linux-kernel - Re: [PATCH] mm: fadvise: Drain all pagevecs if POSIX_FADV

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Date:	Fri, 15 Feb 2013 12:04:04 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Mel Gorman <mgorman@...e.de>, Rob van der Heij <rvdheij@...il.com>,
	Hugh Dickins <hughd@...gle.com>, Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: fadvise: Drain all pagevecs if POSIX_FADV_DONTNEED
 fails to discard all pages

On Thu 14-02-13 12:39:26, Andrew Morton wrote:
> On Thu, 14 Feb 2013 12:03:49 +0000
> Mel Gorman <mgorman@...e.de> wrote:
> 
> > Rob van der Heij reported the following (paraphrased) on private mail.
> > 
> > 	The scenario is that I want to avoid backups to fill up the page
> > 	cache and purge stuff that is more likely to be used again (this is
> > 	with s390x Linux on z/VM, so I don't give it as much memory that
> > 	we don't care anymore). So I have something with LD_PRELOAD that
> > 	intercepts the close() call (from tar, in this case) and issues
> > 	a posix_fadvise() just before closing the file.
> > 
> > 	This mostly works, except for small files (less than 14 pages)
> > 	that remains in page cache after the face.
> 
> Sigh.  We've had the "my backups swamp pagecache" thing for 15 years
> and it's still happening.
> 
> It should be possible nowadays to toss your backup application into a
> container to constrain its pagecache usage.  So we can type
> 
> 	run-in-a-memcg -m 200MB /my/backup/program
> 
> and voila.  Does such a script exist and work?

The script would be as simple as:
cgcreate -g memory:backups/`whoami`
cgset -r memory.limit_in_bytes=200MB backups/`whoami`
cgexec -g memory:backups/`whoami` /my/backup/program

It just expects that admin sets up backups group which allows the user
to create a subgroup (w permission on the directory) and probably set up
some reasonable cap for all backups
[...]
-- 
Michal Hocko
SUSE Labs
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