linux-kernel - Re: [patch 0/4] MAP_NOZERO v2 - VM_NOZERO/MAP

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

Message-ID: <20070704215353.GW9157@hexapodia.org>
Date:	Wed, 4 Jul 2007 14:53:53 -0700
From:	Andy Isaacson <adi@...apodia.org>
To:	Davide Libenzi <davidel@...ilserver.org>
Cc:	Rik van Riel <riel@...hat.com>, Ulrich Drepper <drepper@...il.com>,
	Kyle Moffett <mrmacman_g4@....com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [patch 0/4] MAP_NOZERO v2 - VM_NOZERO/MAP_NOZERO early summer madness

On Mon, Jul 02, 2007 at 06:55:40PM -0400, Rik van Riel wrote:
> You could easily replace the cookie with a pointer to a free
> page pool.

It just occurred to me that something like this is *required* to get the
performance benefit from MAP_NOZERO on a busy system.  With Davide's
current proposal, if there are N jobs with different "nozero cookies"
busy allocating and deallocating pages on a single-NUMA-node system,
then there's only a 1/N chance that the page returned by __alloc_pages
will have the correct cookie, so (N-1)/N percent of the time MAP_NOZERO
will have no positive effect -- 90% of the time for the case of N=10.

(Of course on NUMA systems node affinity will probably make the
situation a bit better.)

So I'm afraid that it sounds like additional complexity would be
required so that __alloc_pages could track and preferentially return
pages with the correct "nozero cookie".

In a mostly unrelated complaint, I note that having a function named
"alloc_zeroed_page_vma" which returns a potentially nonzero page is, um,
unintuitive.  It needs a name which does not claim it returns zeroed
pages.  I'm no good at names, but perhaps "alloc_available_page_vma"?

-andy
-
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/