Re: [PATCH RFC 02/13] mm/huge_memory: fix rejection of swap cache folios with a mapping

[Zi Yan]

On Fri Aug 7, 2026 at 5:17 PM EDT, Kairui Song via B4 Relay wrote:
> From: Kairui Song <kasong@xxxxxxxxxxx>
>
> A folio in the swap cache cannot be split if it has a mapping (shmem).
> The split code only checks for this in __folio_freeze_and_split_unmapped,
> after the folio ref has been frozen and the NR_SHMEM_THPS/NR_FILE_THPS
> counters have been decremented, and returns -EINVAL without unfreezing
> the folio or restoring the counters. That error path is fragile: if it
> is ever taken, the folio is left frozen and stuck, the counters are
> skewed, and the VM_WARN_ON_ONCE_FOLIO would fire for a state that is
> actually legitimate.
>
> Check for this case up front in folio_check_splittable and return
> -EINVAL before any state is modified. Under DEBUG_VM, the existing
> "Tried to split an unsplittable folio" warning in __folio_split
> reports the rejection.

Should we return -EBUSY instead? -EINVAL means the caller should not
split a swapcache shmem with a mapping and the caller needs to avoid
that. The Fixes tag tells me a caller can split a swapcache shmem with a
mapping, so with -EINVAL, we will want to add checks at callers to avoid
it from happening.

>
> Fixes: 00527733d0dc ("mm/huge_memory: add two new (not yet used) functions for folio_split()")
> Fixes: 714b056c8321 ("mm/huge_memory: convert VM_BUG* to VM_WARN* in __folio_split")
> Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>
> ---
>  mm/huge_memory.c | 26 ++++++++++++++++----------
>  1 file changed, 16 insertions(+), 10 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index ced400f72d43..2fa72158e063 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3878,6 +3878,9 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
>  int folio_check_splittable(struct folio *folio, unsigned int new_order,
>  			   enum split_type split_type)
>  {
> +	bool is_anon = folio_test_anon(folio);
> +	bool is_swapcache = folio_test_swapcache(folio);
> +
>  	VM_WARN_ON_FOLIO(!folio_test_locked(folio), folio);
>  	/*
>  	 * Folios that just got truncated cannot get split. Signal to the
> @@ -3886,11 +3889,11 @@ int folio_check_splittable(struct folio *folio, unsigned int new_order,
>  	 * TODO: this will also currently refuse folios without a mapping in the
>  	 * swapcache (shmem or to-be-anon folios).
>  	 */
> -	if (!folio->mapping && !folio_test_anon(folio))
> +	if (!folio->mapping && !is_anon)
>  		return -EBUSY;
>  
>  	/* order-1 is not supported for anonymous THP. */
> -	if (folio_test_anon(folio) && new_order == 1)
> +	if (is_anon && new_order == 1)
>  		return -EINVAL;
>  
>  	/*
> @@ -3901,7 +3904,7 @@ int folio_check_splittable(struct folio *folio, unsigned int new_order,
>  	 * swapcache folio split. Only uniform split to order-0 can be used
>  	 * here.
>  	 */
> -	if ((split_type == SPLIT_TYPE_NON_UNIFORM || new_order) && folio_test_swapcache(folio)) {
> +	if ((split_type == SPLIT_TYPE_NON_UNIFORM || new_order) && is_swapcache) {
>  		return -EINVAL;
>  	}
>  
> @@ -3911,6 +3914,15 @@ int folio_check_splittable(struct folio *folio, unsigned int new_order,
>  	if (folio_test_writeback(folio))
>  		return -EBUSY;
>  
> +	/*
> +	 * A non-anon swapcache folio that still has a mapping should only
> +	 * be a shmem folio under IO, there is little benefit in splitting
> +	 * them hence not supported. Reject it here up front: the split
> +	 * routine cannot back out cleanly once the folio ref is frozen.
> +	 */
> +	if (!is_anon && is_swapcache && folio->mapping)
> +		return -EINVAL;
> +
>  	return 0;
>  }
>  
> @@ -3983,14 +3995,8 @@ static int __folio_freeze_and_split_unmapped(struct folio *folio, unsigned int n
>  			}
>  		}
>  
> -		if (folio_test_swapcache(folio)) {
> -			if (mapping) {
> -				VM_WARN_ON_ONCE_FOLIO(mapping, folio);
> -				return -EINVAL;
> -			}
> -
> +		if (folio_test_swapcache(folio))
>  			ci = swap_cluster_get_and_lock(folio);
> -		}
>  
>  		/* lock lru list/PageCompound, ref frozen by page_ref_freeze */
>  		if (do_lru)




-- 
Best Regards,
Yan, Zi