linux-kernel - Re: [patch] mm, oom: remove gfp helper function

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Message-ID: <20141127102547.GA18833@dhcp22.suse.cz>
Date:	Thu, 27 Nov 2014 11:25:47 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Qiang Huang <h.huangqiang@...wei.com>,
	Johannes Weiner <hannes@...xchg.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [patch] mm, oom: remove gfp helper function

On Wed 26-11-14 14:17:32, David Rientjes wrote:
> Commit b9921ecdee66 ("mm: add a helper function to check may oom
> condition") was added because the gfp criteria for oom killing was
> checked in both the page allocator and memcg.
> 
> That was true for about nine months, but then commit 0029e19ebf84 ("mm:
> memcontrol: remove explicit OOM parameter in charge path") removed the
> memcg usecase.
> 
> Fold the implementation into its only caller.

I don't care much whether the check is open coded or hidden behind the
helper but I would really appreciate a comment explaining why we care
about these two particular gfp flags. The code is like that since ages
- excavation work would lead us back to 2002 resp. 2003. Let's save
other others people time and do not repeat the same exercise again.

What about a comment like the following?

> Signed-off-by: David Rientjes <rientjes@...gle.com>
> ---
>  include/linux/oom.h | 5 -----
>  mm/page_alloc.c     | 2 +-
>  2 files changed, 1 insertion(+), 6 deletions(-)
> 
[...]
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2706,7 +2706,7 @@ rebalance:
>  	 * running out of options and have to consider going OOM
>  	 */
>  	if (!did_some_progress) {
> -		if (oom_gfp_allowed(gfp_mask)) {
		/*
		 * Do not attempt to trigger OOM killer for !__GFP_FS
		 * allocations because it would be premature to kill
		 * anything just because the reclaim is stuck on
		 * dirty/writeback pages.
		 * __GFP_NORETRY allocations might fail and so the OOM
		 * would be more harmful than useful.
		 */
> +		if ((gfp_mask & __GFP_FS) && !(gfp_mask & __GFP_NORETRY)) {
>  			if (oom_killer_disabled)
>  				goto nopage;
>  			/* Coredumps can quickly deplete all memory reserves */

-- 
Michal Hocko
SUSE Labs
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