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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: Tue, 22 Mar 2022 23:14:56 -0700 (PDT)	[thread overview]
Message-ID: <c03de7ac-63e9-2680-ca5b-8be62e4e177f@ewheeler.net> (raw)
In-Reply-To: <YjfFHvTCENCC29WS@T590>

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?

-Eric



> 
> 
> Thanks, 
> Ming
> 
> 

  reply	other threads:[~2022-03-23  6:15 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 [this message]
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
2022-04-17  0:59                 ` Jens Axboe
2022-04-17 16:32                   ` Eric Wheeler

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