linux-kernel - Re: [patch for-4.20] Revert "mm, thp: consolidate THP gfp handling into alloc_hugepage_direct

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Date:   Fri, 7 Dec 2018 14:27:22 -0800 (PST)
From:   David Rientjes <rientjes@...gle.com>
To:     Vlastimil Babka <vbabka@...e.cz>
cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        mgorman@...hsingularity.net, 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 for-4.20] Revert "mm, thp: consolidate THP gfp handling
 into alloc_hugepage_direct_gfpmask"

On Fri, 7 Dec 2018, Vlastimil Babka wrote:

> > This reverts commit 89c83fb539f95491be80cdd5158e6f0ce329e317.
> > 
> > There are a couple of issues with 89c83fb539f9 independent of its partial 
> > revert in 2f0799a0ffc0 ("mm, thp: restore node-local hugepage 
> > allocations"):
> > 
> > Firstly, the interaction between alloc_hugepage_direct_gfpmask() and 
> > alloc_pages_vma() is racy wrt __GFP_THISNODE and MPOL_BIND.  
> > alloc_hugepage_direct_gfpmask() makes sure not to set __GFP_THISNODE for 
> > an MPOL_BIND policy but the policy used in alloc_pages_vma() may not be 
> > the same for shared vma policies, triggering the WARN_ON_ONCE() in 
> > policy_node().
> 
> AFAICS 2f0799a0ffc0 removed the policy check in
> alloc_hugepage_direct_gfpmask() comlpetely, so it's not racy and the
> warning will always trigger for a MPOL_BIND policy right now?
> 

Yup, looks like you hit it on the head.  This revert should have been done 
alongside 2f0799a0ffc0 ("mm, thp: restore node-local hugepage 
allocations"), I didn't appreciate how invasive the consolidation patch 
was.

I noticed the race in 89c83fb539f9 ("mm, thp: consolidate THP gfp handling 
into alloc_hugepage_direct_gfpmask") that is fixed by the revert, but as 
you noted it didn't cleanup the second part which is the balancing act for 
gfp flags between alloc_hugepage_direct_gfpmask() and alloc_pages_vma().  
Syzbot found this to trigger the WARN_ON_ONCE() you mention.

So we certainly need this patch for 4.20 as a follow-up to 2f0799a0ffc0.  
It's likely better to regroup and discuss NUMA aspects of all thp 
allocations separately with a stable 4.20.