linux-kernel - Re: [PATCH] mm: get rid of __alloc_pages_high

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Message-ID: <alpine.DEB.2.10.1511121245430.10324@chino.kir.corp.google.com>
Date:	Thu, 12 Nov 2015 12:47:45 -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] mm: get rid of __alloc_pages_high_priority

On Thu, 12 Nov 2015, mhocko@...nel.org wrote:

> From: Michal Hocko <mhocko@...e.com>
> 
> __alloc_pages_high_priority doesn't do anything special other than it
> calls get_page_from_freelist and loops around GFP_NOFAIL allocation
> until it succeeds. It would be better if the first part was done in
> __alloc_pages_slowpath where we modify the zonelist because this would
> be easier to read and understand. And do the retry at the very same
> place because retrying without even attempting to do any reclaim 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 helper and open code it into its only user. 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.
> 
> Signed-off-by: Michal Hocko <mhocko@...e.com>
> ---
> 
> Hi,
> I think that this is more a cleanup than any functional change. We
> are rarely screwed so much that __alloc_pages_high_priority would
> fail. Yet I think that __alloc_pages_high_priority is obscuring the
> overal intention more than it is helpful. Another motivation is to
> reduce wait_iff_congested call to a single one in the allocator. I plan
> to do other changes in that area and get rid of it altogether.

I think it's a combination of a cleanup (the inlining of 
__alloc_pages_high_priority) and a functional change (no longer looping 
infinitely around a get_page_from_freelist() call).  I'd suggest doing the 
inlining in one patch and then the reworking of __GFP_NOFAIL when 
ALLOC_NO_WATERMARKS fails just so we could easily revert the latter if 
necessary.
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