linux-kernel - Re: [PATCH 05/17] mm: allow PF

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: <20120511.003951.1470088131186301605.davem@davemloft.net>
Date:	Fri, 11 May 2012 00:39:51 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	mgorman@...e.de
Cc:	akpm@...ux-foundation.org, linux-mm@...ck.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	neilb@...e.de, a.p.zijlstra@...llo.nl, michaelc@...wisc.edu,
	emunson@...bm.net
Subject: Re: [PATCH 05/17] mm: allow PF_MEMALLOC from softirq context

From: Mel Gorman <mgorman@...e.de>
Date: Thu, 10 May 2012 14:44:58 +0100

> This is needed to allow network softirq packet processing to make
> use of PF_MEMALLOC.
> 
> Currently softirq context cannot use PF_MEMALLOC due to it not being
> associated with a task, and therefore not having task flags to fiddle
> with - thus the gfp to alloc flag mapping ignores the task flags when
> in interrupts (hard or soft) context.
> 
> Allowing softirqs to make use of PF_MEMALLOC therefore requires some
> trickery.  We basically borrow the task flags from whatever process
> happens to be preempted by the softirq.
> 
> So we modify the gfp to alloc flags mapping to not exclude task flags
> in softirq context, and modify the softirq code to save, clear and
> restore the PF_MEMALLOC flag.
> 
> The save and clear, ensures the preempted task's PF_MEMALLOC flag
> doesn't leak into the softirq. The restore ensures a softirq's
> PF_MEMALLOC flag cannot leak back into the preempted process.
> 
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> Signed-off-by: Mel Gorman <mgorman@...e.de>

We're now making changes to task->flags from both base and
softirq context, but with non-atomic operations and no other
kind of synchronization.

As far as I can tell, this has to be racy.

If this works via some magic combination of invariants, you
absolutely have to document this, verbosely.
--
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/