linux-kernel - Re: [patch 4/6] x86: finish user fault error path with fatal signal

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Date:	Fri, 26 Jul 2013 14:46:57 -0400
From:	Johannes Weiner <hannes@...xchg.org>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	David Rientjes <rientjes@...gle.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	azurIt <azurit@...ox.sk>, linux-mm@...ck.org,
	cgroups@...r.kernel.org, x86@...nel.org,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [patch 4/6] x86: finish user fault error path with fatal signal

On Fri, Jul 26, 2013 at 03:52:07PM +0200, Michal Hocko wrote:
> On Thu 25-07-13 18:25:36, Johannes Weiner wrote:
> > The x86 fault handler bails in the middle of error handling when the
> > task has a fatal signal pending.  For a subsequent patch this is a
> > problem in OOM situations because it relies on
> > pagefault_out_of_memory() being called even when the task has been
> > killed, to perform proper per-task OOM state unwinding.
> > 
> > Shortcutting the fault like this is a rather minor optimization that
> > saves a few instructions in rare cases.  Just remove it for
> > user-triggered faults.
> 
> OK, I thought that this optimization tries to prevent calling OOM
> because the current might release some memory but that wasn't the
> intention of b80ef10e8 (x86: Move do_page_fault()'s error path under
> unlikely()).

out_of_memory() also checks the caller for pending signals, so it
would not actually invoke the OOM killer if the caller is already
dying.

> > Use the opportunity to split the fault retry handling from actual
> > fault errors and add locking documentation that reads suprisingly
> > similar to ARM's.
> > 
> > Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> 
> Reviewed-by: Michal Hocko <mhocko@...e.cz>

Thanks!
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