Re: [PATCH] ext4: complete dropbehind writeback in task context

[Zhang Yi]

On 10/14/2026 1:32 AM, Tal Zussman wrote:
> On 10/13/26 8:47 AM, Jan Kara wrote:
>> On Mon 12-10-26 14:04:37, Tal Zussman wrote:
>>> ext4_end_bio() ends writeback directly from bio completion, which
>>> usually runs in interrupt context, unless the io_end has unwritten
>>> extents to convert and is completed from ext4's workqueue.
>>> folio_end_dropbehind() only invalidates the folios of an RWF_DONTCACHE
>>> write when it runs in task context, and silently skips them otherwise.
>>> So on ext4, dontcache writes that overwrite blocks that are already
>>> written, or that allocate blocks without unwritten extents, leave their
>>> folios in the page cache. Dropping them was best effort so far, as there
>>> was no way to get task context from bio completion.
>>>
>>> Use the task-context bio completion infrastructure instead: set
>>> BIO_COMPLETE_IN_TASK on bios that carry dropbehind folios in
>>> io_submit_add_bh(), so that bio_endio() runs their completion from a
>>> worker. ext4's iomap buffered I/O path already gets this from iomap, so
>>> only the buffer_head based writeback is affected. Skip the flag when the
>>> io_end already has unwritten extents, as ext4 then defers the completion
>>> itself. A later extent can still make the io_end unwritten after an
>>> earlier bio was built. That bio's completion is then deferred twice,
>>> which is not optimal, but harmless.
>>>
>>> This changes behavior only for bios that carry dropbehind folios: their
>>> completion now runs from a worker instead of in interrupt context.
>>>
>>> On the unpatched kernel a dontcache overwrite of already-written blocks,
>>> or a dontcache write into a new file on a dioread_lock mount, leaves
>>> every folio in the page cache (16384/16384 for a 64M file). With the
>>> patch, none remain in the page cache. A dontcache write into a new file
>>> with unwritten extents already drops its folios on both kernels, as
>>> expected, and a plain write keeps them.
>>>
>>> Signed-off-by: Tal Zussman <tz2294@xxxxxxxxxxxx>
>>
>> I guess this is OK as a short term fix but having two different offloading
>> mechanisms for IO completion is just asking for trouble and we should use
>> BIO_COMPLETE_IN_TASK when unwritten extent conversion is needed as well.
>> But for this fix feel free to add:
>>
>> Reviewed-by: Jan Kara <jack@xxxxxxx>
>>
> 

Ha, I also noticed this issue in the buffer_head path when I was adding
the iomap path earlier. I was planned to start addressing the generic
bio completion converting work across both writeback paths after the
current stage of work wraps up, so thanks for fixing this one separately!

Reviewed-by: Zhang Yi <yi.zhang@xxxxxxxxxx>


> Thanks! Agreed on the above. I saw that there was a TODO added in
> ext4_iomap_writeback_submit() from the new iomap conversion series to
> "switch all defer handlers to the generic bio complete in task framework",
> so I assumed that'll be taken care of as part of the ongoing conversion.
> 

Yes, Jan discussed this optimization direction with me previously. Next,
I'll try to remove ext4's own deferred workqueue and migrate the work
into the generic flow.

Thanks,
Yi.

> - Tal
> 
>> 								Honza
>>
>>> ---
>>>  fs/ext4/page-io.c | 10 ++++++++++
>>>  1 file changed, 10 insertions(+)
>>>
>>> diff --git a/fs/ext4/page-io.c b/fs/ext4/page-io.c
>>> index 734d856a6d47..e556598ce9e1 100644
>>> --- a/fs/ext4/page-io.c
>>> +++ b/fs/ext4/page-io.c
>>> @@ -458,6 +458,16 @@ static void io_submit_add_bh(struct ext4_io_submit *io,
>>>  		io_submit_init_bio(io, inode, folio, bh);
>>>  	if (!bio_add_folio(io->io_bio, folio, bh->b_size, bh_offset(bh)))
>>>  		goto submit_and_retry;
>>> +
>>> +	/*
>>> +	 * Dropbehind folios can only be invalidated in task context. An
>>> +	 * io_end with unwritten extents is completed from ext4's workqueue
>>> +	 * anyway, so only ask the block layer to defer the others.
>>> +	 */
>>> +	if (folio_test_dropbehind(folio) &&
>>> +	    !(io->io_end->flag & EXT4_IO_END_UNWRITTEN))
>>> +		bio_set_flag(io->io_bio, BIO_COMPLETE_IN_TASK);
>>> +
>>>  	wbc_account_cgroup_owner(io->io_wbc, folio, bh->b_size);
>>>  	io->io_next_block++;
>>>  }
>>>
>>> ---
>>> base-commit: 35aefefd7aba0801bab635b11943f932626be3b2
>>> change-id: 20260924-ext4-dontcache-complete-in-task-2340ed5f3114
>>>
>>> Best regards,
>>> --  
>>> Tal Zussman <tz2294@xxxxxxxxxxxx>
>>>
>> -- 
>> Jan Kara <jack@xxxxxxxx>
>> SUSE Labs, CR
>>
> 
>