Messages in this thread | | | Date | Thu, 26 Feb 2015 00:55:40 +0100 | From | Vlastimil Babka <> | Subject | Re: [patch v2 for-4.0] mm, thp: really limit transparent hugepage allocation to local node |
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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.
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