linux-kernel - Re: [PATCH] remove throttle_vm

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Date:	Fri, 05 Oct 2007 15:23:16 -0400
From:	Trond Myklebust <trond.myklebust@....uio.no>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Miklos Szeredi <miklos@...redi.hu>, akpm@...ux-foundation.org,
	wfg@...l.ustc.edu.cn, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] remove throttle_vm_writeout()

On Fri, 2007-10-05 at 15:20 -0400, Trond Myklebust wrote:
> On Fri, 2007-10-05 at 20:32 +0200, Peter Zijlstra wrote:
> > Well, the thing is, we throttle pageout in throttle_vm_writeout(). As it
> > stand we can deadlock there because it just waits for the numbers to
> > drop, and unstable pages don't automagically dissapear. Only
> > write_inodes() - normally called from balance_dirty_pages() will call
> > COMMIT.
> > 
> > So my thought was that calling pageout() on an unstable page would do
> > the COMMIT - we're low on memory, otherwise we would not be paging, so
> > getting rid of unstable pages seems to make sense to me.
> 
> Why not rather track which mappings have large numbers of outstanding
> unstable writes at the VM level, and then add some form of callback to
> allow it to notify the filesystem when it needs to flush them out?
> 
> Cheers
>   Trond

BTW: Please note that at least in the case of NFS, you will have to
allow for the fact that the filesystem may not be _able_ to cause the
numbers to drop. If the server is unavailable, then we're may be stuck
in unstable page limbo for quite some time.

Trond

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