linux-kernel - Re: [RFC] [PATCH] drop

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Date:	Thu, 28 Apr 2011 16:22:10 -0700
From:	Joel Becker <jlbec@...lplan.org>
To:	Andrea Righi <andrea@...terlinux.com>
Cc:	Dave Chinner <david@...morbit.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Arnd Bergmann <arnd@...db.de>, linux-fsdevel@...r.kernel.org,
	linux-api@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC] [PATCH] drop_pagecache syscall

On Wed, Apr 27, 2011 at 11:01:28AM +0200, Andrea Righi wrote:
> On Wed, Apr 27, 2011 at 10:14:53AM +1000, Dave Chinner wrote:
> > On Tue, Apr 26, 2011 at 11:35:27PM +0200, Andrea Righi wrote:
> > > Introduce sys_drop_pagecache() system call to drop the page cache pages of
> > > a single filesystem.
<snip>
> > > This functionality can be used by all the applications that want to have a
> > > better control over the page cache management (for example to immediately drop
> > > pages that for sure will not be reused in the near future, without calling
> > > posix_fadvise() for all the files they've touched), or to provide a more fine
> > > grained debugging feature usable by the filesystem benchmarks.
> > > 
> > > The system call does not require root privileges and it can be called by any
> > > unprivileged application. For example, we can write a userspace tool to run
> > > something like this:
> > > 
> > >   $ drop-pagecache /path/file_or_dir
> > 
> > That's a potential DOS vector, I think. Drop the pagecache in a hard
> > loop on the root fs of a busy server and watch it crawl...
> 
> Yes, probably we could allow only the CAP_SYS_ADMIN tasks to execute
> this syscall.

	The majority of apps that want this do not run as root.  Do we
want them all setuid?  ;-)

Joel

-- 

"If the human brain were so simple we could understand it, we would
 be so simple that we could not."
	- W. A. Clouston

			http://www.jlbec.org/
			jlbec@...lplan.org
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