Re: [RFC PATCH 1/3] mm/huge_memory: prevent NULL pointer dereference in try_folio_split_to_order()

[Zi Yan <ziy@nvidia.com>]

On 19 Nov 2025, at 23:28, Balbir Singh wrote:

> On 11/20/25 14:59, Zi Yan wrote:
>> folio_split_supported() used in try_folio_split_to_order() requires
>> folio->mapping to be non NULL, but current try_folio_split_to_order() does
>> not check it. Add the check to prevent NULL pointer dereference.
>>
>> There is no issue in the current code, since try_folio_split_to_order() is
>> only used in truncate_inode_partial_folio(), where folio->mapping is not
>> NULL.
>>
>
> Just reading through the description does not clarify one thing
>
> What is the race between just truncated and trying to split them - is there a common lock
> that needs to be held? Is it the subsequent call in truncate_inode_partial_folio()
> that causes the race?
>
> IOW, if a folio is not anonymous and does not have a mapping, how is it
> being passed to this function?

Quote David’s explanation[1] (note: shmem is not anonymous):

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().

[1] https://lore.kernel.org/all/14253d62-0a85-4f61-aed6-72da17bcef77@kernel.org/
>
>> Signed-off-by: Zi Yan <ziy@nvidia.com>
>> ---
>>  include/linux/huge_mm.h | 7 +++++++
>>  1 file changed, 7 insertions(+)
>>
>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
>> index 1d439de1ca2c..0d55354e3a34 100644
>> --- a/include/linux/huge_mm.h
>> +++ b/include/linux/huge_mm.h
>> @@ -407,6 +407,13 @@ static inline int split_huge_page_to_order(struct page *page, unsigned int new_o
>>  static inline int try_folio_split_to_order(struct folio *folio,
>>  		struct page *page, unsigned int new_order)
>>  {
>> +	/*
>> +	 * Folios that just got truncated cannot get split. Signal to the
>> +	 * caller that there was a race.
>> +	 */
>> +	if (!folio_test_anon(folio) && !folio->mapping)
>> +		return -EBUSY;
>> +
>>  	if (!folio_split_supported(folio, new_order, SPLIT_TYPE_NON_UNIFORM, /* warns= */ false))
>>  		return split_huge_page_to_order(&folio->page, new_order);
>>  	return folio_split(folio, new_order, page, NULL);


Best Regards,
Yan, Zi