Re: [PATCH/RFC] btrfs: fix folio lock leak in writepage_delalloc() for folios dirtied behind btrfs' back

[Christian Borntraeger <borntraeger@linux.ibm.com>]

Am 22.07.26 um 11:35 schrieb Qu Wenruo:
> 
> 
> 在 2026/7/22 18:59, Christian Borntraeger 写道:
>> Am 22.07.26 um 10:59 schrieb Qu Wenruo:
>>>
>>>
>>> 在 2026/7/22 18:05, Christian Borntraeger 写道:
>>>> Am 21.07.26 um 23:07 schrieb Qu Wenruo:
>>>>
>>>> First, thank you for taking the time to look into this and trying to understand
>>>> things and trying to explain things. this is highly appreciated.
>>>> I am still trying to fully understand this myself. A question:
>>>>
>>>>> 在 2026/7/22 04:41, Christian Borntraeger 写道:
>>>>>> A folio can carry the folio-level dirty flag while its btrfs subpage
>>>>>> dirty bitmap is empty: btrfs data mappings use filemap_dirty_folio(),
>>>>>> so a generic folio_mark_dirty() call sets only the folio flag and the
>>>>>> xarray tag, without setting any subpage dirty bit and without a
>>>>>> delalloc reservation.  The typical source is set_page_dirty_lock() on
>>>>>> a GUP pin,
>>>>>
>>>>> Shouldn't such folio got its ->page_mkwrite() callback get called first?
>>>>
>>>> Isnt that called implicitely at pin time?
>>>
>>> I have to admit, I'm not an expert on the MM part, I'm mostly a simple user of the existing MM interfaces.
>>>
>>> AFAIK, the last time I brought this thing up, Christoph mentioned that dirtying-folio-without-notifying-fs is a bug, and fs should not and is not able to handle such situation anyway.
>>>
>>> And that idea makes a lot of sense to me.
>>>
>>> So adding MM list for more help.
>> So I now have an userspace O_DIRECT reproducer outside of KVM. (attached) which gave me (on an s390 system, though)
> 
> Thanks a lot, I can also reproduce it on arm64 (64K page size).
> 
> This is super bad, as we have just removed a lot of folio ordered related code to detect such problem.
> 
> I'll also check if it's some recent btrfs changes making it worse.


The RFC patch from yesterday seems to fix _THIS_ problem, but as you outlined, there might be other issues that break and are not handled by my patch.