linux-kernel - Re: [patch 1/2 for-4.20] mm, thp: restore node-local hugepage allocations

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Date:   Tue, 4 Dec 2018 13:56:30 -0800 (PST)
From:   David Rientjes <rientjes@...gle.com>
To:     Michal Hocko <mhocko@...nel.org>
cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>, ying.huang@...el.com,
        s.priebe@...fihost.ag, mgorman@...hsingularity.net,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        alex.williamson@...hat.com, lkp@...org, kirill@...temov.name,
        Andrew Morton <akpm@...ux-foundation.org>,
        zi.yan@...rutgers.edu, Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [patch 1/2 for-4.20] mm, thp: restore node-local hugepage
 allocations

On Tue, 4 Dec 2018, Michal Hocko wrote:

> > This is a full revert of ac5b2c18911f ("mm: thp: relax __GFP_THISNODE for
> > MADV_HUGEPAGE mappings") and a partial revert of 89c83fb539f9 ("mm, thp:
> > consolidate THP gfp handling into alloc_hugepage_direct_gfpmask").
> > 
> > By not setting __GFP_THISNODE, applications can allocate remote hugepages
> > when the local node is fragmented or low on memory when either the thp
> > defrag setting is "always" or the vma has been madvised with
> > MADV_HUGEPAGE.
> > 
> > Remote access to hugepages often has much higher latency than local pages
> > of the native page size.  On Haswell, ac5b2c18911f was shown to have a
> > 13.9% access regression after this commit for binaries that remap their
> > text segment to be backed by transparent hugepages.
> > 
> > The intent of ac5b2c18911f is to address an issue where a local node is
> > low on memory or fragmented such that a hugepage cannot be allocated.  In
> > every scenario where this was described as a fix, there is abundant and
> > unfragmented remote memory available to allocate from, even with a greater
> > access latency.
> > 
> > If remote memory is also low or fragmented, not setting __GFP_THISNODE was
> > also measured on Haswell to have a 40% regression in allocation latency.
> > 
> > Restore __GFP_THISNODE for thp allocations.
> > 
> > Fixes: ac5b2c18911f ("mm: thp: relax __GFP_THISNODE for MADV_HUGEPAGE mappings")
> > Fixes: 89c83fb539f9 ("mm, thp: consolidate THP gfp handling into alloc_hugepage_direct_gfpmask")
> 
> At minimum do not remove the cleanup part which consolidates the gfp
> hadnling to a single place. There is no real reason to have the
> __GFP_THISNODE ugliness outside of alloc_hugepage_direct_gfpmask.
> 

The __GFP_THISNODE usage is still confined to 
alloc_hugepage_direct_gfpmask() for the thp fault path, we no longer set 
it in alloc_pages_vma() as done before the cleanup.