc7dc5 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -747,7 +747,11 @@ static inline bool vma_start_read(struct mm_struct *mm, struct vm_area_struct *v
 
 
 	rwsem_acquire_read(&vma->vmlock_dep_map, 0, 0, _RET_IP_);
-	/* Limit at VMA_REF_LIMIT to leave one count for a writer */
+	/*
+	 * Limit at VMA_REF_LIMIT to leave one count for a writer.
+	 * If VMA_LOCK_OFFSET is set, __refcount_inc_not_zero_limited() will fail
+	 * because VMA_REF_LIMIT is less than VMA_LOCK_OFFSET.
+	 */
 	if (unlikely(!__refcount_inc_not_zero_limited(&vma->vm_refcnt, &oldcnt,
 						      VMA_REF_LIMIT))) {
 		rwsem_release(&vma->vmlock_dep_map, _RET_IP_);
@@ -766,8 +770,7 @@ static inline bool vma_start_read(struct mm_struct *mm, struct vm_area_struct *v
 	 * after it has been unlocked.
 	 * This pairs with RELEASE semantics in vma_end_write_all().
 	 */
-	if (unlikely(oldcnt & VMA_LOCK_OFFSET ||
-		     vma->vm_lock_seq == raw_read_seqcount(&mm->mm_lock_seq))) {
+	if (unlikely(vma->vm_lock_seq == raw_read_seqcount(&mm->mm_lock_seq))) {
 		vma_refcount_put(vma);
 		return false;
 	}

base-commit: f349e79bfbf3abfade8011797ff6d0d47b67dab7
-- 
2.47.1.613.gc27f4b7a9f-goog