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

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Date:   Sat, 16 Apr 2022 13:05:23 -0700 (PDT)
From:   Eric Wheeler <linux-block@...ts.ewheeler.net>
To:     Christoph Hellwig <hch@...radead.org>
cc:     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 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?

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?

Thanks!

--
Eric Wheeler