linux-ext4 - Re: loop: it looks like REQ_OP_FLUSH could return before IO completion.

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Message-ID: <d867fce8-e252-a621-cadb-c658dd2906a@ewheeler.net>
Date:   Sun, 17 Apr 2022 09:32:07 -0700 (PDT)
From:   Eric Wheeler <linux-block@...ts.ewheeler.net>
To:     Jens Axboe <axboe@...nel.dk>
cc:     Christoph Hellwig <hch@...radead.org>,
        Ming Lei <ming.lei@...hat.com>, linux-block@...r.kernel.org,
        linux-ext4@...r.kernel.org
Subject: Re: loop: it looks like REQ_OP_FLUSH could return before IO
 completion.

On Sat, 16 Apr 2022, Jens Axboe wrote:
> On 4/16/22 2:05 PM, Eric Wheeler wrote:
> > On Fri, 15 Apr 2022, Christoph Hellwig wrote:
> >> On Fri, Apr 15, 2022 at 10:29:34PM +0800, Ming Lei wrote:
> >>> If ext4 expects the following order, it is ext4's responsibility to
> >>> maintain the order, and block layer may re-order all these IOs at will,
> >>> so do not expect IOs are issued to device in submission order
> >>
> >> Yes, and it has been so since REQ_FLUSH (which later became
> >> REQ_OP_FLUSH) replaced REQ_BARRIER 12 years ago:
> >>
> >> commit 28e7d1845216538303bb95d679d8fd4de50e2f1a
> >> Author: Tejun Heo <tj@...nel.org>
> >> Date:   Fri Sep 3 11:56:16 2010 +0200
> >>
> >> block: drop barrier ordering by queue draining
> >>     
> >>     Filesystems will take all the responsibilities for ordering requests
> >>     around commit writes and will only indicate how the commit writes
> >>     themselves should be handled by block layers.  This patch drops
> >>     barrier ordering by queue draining from block layer.
> > 
> > Thanks Christoph. I think this answers my original question, too.
> > 
> > You may have already answered this implicitly above.  If you would be so 
> > kind as to confirm my or correct my understanding with a few more 
> > questions:
> > 
> > 1. Is the only way for a filesystem to know if one IO completed before a 
> >    second IO to track the first IO's completion and submit the second IO 
> >    when the first IO's completes (eg a journal commit followed by the 
> >    subsequent metadata update)?  If not, then what block-layer mechanism 
> >    should be used?
> 
> You either need to have a callback or wait on the IO, there's no other
> way.
> 
> > 2. Are there any IO ordering flags or mechanisms in the block layer at 
> >    this point---or---is it simply that all IOs entering the block layer 
> >    can always be re-ordered before reaching the media?
> 
> No, no ordering flags are provided for this kind of use case. Any IO can
> be reordered, hence the only reliable solution is to ensure the previous
> have completed.

Perfect, thanks Jens!

> 
> -- 
> Jens Axboe
> 
> 



--
Eric Wheeler