linux-kernel - Re: suspicious __GFP

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Message-ID: <20170804075636.GD26029@dhcp22.suse.cz>
Date:   Fri, 4 Aug 2017 09:56:36 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Paul Moore <paul@...l-moore.com>
Cc:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>, mgorman@...e.de,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        selinux@...ho.nsa.gov
Subject: Re: suspicious __GFP_NOMEMALLOC in selinux

On Thu 03-08-17 14:17:26, Paul Moore wrote:
> On Thu, Aug 3, 2017 at 7:05 AM, Michal Hocko <mhocko@...nel.org> wrote:
> > On Thu 03-08-17 19:44:46, Tetsuo Handa wrote:
[...]
> >> When allocating thread is selected as an OOM victim, it gets TIF_MEMDIE.
> >> Since that function might be called from !in_interrupt() context, it is
> >> possible that gfp_pfmemalloc_allowed() returns true due to TIF_MEMDIE and
> >> the OOM victim will dip into memory reserves even when allocation failure
> >> is not a problem.
> >
> > Yes this is possible but I do not see any major problem with that.
> > I wouldn't add __GFP_NOMEMALLOC unless there is a real runaway of some
> > sort that could be abused.
> 
> Adding __GFP_NOMEMALLOC would not hurt anything would it?

I is not harmfull but I fail to see how it would be useful either and as
such it just adds a pointless gfp flag and confusion to whoever tries to
modify the code in future. Really the main purpose of __GFP_NOMEMALLOC
is to override the process scope PF_MEMALLOC. As such it is quite a hack
and the fewer users we have the better.

Btw. Should I resend the patch or somebody will take it from this email
thread?
-- 
Michal Hocko
SUSE Labs