Re: [PATCH v2 1/2] mm: vmscan: remove folio_test_private() check in pageout()

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 2025/9/19 09:06, Shakeel Butt wrote:
> On Thu, Sep 18, 2025 at 05:36:17PM +0800, Baolin Wang wrote:
>>
>>
>> On 2025/9/18 14:00, David Hildenbrand wrote:
>>> On 18.09.25 05:46, Baolin Wang wrote:
>>>> The folio_test_private() check in pageout() was introduced by commit
>>>> ce91b575332b ("orphaned pagecache memleak fix") in 2005 (checked from
>>>> a history tree[1]). As the commit message mentioned, it was to address
>>>> the issue where reiserfs pagecache may be truncated while still pinned.
>>>> To further explain, the truncation removes the page->mapping, but the
>>>> page is still listed in the VM queues because it still has buffers.
>>>>
>>>> In 2008, commit a2b345642f530 ("Fix dirty page accounting leak with ext3
>>>> data=journal") seems to be dealing with a similar issue, where the page
>>>> becomes dirty after truncation, and it provides a very useful call stack:
>>>> truncate_complete_page()
>>>>         cancel_dirty_page() // PG_dirty cleared, decr. dirty pages
>>>>         do_invalidatepage()
>>>>           ext3_invalidatepage()
>>>>             journal_invalidatepage()
>>>>               journal_unmap_buffer()
>>>>                 __dispose_buffer()
>>>>                   __journal_unfile_buffer()
>>>>                     __journal_temp_unlink_buffer()
>>>>                       mark_buffer_dirty(); // PG_dirty set, incr.
>>>> dirty pages
>>>>
>>>> In this commit a2b345642f530, we forcefully clear the page's dirty flag
>>>> during truncation (in truncate_complete_page()).
>>>>
>>>> Now it seems this was just a peculiar usage specific to reiserfs. Maybe
>>>> reiserfs had some extra refcount on these pages, which caused them
>>>> to pass
>>>> the is_page_cache_freeable() check. With the fix provided by commit
>>>> a2b345642f530
>>>> and reiserfs being removed in 2024 by commit fb6f20ecb121 ("reiserfs: The
>>>> last commit"), such a case is unlikely to occur again. So let's
>>>> remove the
>>>> redundant folio_test_private() checks and related buffer_head
>>>> release logic,
>>>> and just leave a warning here to catch such a bug.
>>>>
>>>> [1] https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git
>>>> Acked-by: David Hildenbrand <david@redhat.com>
>>>> Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
>>>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>>>> ---
>>>>    mm/vmscan.c | 12 +++---------
>>>>    1 file changed, 3 insertions(+), 9 deletions(-)
>>>>
>>>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>>>> index f1fc36729ddd..930add6d90ab 100644
>>>> --- a/mm/vmscan.c
>>>> +++ b/mm/vmscan.c
>>>> @@ -701,16 +701,10 @@ static pageout_t pageout(struct folio *folio,
>>>> struct address_space *mapping,
>>>>            return PAGE_KEEP;
>>>>        if (!mapping) {
>>>>            /*
>>>> -         * Some data journaling orphaned folios can have
>>>> -         * folio->mapping == NULL while being dirty with clean buffers.
>>>> +         * Is it still possible to have a dirty folio with
>>>> +         * a NULL mapping? I think not.
>>>>             */
>>>
>>> I would rephrase slightly (removing the "I think not"):
>>>
>>> /*
>>>    * We should no longer have dirty folios with clean buffers and a NULL
>>>    * mapping. However, let's be careful for now.
>>>    */
>>
>> LGTM.
>>
>> Andrew, could you help squash these comments into this patch? Thanks.
>>
>>>> -        if (folio_test_private(folio)) {
>>>> -            if (try_to_free_buffers(folio)) {
>>>> -                folio_clear_dirty(folio);
>>>> -                pr_info("%s: orphaned folio\n", __func__);
>>>> -                return PAGE_CLEAN;
>>>> -            }
>>>> -        }
>>>> +        VM_WARN_ON_FOLIO(true, folio);
> 
> Unexpected but better to use VM_WARN_ON_ONCE_FOLIO here.

Um, I don't think it makes much difference, because we should no longer 
hit this.