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

[Barry Song]

On Fri, Aug 21, 2026 at 2:55 AM Kairui Song via B4 Relay
<devnull+kasong.tencent.com@xxxxxxxxxx> 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>

LGTM,
Reviewed-by: Barry Song <baohua@xxxxxxxxxx>

[...]
>
> -               if (folio_test_swapcache(folio)) {
> -                       if (mapping) {
> -                               VM_WARN_ON_ONCE_FOLIO(mapping, folio);
> -                               return -EINVAL;

BTW, I’m really curious whether you’ve ever encountered this warning?

Best Regards
Barry