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

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Message-Id: <20120327172622.827fd6ab.akpm@linux-foundation.org>
Date:	Tue, 27 Mar 2012 17:26:22 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Dave Jones <davej@...hat.com>
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, 27 Mar 2012 20:15:50 -0400 Dave Jones <davej@...hat.com> wrote:

> 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.
> 

/*
 * PAGE_ALLOC_COSTLY_ORDER is the order at which allocations are deemed
 * costly to service.  That is between allocation orders which should
 * coelesce naturally under reasonable reclaim pressure and those which
 * will not.
 */
#define PAGE_ALLOC_COSTLY_ORDER 3


Death to magic numbers :(
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