Re: [PATCH 1/1] mm: remove extra check for VMA_LOCK_OFFSET when read-locking a vma

[Wei Yang]

On Mon, Jan 06, 2025 at 07:04:15PM -0800, Suren Baghdasaryan wrote:
>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@xxxxxxxxx>
>Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>

Reviewed-by: Wei Yang <richard.weiyang@xxxxxxxxx>

>---
>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

-- 
Wei Yang
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