linux-kernel - Re: suppress page allocation failure warnings from sys

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Date:	Tue, 27 Mar 2012 20:15:50 -0400
From:	Dave Jones <davej@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	viro@...iv.linux.org.uk,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: suppress page allocation failure warnings from sys_listxattr

On Tue, Mar 27, 2012 at 03:51:49PM -0700, Andrew Morton wrote:
 > On Tue, 13 Mar 2012 14:22:20 -0400
 > Dave Jones <davej@...hat.com> wrote:
 > 
 > > This size is user controllable, and so it's trivial for someone to trigger a
 > > stream of order:4 page allocation errors.
 > > 
 > > Signed-off-by: Dave Jones <davej@...hat.com>
 > > 
 > > --- 
 > > There's also a similar problem in setxattr, but I'm not sure how we want
 > > to pass NOWARN down to memdup_user. Thoughts ?
 > > 
 > > diff --git a/fs/xattr.c b/fs/xattr.c
 > > index 82f4337..544df90 100644
 > > --- a/fs/xattr.c
 > > +++ b/fs/xattr.c
 > > @@ -496,7 +496,7 @@ listxattr(struct dentry *d, char __user *list, size_t size)
 > >  	if (size) {
 > >  		if (size > XATTR_LIST_MAX)
 > >  			size = XATTR_LIST_MAX;
 > > -		klist = kmalloc(size, GFP_KERNEL);
 > > +		klist = kmalloc(size, __GFP_NOWARN | GFP_KERNEL);
 > >  		if (!klist)
 > >  			return -ENOMEM;
 > >  	}
 > 
 > hm.  The patch is good, but one would hope that it isn't "trivial" to
 > trigger a page allocation failure for a kmalloc(65536, GFP_KERNEL) -
 > the VM is supposed to be able to handle that.
 > 
 > Is it really *that* easy, or is Something Unusual happening with that
 > machine?

Well, the unusual thing was that I was fuzzing system calls for a few hours.

My fuzzing tool was able to trigger these very easily after an hour or two
of uptime and memory had fragmented a little, so yeah, quite trivial.

	Dave
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