linux-ext4 - Re: [PATCH 1/4] ext4: Fix deadlock during page writeback

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Date:	Mon, 4 Jul 2016 17:47:09 +0200
From:	Jan Kara <jack@...e.cz>
To:	Theodore Ts'o <tytso@....edu>
Cc:	Jan Kara <jack@...e.cz>, linux-ext4@...r.kernel.org,
	Eryu Guan <eguan@...hat.com>, stable@...r.kernel.org
Subject: Re: [PATCH 1/4] ext4: Fix deadlock during page writeback

On Mon 04-07-16 11:20:43, Ted Tso wrote:
> On Mon, Jul 04, 2016 at 04:00:12PM +0200, Jan Kara wrote:
> > OK, but we do all the block mappings, page locking etc. while the handle is
> > started so it is not exactly a really short lived handle. The patch adds
> > there a submission of a bio (we have the IO plugged so it will just add the
> > bio to the list of submitted bios), unlock locked pages, drop refcount to
> > ioend (unless IO is already completed, only refcount update is done, if IO
> > is completed we defer any real work to workqueue anyway). So although we
> > add some work which is done while the handle is still running, it is not
> > that much.
> 
> Good point that the block device is plugged.  Ultimately I suspect the
> way to fix the scalability problem will be move to dioread nolock as
> the default, and use separate transaction to map the blocks using the
> uninitialized flags, and then do a separate transaction to convert
> them afterwards.

This is what already happens currently - we only reserve a handle for
conversion during writeback but that reservation fluently moves between
running transactions until a point where the reserved handle is started -
then the handle is pinned to the currently running transaction - and this
happens only in the completion handler after IO is completed.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR
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