Re: [PATCH] mm/huge_memory: fix NULL pointer deference when splitting shmem folio in swap cache

[Zi Yan <ziy@nvidia.com>]

On 19 Nov 2025, at 9:46, David Hildenbrand (Red Hat) wrote:

> On 19.11.25 15:37, David Hildenbrand (Red Hat) wrote:
>>>> Given folio_test_swapcache() might have false positives,
>>>> I assume we'd need a
>>>>
>>>> 	folio_test_swapbacked() && folio_test_swapcache(folio)
>>>>
>>>> To detect large large shmem folios in the swapcache in all cases here.
>>>>
>>>> Something like the following would hopefully do:
>>>>
>>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>>> index 2f2a521e5d683..57aab66bedbea 100644
>>>> --- a/mm/huge_memory.c
>>>> +++ b/mm/huge_memory.c
>>>> @@ -3515,6 +3515,13 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
>>>>           return ret;
>>>>    }
>>>>    +static bool folio_test_shmem_swapcache(struct folio *folio)
>>>> +{
>>>> +       VM_WARN_ON_ONCE_FOLIO(folio_test_anon(folio), folio);
>>>> +       /* These folios do not have folio->mapping set. */
>>>> +       return folio_test_swapbacked(folio) && folio_test_swapcache(folio);
>>>> +}
>>>> +
>>>>    bool non_uniform_split_supported(struct folio *folio, unsigned int new_order,
>>>>                   bool warns)
>>>>    {
>>>> @@ -3524,6 +3531,9 @@ bool non_uniform_split_supported(struct folio *folio, unsigned int new_order,
>>>>                                   "Cannot split to order-1 folio");
>>>>                   if (new_order == 1)
>>>>                           return false;
>>>> +       } else if (folio_test_shmem_swapcache(folio)) {
>>>> +               /* TODO: support shmem folios that are in the swapcache. */
>>>> +               return false;
>>>
>>> With this, truncated shmem returns -EINVALID instead of -EBUSY now.
>>> Can s390_wiggle_split_folio() such folios?
>>
>> [noting that s390_wiggle_split_folio() was just one caller where I new
>> the return value differs. I suspect there might be more.]
>>
>> I am still not clear on that one.
>>
>> s390x obtains the folio while walking the page tables. In case it gets
>> -EBUSY it simply retries to obtain the folio from the page tables.
>>
>> So assuming there was concurrent truncation and we returned -EBUSY, it
>> would just retry walking the page tables (trigger a fault to map a
>> folio) and retry with that one.
>>
>> I would assume that the shmem folio in the swapcache could never have
>> worked before, and that there is no way to make progress really.
>>
>> In other words: do we know how we can end up with a shmem folio that is
>> in the swapcache and does not have folio->mapping set?
>>
>> Could that think still be mapped into the page tables? (I hope not, but
>> right now I am confused how that can happen )
>>
>
> Ah, my memory comes back.
>
> vmscan triggers shmem_writeout() after unmapping the folio and after making sure that there are no unexpected folio references.
>
> shmem_writeout() will do the shmem_delete_from_page_cache() where we set folio->mapping = NULL.
>
> So anything walking the page tables (like s390x) could never find it.
>
>
> Such shmem folios really cannot get split right now until we either reclaimed them (-> freed) or until shmem_swapin_folio() re-obtained them from the swapcache to re-add them to the swapcache through shmem_add_to_page_cache().
>
> So maybe we can just make our life easy and just keep returning -EBUSY for this scenario for the time being?

Exactly, also an easy backport.

>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 2f2a521e5d683..5ce86882b2727 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3619,6 +3619,16 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>         if (folio != page_folio(split_at) || folio != page_folio(lock_at))
>                 return -EINVAL;
>  +       /*
> +        * Folios that just got truncated cannot get split. Signal to the
> +        * caller that there was a race.
> +        *
> +        * TODO: this will also currently refuse shmem folios that are in
> +        * the swapcache.
> +        */
> +       if (!is_anon && !folio->mapping)
> +               return -EBUSY;
> +
>         if (new_order >= folio_order(folio))
>                 return -EINVAL;
>  @@ -3659,17 +3669,7 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>                 gfp_t gfp;
>                  mapping = folio->mapping;
> -
> -               /* Truncated ? */
> -               /*
> -                * TODO: add support for large shmem folio in swap cache.
> -                * When shmem is in swap cache, mapping is NULL and
> -                * folio_test_swapcache() is true.
> -                */
> -               if (!mapping) {
> -                       ret = -EBUSY;
> -                       goto out;
> -               }
> +               VM_WARN_ON_ONCE_FOLIO(!mapping, folio);
>                  min_order = mapping_min_folio_order(folio->mapping);
>                 if (new_order < min_order) {
>
>
> -- 
> Cheers
>
> David


Best Regards,
Yan, Zi