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From: Eric Wheeler <>
To: Christoph Hellwig <>
Cc: Ming Lei <>,,
Subject: Re: loop: it looks like REQ_OP_FLUSH could return before IO completion.
Date: Sat, 16 Apr 2022 13:05:23 -0700 (PDT)	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <YlpRrLmwe/>

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 <>
> 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 

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?


Eric Wheeler

  reply	other threads:[~2022-04-16 20:05 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-03-19 17:14 loop: it looks like REQ_OP_FLUSH could return before IO completion Eric Wheeler
2022-03-21  0:21 ` Ming Lei
2022-03-23  6:14   ` Eric Wheeler
2022-04-13 22:49     ` Eric Wheeler
2022-04-14  0:15       ` Ming Lei
2022-04-14  0:28       ` Ming Lei
2022-04-15  2:10         ` Eric Wheeler
2022-04-15 14:29           ` Ming Lei
2022-04-16  5:18             ` Christoph Hellwig
2022-04-16 20:05               ` Eric Wheeler [this message]
2022-04-17  0:59                 ` Jens Axboe
2022-04-17 16:32                   ` Eric Wheeler

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