linux-kernel - Re: [PATCH v2.4 0/3] mm/fs: Remove unnecessary waiting for stable pages

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Date:	Wed, 16 Jan 2013 20:43:52 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Darrick J. Wong" <darrick.wong@...cle.com>
Cc:	axboe@...nel.dk, lucho@...kov.net, jack@...e.cz, ericvh@...il.com,
	tytso@....edu, viro@...iv.linux.org.uk, rminnich@...dia.gov,
	martin.petersen@...cle.com, neilb@...e.de, david@...morbit.com,
	gnehzuil.liu@...il.com, linux-kernel@...r.kernel.org,
	hch@...radead.org, linux-fsdevel@...r.kernel.org,
	adilger.kernel@...ger.ca, bharrosh@...asas.com, jlayton@...ba.org,
	linux-ext4@...r.kernel.org, hirofumi@...l.parknet.co.jp
Subject: Re: [PATCH v2.4 0/3] mm/fs: Remove unnecessary waiting for stable
 pages

On Wed, 16 Jan 2013 18:49:02 -0800 "Darrick J. Wong" <darrick.wong@...cle.com> wrote:

> > 
> > The problem back in 2001 was that we held lock_page() across the
> > duration of page writeback, so if another thread came in and tried to
> > dirty the page, it would block on lock_page() until IO completion.  I
> > can't remember whether writeback would also block read().  Maybe it did,
> > in which case the effects of this patchset won't be as dramatic as were
> > the effects of splitting PG_lock into PG_lock and PG_writeback.
> 
> Now that you've stirred my memory, I /do/ dimly recall that Linux waited for
> writeback back in the old days.  At least we'll be back to that.

Not really.  2.4 did writeback by walking a standalone list of
buffer_heads, without locking their containing page.  I removed all
that and did writeback of the page instead.  That immediately caused
this problem, because the 2.4 writepage held lock_page() across
writeout.  So I changed that to drop lock_page() immediately after
submission and added PG_writeback to flag the under-writeback state. 
The second change went in pretty much immediately - all within the
same 2.5.x release, probably.

> As a side note, the average latency of a write to a non-DIF disk dropped down
> to nearly nothing.

Some hard numbers in the changelog would be nice.  Did you try dbench-on-ext2?

> > 
> > Do we generate nice kernel messages (at mount or device-probe time)
> > which will permit people to work out which strategy their device/fs is
> > using?
> 
> No.  /sys/devices/virtual/bdi/*/stable_pages_required will tell you
> stable pages are on or not, but so far only ext3 uses snapshots and the rest
> just wait.  Do you think a printk would be useful?

Nope, if we can query the mode under /sys then that should be sufficient.
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