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From: Eric Wheeler <linux-block@lists.ewheeler.net>
To: Ming Lei <ming.lei@redhat.com>
Cc: linux-block@vger.kernel.org
Subject: Re: loop: it looks like REQ_OP_FLUSH could return before IO completion.
Date: Wed, 13 Apr 2022 15:49:07 -0700 (PDT)	[thread overview]
Message-ID: <bd5f9817-c65e-7915-18b-9c68bb34488e@ewheeler.net> (raw)
In-Reply-To: <c03de7ac-63e9-2680-ca5b-8be62e4e177f@ewheeler.net>

On Tue, 22 Mar 2022, Eric Wheeler wrote:
> On Mon, 21 Mar 2022, Ming Lei wrote:
> > On Sat, Mar 19, 2022 at 10:14:29AM -0700, Eric Wheeler wrote:
> > > Hello all,
> > > 
> > > In loop.c do_req_filebacked() for REQ_OP_FLUSH, lo_req_flush() is called: 
> > > it does not appear that lo_req_flush() does anything to make sure 
> > > ki_complete has been called for pending work, it just calls vfs_fsync().
> > > 
> > > Is this a consistency problem?
> > 
> > No. What FLUSH command provides is just flushing cache in device side to
> > storage medium, so it is nothing to do with pending request.
> 
> If a flush follows a series of writes, would it be best if the flush 
> happened _after_ those writes complete?  Then then the storage medium will 
> be sure to flush what was intended to be written.
> 
> It seems that this series of events could lead to inconsistent data:
> 	loop		->	filesystem
> 	write a
> 	write b
> 	flush
> 				write a
> 				flush
> 				write b
> 				crash, b is lost
> 
> If write+flush ordering is _not_ important, then can you help me 
> understand why?
> 

Hi Ming, just checking in: did you see the message above?

Do you really mean to say that reordering writes around a flush is safe 
in the presence of a crash?


--
Eric Wheeler



> -Eric
> 
> 
> 
> > 
> > 
> > Thanks, 
> > Ming
> > 
> > 
> 

  reply	other threads:[~2022-04-13 22:49 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 [this message]
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
2022-04-17  0:59                 ` Jens Axboe
2022-04-17 16:32                   ` Eric Wheeler

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