Re: [PATCH v4 01/10] mm: vmscan: add validation before spliting shmem large folio

[David Hildenbrand <david@redhat.com>]

On 07.08.24 09:31, Baolin Wang wrote:
> Page reclaim will not scan anon LRU if no swap space, however MADV_PAGEOUT
> can still split shmem large folios even without a swap device. Thus add
> swap available space validation before spliting shmem large folio to
> avoid redundant split.
> 
> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> ---
>   mm/vmscan.c | 8 ++++++++
>   1 file changed, 8 insertions(+)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 31d13462571e..796f65781f4f 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1259,6 +1259,14 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
>   			}
>   		} else if (folio_test_swapbacked(folio) &&
>   			   folio_test_large(folio)) {
> +
> +			/*
> +			 * Do not split shmem folio if no swap memory
> +			 * available.
> +			 */
> +			if (!total_swap_pages)
> +				goto activate_locked;
> +
>   			/* Split shmem folio */
>   			if (split_folio_to_list(folio, folio_list))
>   				goto keep_locked;

Reminds me of

commit 9a976f0c847b67d22ed694556a3626ed92da0422
Author: Luis Chamberlain <mcgrof@kernel.org>
Date:   Thu Mar 9 15:05:43 2023 -0800

     shmem: skip page split if we're not reclaiming
     
     In theory when info->flags & VM_LOCKED we should not be getting
     shem_writepage() called so we should be verifying this with a
     WARN_ON_ONCE().  Since we should not be swapping then best to ensure we
     also don't do the folio split earlier too.  So just move the check early
     to avoid folio splits in case its a dubious call.
     
     We also have a similar early bail when !total_swap_pages so just move that
     earlier to avoid the possible folio split in the same situation.


But indeed, pageout() -> writepage() is called *after* the split in the vmscan path.

In that "noswap" context, I wonder if we also want to skip folios part of shmem
with disabled swapping?

But now I am wondering under which circumstances we end up calling
shmem_writepage() with a large folio. And I think the answer is the comment of
folio_test_large(): via drivers/gpu/drm/i915/gem/i915_gem_shmem.c.


... so if shmem_writepage() handles+checks that, could we do

diff --git a/mm/vmscan.c b/mm/vmscan.c
index a332cb80e928..7dfa3d6e8ba7 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1257,11 +1257,6 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
                                                 goto activate_locked_split;
                                 }
                         }
-               } else if (folio_test_swapbacked(folio) &&
-                          folio_test_large(folio)) {
-                       /* Split shmem folio */
-                       if (split_folio_to_list(folio, folio_list))
-                               goto keep_locked;
                 }
  
                 /*

instead?

-- 
Cheers,

David / dhildenb