linux-kernel - Re: f2fs async buffered write patch

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Message-ID: <9438ac5c-6370-9df6-7259-c0914391dd74@kernel.dk>
Date:   Tue, 20 Jun 2023 07:09:08 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Jaegeuk Kim <jaegeuk@...nel.org>
Cc:     Yangtao Li <frank.li@...o.com>, Lu Hongfei <luhongfei@...o.com>,
        linux-f2fs-devel@...ts.sourceforge.net,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: f2fs async buffered write patch

On 6/20/23 12:16?AM, Jaegeuk Kim wrote:
> On 06/19, Jens Axboe wrote:
>> Hi,
>>
>> I came across this patch in a news posting:
>>
>> https://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs.git/commit/?h=dev&id=d618126911829523e35a61f4a5a4ad159b1b2c8d
>>
>> which has me a bit worried. As far as I can tell, all that patch does is
>> set FMODE_BUF_WASYNC, and then just hope that the lower layers handle
>> the rest?
>>
>> What happens if iocb->ki_flags & IOCB_NOWAIT is true, and now we do:
>>
>> generic_perform_write(iocb, from)
>> 	...
>> 	->write_begin() <- does this block?
>> 	...
>> 	->write_end() <- or this one?
>> 	...
>> 	balance_dirty_pages_ratelimited() <- this one surely does...
>>
>> If you look just one level down the latter to
>> balance_dirty_pages_ratelimited_flags(), you'll even see the 'flags'
>> argument documented there.
>>
>> This looks pretty haphazard and cannot possibly work as-is, so please
>> get this reverted until f2fs is converted to iomap, or IOCB_NOWAIT is
>> handled by generic_perform_write() and below.
> 
> Thank you for pointing that out. It seems I haven't reviewed it carefully.
> Hence I removed it from -next, and hope to have some time to convert iomap
> soon.

Thanks - would be great to get FMODE_BUF_WASYNC enabled obviously, just
needs a bit more work to get there.

-- 
Jens Axboe