Re: [PATCH v3 02/18] mm/huge_memory: fix rejection of swap cache folios with a mapping

[David Hildenbrand (Arm)]

On 8/20/26 20:55, 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 does a defensive check 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. It
> rejects the split and returns -EINVAL without unfreezing the folio or
> restoring the counters. That error path is buggy: if it is ever taken,
> it leaves the folio frozen and stuck, skews the counters, and fires
> the VM_WARN_ON_ONCE_FOLIO for a state that is actually legitimate.
> 
> Check for this case up front in folio_check_splittable and return
> -EBUSY before any state is modified, so the split routine always backs
> out cleanly.
> 
> Also fix a bracket style issue that checkpatch.pl keeps complaining
> about.
> 
> 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")
> Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>
> Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>
> ---
>  mm/huge_memory.c | 27 ++++++++++++++++-----------
>  1 file changed, 16 insertions(+), 11 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index ced400f72d43..a6759a14e057 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);

Both const please.

Acked-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>

-- 
Cheers,

David