linux-kernel - Re: [PATCH 2/2] mm: do not loop over ALLOC_NO

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Message-ID: <alpine.DEB.2.10.1511161318180.11456@chino.kir.corp.google.com>
Date:	Mon, 16 Nov 2015 13:18:53 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	mhocko@...nel.org
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>,
	Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH 2/2] mm: do not loop over ALLOC_NO_WATERMARKS without
 triggering reclaim

On Mon, 16 Nov 2015, mhocko@...nel.org wrote:

> From: Michal Hocko <mhocko@...e.com>
> 
> __alloc_pages_slowpath is looping over ALLOC_NO_WATERMARKS requests if
> __GFP_NOFAIL is requested. This is fragile because we are basically
> relying on somebody else to make the reclaim (be it the direct reclaim
> or OOM killer) for us. The caller might be holding resources (e.g.
> locks) which block other other reclaimers from making any progress for
> example. Remove the retry loop and rely on __alloc_pages_slowpath to
> invoke all allowed reclaim steps and retry logic.
> 
> We have to be careful about __GFP_NOFAIL allocations from the
> PF_MEMALLOC context even though this is a very bad idea to begin with
> because no progress can be gurateed at all.  We shouldn't break the
> __GFP_NOFAIL semantic here though. It could be argued that this is
> essentially GFP_NOWAIT context which we do not support but PF_MEMALLOC
> is much harder to check for existing users because they might happen
> deep down the code path performed much later after setting the flag
> so we cannot really rule out there is no kernel path triggering this
> combination.
> 
> Acked-by: Mel Gorman <mgorman@...e.de>
> Signed-off-by: Michal Hocko <mhocko@...e.com>

Acked-by: David Rientjes <rientjes@...gle.com>

It'll be scary if anything actually relies on this, but I think it's more 
correct.
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