Message-ID: <87k2x6q6n0.fsf@linux.vnet.ibm.com>
Date:	Tue, 21 Apr 2015 13:01:47 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	Vlastimil Babka <vbabka@...e.cz>,
	David Rientjes <rientjes@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Greg Thelen <gthelen@...gle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [patch v2 for-4.0] mm, thp: really limit transparent hugepage allocation to local node

Vlastimil Babka <vbabka@...e.cz> writes:

> On 25.2.2015 22:24, David Rientjes wrote:
>>
>>> alloc_pages_preferred_node() variant, change the exact_node() variant to pass
>>> __GFP_THISNODE, and audit and adjust all callers accordingly.
>>>
>> Sounds like that should be done as part of a cleanup after the 4.0 issues
>> are addressed.  alloc_pages_exact_node() does seem to suggest that we want
>> exactly that node, implying __GFP_THISNODE behavior already, so it would
>> be good to avoid having this come up again in the future.
>
> Oh lovely, just found out that there's alloc_pages_node which should be the
> preferred-only version, but in fact does not differ from 
> alloc_pages_exact_node
> in any relevant way. I agree we should do some larger cleanup for next 
> version.
>
>>> Also, you pass __GFP_NOWARN but that should be covered by GFP_TRANSHUGE
>>> already. Of course, nothing guarantees that hugepage == true implies that gfp
>>> == GFP_TRANSHUGE... but current in-tree callers conform to that.
>>>
>> Ah, good point, and it includes __GFP_NORETRY as well which means that
>> this patch is busted.  It won't try compaction or direct reclaim in the
>> page allocator slowpath because of this:
>>
>> 	/*
>> 	 * GFP_THISNODE (meaning __GFP_THISNODE, __GFP_NORETRY and
>> 	 * __GFP_NOWARN set) should not cause reclaim since the subsystem
>> 	 * (f.e. slab) using GFP_THISNODE may choose to trigger reclaim
>> 	 * using a larger set of nodes after it has established that the
>> 	 * allowed per node queues are empty and that nodes are
>> 	 * over allocated.
>> 	 */
>> 	if (IS_ENABLED(CONFIG_NUMA) &&
>> 	    (gfp_mask & GFP_THISNODE) == GFP_THISNODE)
>> 		goto nopage;
>>
>> Hmm.  It would be disappointing to have to pass the nodemask of the exact
>> node that we want to allocate from into the page allocator to avoid using
>> __GFP_THISNODE.
>
> Yeah.
>
>>
>> There's a sneaky way around it by just removing __GFP_NORETRY from
>> GFP_TRANSHUGE so the condition above fails and since the page allocator
>> won't retry for such a high-order allocation, but that probably just
>> papers over this stuff too much already.  I think what we want to do is
>
> Alternatively alloc_pages_exact_node() adds __GFP_THISNODE just to
> node_zonelist() call and not to __alloc_pages() gfp_mask proper? Unless 
> __GFP_THISNODE
> was given *also* in the incoming gfp_mask, this should give us the right 
> combination?
> But it's also subtle....
>
>> cause the slab allocators to not use __GFP_WAIT if they want to avoid
>> reclaim.
>
> Yes, the fewer subtle heuristics we have that include combinations of 
> flags (*cough*
> GFP_TRANSHUGE *cough*), the better.
>
>> This is probably going to be a much more invasive patch than originally
>> thought.
>
> Right, we might be changing behavior not just for slab allocators, but 
> also others using such
> combination of flags.

Any update on this ? Did we reach a conclusion on how to go forward here
?

-aneesh

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