linux-kernel - Re: [RFC][PATCH 2/3] mm: slab: move around slab ->freelist for cmpxchg

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Date:	Thu, 12 Dec 2013 15:40:38 -0800
From:	Andi Kleen <ak@...ux.intel.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Dave Hansen <dave@...1.net>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, kirill.shutemov@...ux.intel.com
Subject: Re: [RFC][PATCH 2/3] mm: slab: move around slab ->freelist for
 cmpxchg

On Thu, Dec 12, 2013 at 05:46:02PM +0000, Christoph Lameter wrote:
> On Wed, 11 Dec 2013, Dave Hansen wrote:
> 
> >
> > The write-argument to cmpxchg_double() must be 16-byte aligned.
> > We used to align 'struct page' itself in order to guarantee this,
> > but that wastes 8-bytes per page.  Instead, we take 8-bytes
> > internal to the page before page->counters and move freelist
> > between there and the existing 8-bytes after counters.  That way,
> > no matter how 'stuct page' itself is aligned, we can ensure that
> > we have a 16-byte area with which to to this cmpxchg.
> 
> Well this adds additional branching to the fast paths.

The branch should be predictible and compare the cost of a branch
(near nothing on a modern OOO CPU with low IPC code like this when
predicted) to the cost of a cache miss (due to larger struct page)

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/