linux-kernel - Re: [PATCH 1/4] Cache xattr security drop check for write

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Date:	Sat, 28 May 2011 12:42:44 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Andi Kleen <ak@...ux.intel.com>, chris.mason@...cle.com,
	josef@...hat.com, agruen@...bit.com,
	"Serge E. Hallyn" <serue@...ibm.com>
Subject: Re: [PATCH 1/4] Cache xattr security drop check for write

On Fri, May 27, 2011 at 03:54:02PM -0700, Andi Kleen wrote:
> From: Andi Kleen <ak@...ux.intel.com>
> 
> Some recent benchmarking on btrfs showed that a major scaling bottleneck
> on large systems on btrfs is currently the xattr lookup on every write.
> 
> Why xattr lookup on every write I hear you ask?
> 
> write wants to drop suid and security related xattrs that could set o
> capabilities for executables.  To do that it currently looks up
> security.capability on EVERY write (even for non executables) to decide
> whether to drop it or not.

Hm...
	a) is_sgid() is a bad name for that - at the very least s/g/x/, since
anybody would read your variant as "check if it's set-group-id".
	b) I'd add a helper for filesystems to use, rather than messing
with the flags directly.
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