linux-kernel - Re: [PATCH 2/2] mm, oom: do not enfore OOM killer for __GFP

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Message-ID: <f7ac6711-4c9b-5603-7901-ae90a56a0d1a@suse.cz>
Date:   Fri, 2 Dec 2016 08:23:24 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Michal Hocko <mhocko@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        Johannes Weiner <hannes@...xchg.org>,
        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 2/2] mm, oom: do not enfore OOM killer for __GFP_NOFAIL
 automatically

On 12/01/2016 04:25 PM, Michal Hocko wrote:
> From: Michal Hocko <mhocko@...e.com>
>
> __alloc_pages_may_oom makes sure to skip the OOM killer depending on
> the allocation request. This includes lowmem requests, costly high
> order requests and others. For a long time __GFP_NOFAIL acted as an
> override for all those rules. This is not documented and it can be quite
> surprising as well. E.g. GFP_NOFS requests are not invoking the OOM
> killer but GFP_NOFS|__GFP_NOFAIL does so if we try to convert some of
> the existing open coded loops around allocator to nofail request (and we
> have done that in the past) then such a change would have a non trivial
> side effect which is not obvious. Note that the primary motivation for
> skipping the OOM killer is to prevent from pre-mature invocation.
>
> The exception has been added by 82553a937f12 ("oom: invoke oom killer
> for __GFP_NOFAIL"). The changelog points out that the oom killer has to
> be invoked otherwise the request would be looping for ever. But this
> argument is rather weak because the OOM killer doesn't really guarantee
> any forward progress for those exceptional cases - e.g. it will hardly
> help to form costly order - I believe we certainly do not want to kill
> all processes and eventually panic the system just because there is a
> nasty driver asking for order-9 page with GFP_NOFAIL not realizing all
> the consequences - it is much better this request would loop for ever
> than the massive system disruption, lowmem is also highly unlikely to be
> freed during OOM killer and GFP_NOFS request could trigger while there
> is still a lot of memory pinned by filesystems.
>
> This patch simply removes the __GFP_NOFAIL special case in order to have
> a more clear semantic without surprising side effects. Instead we do
> allow nofail requests to access memory reserves to move forward in both
> cases when the OOM killer is invoked and when it should be supressed.
> __alloc_pages_nowmark helper has been introduced for that purpose.
>
> Signed-off-by: Michal Hocko <mhocko@...e.com>

Acked-by: Vlastimil Babka <vbabka@...e.cz>