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

[Andrew Morton <akpm@linux-foundation.org>]

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