linux-kernel - Re: [PATCH] mm: consolidate GFP_NOFAIL checks in the allocator slowpath

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Message-ID: <20161214203530.GA18561@cmpxchg.org>
Date:   Wed, 14 Dec 2016 15:35:30 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        Mel Gorman <mgorman@...e.de>,
        David Rientjes <rientjes@...gle.com>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>,
        Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH] mm: consolidate GFP_NOFAIL checks in the allocator
 slowpath

On Wed, Dec 14, 2016 at 04:07:06PM +0100, Michal Hocko wrote:
> From: Michal Hocko <mhocko@...e.com>
> 
> Tetsuo Handa has pointed out that 0a0337e0d1d1 ("mm, oom: rework oom
> detection") has subtly changed semantic for costly high order requests
> with __GFP_NOFAIL and withtout __GFP_REPEAT and those can fail right now.
> My code inspection didn't reveal any such users in the tree but it is
> true that this might lead to unexpected allocation failures and
> subsequent OOPs.
> 
> __alloc_pages_slowpath wrt. GFP_NOFAIL is hard to follow currently.
> There are few special cases but we are lacking a catch all place to be
> sure we will not miss any case where the non failing allocation might
> fail. This patch reorganizes the code a bit and puts all those special
> cases under nopage label which is the generic go-to-fail path. Non
> failing allocations are retried or those that cannot retry like
> non-sleeping allocation go to the failure point directly. This should
> make the code flow much easier to follow and make it less error prone
> for future changes.
> 
> While we are there we have to move the stall check up to catch
> potentially looping non-failing allocations.
> 
> Signed-off-by: Michal Hocko <mhocko@...e.com>
> Acked-by: Vlastimil Babka <vbabka@...e.cz>

It's not the nicest thing that we have to duplicate all the conditions
to warn on, but it's preferable over unreliable GFP_NOFAIL handling.
Consolidating the handling of this flag makes a lot of sense to me.

Acked-by: Johannes Weiner <hannes@...xchg.org>