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

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

Am 27.07.26 um 10:41 schrieb Qu Wenruo:
> 
> 
> 在 2026/7/27 17:41, Christian Borntraeger 写道:
>> Am 25.07.26 um 08:26 schrieb Boris Burkov:
>>> On Fri, Jul 24, 2026 at 08:10:36AM +0930, Qu Wenruo wrote:
>>>>
>>>>
>>>> 在 2026/7/23 21:30, Matthew Wilcox 写道:
>>>>> On Thu, Jul 23, 2026 at 10:12:27AM +0930, Qu Wenruo wrote:
>>>>>>
>>>>>>
>>>>>> 在 2026/7/22 22:27, Matthew Wilcox 写道:
>>>>>>> On Wed, Jul 22, 2026 at 11:29:36AM +0200, Christian Borntraeger wrote:
>>>>>>>>     *   4. Thread B loops sync_file_range(WRITE|WAIT) on the target file.
>>>>>>>>     *      Whenever a full clean cycle (clear_page_dirty_for_io(),
>>>>>>>>     *      writeback, bits cleared) completes inside thread A's
>>>>>>>>     *      submission->completion window, the completion-time
>>>>>>>>     *      set_page_dirty_lock() hits a *clean* folio: filemap_dirty_folio()
>>>>>>>>     *      sets only the folio flag and the xarray tag - no btrfs subpage
>>>>>>>>     *      dirty bit, no delalloc reservation.  See the 20-year- old comment
>>>>>>>>     *      above bio_set_pages_dirty() in block/bio.c describing exactly
>>>>>>>>     *      this ("other code (eg, flusher threads) could clean the pages").
>>>>>>>
>>>>>>> There's your problem.  filemap_dirty_folio() documents that btrfs is
>>>>>>> doing it wrongly:
>>>>>>>
>>>>>>>     * Filesystems which do not use buffer heads should call this function
>>>>>>>     * from their dirty_folio address space operation.  It ignores the
>>>>>>>     * contents of folio_get_private(), so if the filesystem marks individual
>>>>>>>     * blocks as dirty, the filesystem should handle that itself.
>>>>>>>
>>>>>>> fs/btrfs/inode.c:       .dirty_folio    = filemap_dirty_folio,
>>>>>>>
>>>>>>> so btrfs should have its own btrfs_dirty_folio() which does whatever
>>>>>>> metadata updates it needs to and then call filemap_dirty_folio() to
>>>>>>> take care of the page cache business.  See iomap_dirty_folio() as
>>>>>>> an example, but many other filesystems also do this.
>>>>>>
>>>>>> Thanks a lot for the advice.
>>>>>>
>>>>>> However it looks like the sub-folio dirty block tracking is a little
>>>>>> different between iomap and btrfs.
>>>>>
>>>>> My point is not that "you should do it the exact same way as iomap".
>>>>> Rather "the dirty_folio op is the entry point to tell the filesystem
>>>>> that a folio is being dirtied".
>>>> And since dirty_folio() is not allowed to sleep, we should introduce some
>>>> extra mechanism, e.g. page private 2/checked, to notify the fs that the
>>>> folio is marked dirty without proper preparation.
>>>>
>>>> Then during writeback, detect such folio and do needed preparation for it
>>>> since at writeback we're allowed to sleep.
>>>>
>>>> That sounds feasible, but I haven't seen anyone doing that (including the
>>>> older btrfs cow fixup).
>>>>
>>>> Will explore that path. Thanks a lot again for the dirty_folio() help.
>>>>
>>>> Thanks,
>>>> Qu
>>>
>>> Here is my proposal for a candidate fix. It passes the reproducer in
>>> this thread as well as several more intense reproducers (alluded to but
>>> not yet included)
>>>
>>> https://lore.kernel.org/linux- btrfs/6758d4f27be0bbdb865cee7dd5adc435c969f4a3.1784960646.git.boris@bur.io/T/#u
>> I am willing to give it a try, but that does not seem to apply on 7.2-rc4
>>
> 
> The latest version is out:
> 
> https://lore.kernel.org/linux-btrfs/9f7a81cdd022d9e3f69d347a17c0a25438ccec14.1785131713.git.boris@bur.io/
> 
> And you can apply it on the latest btrfs for-next branch without any conflict:
> 
> https://github.com/btrfs/linux.git for-next
> 

Ok that seems to work for me avoiding the lockup issues.

I do have another issue (performance) with secure KVM memory on a large btrfs file but I think
I will report this differently when I have something to tell/show