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

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

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);
>>           return PAGE_KEEP;
>>       }
> 
>