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

["David Hildenbrand (Red Hat)" <david@kernel.org>]

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?

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