Re: [PATCH v4 4/4] mm: use vma_start_write_killable() in process_vma_walk_lock()

["Lorenzo Stoakes (Oracle)" <ljs@kernel.org>]

On Sat, Mar 21, 2026 at 10:43:08PM -0700, Suren Baghdasaryan wrote:
> Replace vma_start_write() with vma_start_write_killable() when
> process_vma_walk_lock() is used with PGWALK_WRLOCK option.
> Adjust its direct and indirect users to check for a possible error
> and handle it. Ensure users handle EINTR correctly and do not ignore
> it.
>
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> ---
>  fs/proc/task_mmu.c |  5 ++++-
>  mm/mempolicy.c     |  1 +
>  mm/pagewalk.c      | 20 ++++++++++++++------
>  3 files changed, 19 insertions(+), 7 deletions(-)
>
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index e091931d7ca1..2fe3d11aad03 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -1797,6 +1797,7 @@ static ssize_t clear_refs_write(struct file *file, const char __user *buf,
>  		struct clear_refs_private cp = {
>  			.type = type,
>  		};
> +		int err;

Maybe better to make it a ssize_t given return type of function?

>
>  		if (mmap_write_lock_killable(mm)) {
>  			count = -EINTR;
> @@ -1824,7 +1825,9 @@ static ssize_t clear_refs_write(struct file *file, const char __user *buf,
>  						0, mm, 0, -1UL);
>  			mmu_notifier_invalidate_range_start(&range);
>  		}
> -		walk_page_range(mm, 0, -1, &clear_refs_walk_ops, &cp);
> +		err = walk_page_range(mm, 0, -1, &clear_refs_walk_ops, &cp);
> +		if (err)
> +			count = err;

Hmm this is gross, but it's an established pattern here, ugh.

Now we have an err though, could we update:

		if (mmap_write_lock_killable(mm)) {
-			count = -EINTR;
+			err = -EINTR;
			goto out_mm;
		}

Then we can just do:

+		err = walk_page_range(mm, 0, -1, &clear_refs_walk_ops, &cp);

And at the end do:

	return err ?: count;

Which possibly _necessitates_ err being a ssize_t.

>  		if (type == CLEAR_REFS_SOFT_DIRTY) {
>  			mmu_notifier_invalidate_range_end(&range);
>  			flush_tlb_mm(mm);
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 929e843543cf..bb5b0e83ce0f 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -969,6 +969,7 @@ static const struct mm_walk_ops queue_pages_lock_vma_walk_ops = {
>   *      (a hugetlbfs page or a transparent huge page being counted as 1).
>   * -EIO - a misplaced page found, when MPOL_MF_STRICT specified without MOVEs.
>   * -EFAULT - a hole in the memory range, when MPOL_MF_DISCONTIG_OK unspecified.
> + * -EINTR - walk got terminated due to pending fatal signal.

Thanks!

>   */
>  static long
>  queue_pages_range(struct mm_struct *mm, unsigned long start, unsigned long end,
> diff --git a/mm/pagewalk.c b/mm/pagewalk.c
> index eda74273c8ec..a42cd6a6d812 100644
> --- a/mm/pagewalk.c
> +++ b/mm/pagewalk.c
> @@ -438,14 +438,13 @@ static inline void process_mm_walk_lock(struct mm_struct *mm,
>  		mmap_assert_write_locked(mm);
>  }
>
> -static inline void process_vma_walk_lock(struct vm_area_struct *vma,
> +static inline int process_vma_walk_lock(struct vm_area_struct *vma,

NIT: Don't need this to be an inline any longer. May as well fix up while we're
here.

>  					 enum page_walk_lock walk_lock)
>  {
>  #ifdef CONFIG_PER_VMA_LOCK
>  	switch (walk_lock) {
>  	case PGWALK_WRLOCK:
> -		vma_start_write(vma);
> -		break;
> +		return vma_start_write_killable(vma);

LGTM

>  	case PGWALK_WRLOCK_VERIFY:
>  		vma_assert_write_locked(vma);
>  		break;
> @@ -457,6 +456,7 @@ static inline void process_vma_walk_lock(struct vm_area_struct *vma,
>  		break;
>  	}
>  #endif
> +	return 0;
>  }
>
>  /*
> @@ -500,7 +500,9 @@ int walk_page_range_mm_unsafe(struct mm_struct *mm, unsigned long start,
>  			if (ops->pte_hole)
>  				err = ops->pte_hole(start, next, -1, &walk);
>  		} else { /* inside vma */
> -			process_vma_walk_lock(vma, ops->walk_lock);
> +			err = process_vma_walk_lock(vma, ops->walk_lock);
> +			if (err)
> +				break;

In every other case we set walk.vma = vma or NULL. Is it a problem not setting
it at all in this code path?

>  			walk.vma = vma;
>  			next = min(end, vma->vm_end);
>  			vma = find_vma(mm, vma->vm_end);
> @@ -717,6 +719,7 @@ int walk_page_range_vma_unsafe(struct vm_area_struct *vma, unsigned long start,
>  		.vma		= vma,
>  		.private	= private,
>  	};
> +	int err;
>
>  	if (start >= end || !walk.mm)
>  		return -EINVAL;
> @@ -724,7 +727,9 @@ int walk_page_range_vma_unsafe(struct vm_area_struct *vma, unsigned long start,
>  		return -EINVAL;
>
>  	process_mm_walk_lock(walk.mm, ops->walk_lock);
> -	process_vma_walk_lock(vma, ops->walk_lock);
> +	err = process_vma_walk_lock(vma, ops->walk_lock);
> +	if (err)
> +		return err;

LGTM

>  	return __walk_page_range(start, end, &walk);
>  }
>
> @@ -747,6 +752,7 @@ int walk_page_vma(struct vm_area_struct *vma, const struct mm_walk_ops *ops,
>  		.vma		= vma,
>  		.private	= private,
>  	};
> +	int err;
>
>  	if (!walk.mm)
>  		return -EINVAL;
> @@ -754,7 +760,9 @@ int walk_page_vma(struct vm_area_struct *vma, const struct mm_walk_ops *ops,
>  		return -EINVAL;
>
>  	process_mm_walk_lock(walk.mm, ops->walk_lock);
> -	process_vma_walk_lock(vma, ops->walk_lock);
> +	err = process_vma_walk_lock(vma, ops->walk_lock);
> +	if (err)
> +		return err;

LGTM

>  	return __walk_page_range(vma->vm_start, vma->vm_end, &walk);
>  }
>
> --
> 2.53.0.1018.g2bb0e51243-goog
>

Thanks, Lorenzo