linux-kernel - suspicious __GFP_NOMEMALLOC in selinux

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Message-ID: <20170802105018.GA2529@dhcp22.suse.cz>
Date:   Wed, 2 Aug 2017 12:50:18 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Jeff Vander Stoep <jeffv@...gle.com>
Cc:     Paul Moore <pmoore@...hat.com>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: suspicious __GFP_NOMEMALLOC in selinux

Hi,
while doing something completely unrelated to selinux I've noticed a
really strange __GFP_NOMEMALLOC usage pattern in selinux, especially
GFP_ATOMIC | __GFP_NOMEMALLOC doesn't make much sense to me. GFP_ATOMIC
on its own allows to access memory reserves while the later flag tells
we cannot use memory reserves at all. The primary usecase for
__GFP_NOMEMALLOC is to override a global PF_MEMALLOC should there be a
need.

It all leads to fa1aa143ac4a ("selinux: extended permissions for
ioctls") which doesn't explain this aspect so let me ask. Why is the
flag used at all? Moreover shouldn't GFP_ATOMIC be actually GFP_NOWAIT.
What makes this path important to access memory reserves?

Thanks
-- 
Michal Hocko
SUSE Labs