linux-kernel - Re: [patch v2 for-4.20] mm, thp: restore node-local hugepage allocations

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Date:   Thu, 6 Dec 2018 13:42:05 -0800 (PST)
From:   David Rientjes <rientjes@...gle.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
cc:     Andrea Arcangeli <aarcange@...hat.com>,
        mgorman@...hsingularity.net, Vlastimil Babka <vbabka@...e.cz>,
        mhocko@...nel.org, ying.huang@...el.com, s.priebe@...fihost.ag,
        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
Subject: Re: [patch v2 for-4.20] mm, thp: restore node-local hugepage
 allocations

On Wed, 5 Dec 2018, David Rientjes 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")
> Signed-off-by: David Rientjes <rientjes@...gle.com>

We've identified a couple more regressions wrt 89c83fb539f9 ("mm, thp: 
consolidate THP gfp handling into alloc_hugepage_direct_gfpmask") in 
automated testing so I'm going to be proposing a full revert of that 
commit for 4.20 as a follow-up to this.