Re: [PATCH] mm: Tweak __vma_enter_locked()

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Wed, Nov 19, 2025 at 04:26:35AM +0000, Matthew Wilcox (Oracle) wrote:
> Move the commentary on how __vma_enter_locked() behaves from the
> body of __vma_start_write() to the head of __vma_enter_locked() and
> merge it with the existing documentation.  Also add a call to
> mmap_assert_write_locked().
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

LGTM, so:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

> Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> Cc: Suren Baghdasaryan <surenb@google.com>
> ---
>  mm/mmap_lock.c | 13 +++++--------
>  1 file changed, 5 insertions(+), 8 deletions(-)
>
> diff --git a/mm/mmap_lock.c b/mm/mmap_lock.c
> index f2532af6208c..e6e5570d1ec7 100644
> --- a/mm/mmap_lock.c
> +++ b/mm/mmap_lock.c
> @@ -46,9 +46,9 @@ EXPORT_SYMBOL(__mmap_lock_do_trace_released);
>  #ifdef CONFIG_MMU
>  #ifdef CONFIG_PER_VMA_LOCK
>  /*
> - * Return value: 0 if vma detached,
> - * 1 if vma attached with no readers,
> - * -EINTR if signal received,
> + * __vma_enter_locked() returns 0 immediately if the vma is not
> + * attached, otherwise it waits for any current readers to finish and
> + * returns 1.  Returns -EINTR if a signal is received while waiting.
>   */
>  static inline int __vma_enter_locked(struct vm_area_struct *vma,
>  		bool detaching, int state)
> @@ -56,6 +56,8 @@ static inline int __vma_enter_locked(struct vm_area_struct *vma,
>  	int err;
>  	unsigned int tgt_refcnt = VMA_LOCK_OFFSET;
>
> +	mmap_assert_write_locked(vma->vm_mm);

This is nice, actually was thinking the other day we need more asserts like this
in these lock functions.

> +
>  	/* Additional refcnt if the vma is attached. */
>  	if (!detaching)
>  		tgt_refcnt++;
> @@ -91,11 +93,6 @@ int __vma_start_write(struct vm_area_struct *vma, unsigned int mm_lock_seq,
>  {
>  	int locked;
>
> -	/*
> -	 * __vma_enter_locked() returns false immediately if the vma is not
> -	 * attached, otherwise it waits until refcnt is indicating that vma
> -	 * is attached with no readers.
> -	 */
>  	locked = __vma_enter_locked(vma, false, state);
>  	if (locked < 0)
>  		return locked;
> --
> 2.47.2
>
>