Re: [PATCH] mm/huge_memory: update file PMD counter before folio_put()

[Yin Tirui <yintirui@huawei.com>]

On 5/26/2026 7:05 PM, Lorenzo Stoakes wrote:
> On Tue, May 26, 2026 at 06:13:37PM +0800, Yin Tirui wrote:
>> __split_huge_pmd_locked() updates the file/shmem RSS counter after
>> dropping the PMD mapping's folio reference. If folio_put() drops the
>> last reference, mm_counter_file() can later read freed folio state via
>> folio_test_swapbacked().
>>
>> Move the counter update before folio_put().
>>
>> Fixes: fadae2953072 ("thp: use mm_file_counter to determine update which rss counter")
> That's an old commit :) I mean I suspect we're probably not actually ever
> dropping the folio ref to 0 here since we never had a report since ~2018.
>
> The page cache keeping a reference I guess?
>
> But doesn't mean we shouldn't fix this on principal/there being some way
> this could happen.
Yes, agreed.
>
>> Cc: <stable@vger.kernel.org>
>> Signed-off-by: Yin Tirui <yintirui@huawei.com>
> LGTM, so:
>
> Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Thanks!
>> ---
>>   mm/huge_memory.c | 2 ++
>>   1 file changed, 2 insertions(+)
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 0135c29a4372..a5f4a48b7b77 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -3145,7 +3145,9 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>>   			if (!folio_test_referenced(folio) && pmd_young(old_pmd))
>>   				folio_set_referenced(folio);
>>   			folio_remove_rmap_pmd(folio, page, vma);
>> +			add_mm_counter(mm, mm_counter_file(folio), -HPAGE_PMD_NR);
>>   			folio_put(folio);
>> +			return;
> Hmm, sucks to duplicate like this, but for purposes of backport and getting
> this resolved fine, we can clean it up later.
Yes, that was my intention as well: keep this patch minimal for backport.
I will post the related refactoring patch soon.
>>   		}
>>   		add_mm_counter(mm, mm_counter_file(folio), -HPAGE_PMD_NR);
>>   		return;
>> --
>> 2.43.0
>>
> Cheers, Lorenzo
>
-- 
Yin Tirui