linux-kernel - Re: sync_file_range(SYNC_FILE_RANGE

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Date:	Mon, 16 Jun 2008 21:54:06 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	Hugh Dickins <hugh@...itas.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Lee Schermerhorn <lee.schermerhorn@...com>,
	Pavel Machek <pavel@...e.cz>, mtk.manpages@...il.com,
	kernel list <linux-kernel@...r.kernel.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>
Subject: Re: sync_file_range(SYNC_FILE_RANGE_WRITE) blocks?

On Mon, 16 Jun 2008 16:53:51 -0400 Rik van Riel <riel@...hat.com> wrote:

> On Mon, 2 Jun 2008 13:40:28 +0100 (BST)
> Hugh Dickins <hugh@...itas.com> wrote:
> > On Mon, 2 Jun 2008, Jens Axboe wrote:
> > > On Sun, Jun 01 2008, Andrew Morton wrote:
> > > > 
> > > > Well if you're asking the syscall to shove more data into the block
> > > > layer than it can concurrently handle, sure, the block layer will
> > > > block.  It's tunable...
> > > 
> > > Ehm, lets get the history right, please :-)
> > > 
> > > The block layer pretty much doesn't care about how large the queue
> > > size is, it's largely at 128 to prevent the vm from shitting itself
> > > like it has done in the past (and continues to do I guess, though
> > > your reply leaves me wondering).
> > > 
> > > So you think the vm will be fine with a huge number of requests?
> > > It wont go nuts scanning and reclaiming, wasting oodles of CPU
> > > cycles?
> > 
> > Interesting.  I wonder.  I may be quite wrong (Cc'ed Rik and Lee
> > who I think are currently most in touch with what goes on there),
> > but my impression is that whereas vmscan.c takes pages off LRU
> > while it's doing writeback on them, and arranges for them to go
> > back to the reclaimable tail of the LRU
> 
> The problem with lots of CFQ requests is simpler.
> 
> If you look at balance_dirty_pages() you will see that it only
> takes dirty and NFS unstable pages into account when checking
> the dirty limit.  The pages that are in flight (under IO) are
> not counted at all.

That would be totally busted.  All that nr_writeback logic in there is very
much supposed to handle under-writeback pages?

> If you have 8192 CFQ requests and large streaming IO, most of
> the IOs will be mergeable and it is possible to pin all of
> memory in in-flight (PG_writeback - and other?) pages.
> 
> I suspect that balance_dirty_pages() will have to take the
> writeback pages into account, though that may cause problems
> with FUSE.  Maybe it should at least wait for some IO to
> complete?

Worried.  One of us is missing something here.
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