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

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Message-ID: <20181210083409.GJ1286@dhcp22.suse.cz>
Date:   Mon, 10 Dec 2018 09:34:09 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     David Rientjes <rientjes@...gle.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        mgorman@...hsingularity.net, Vlastimil Babka <vbabka@...e.cz>,
        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 07-12-18 15:05:28, David Rientjes wrote:
> On Fri, 7 Dec 2018, Michal Hocko wrote:
[...]
> > > Secondly, prior to 89c83fb539f9, alloc_pages_vma() implemented a somewhat 
> > > different policy for hugepage allocations, which were allocated through 
> > > alloc_hugepage_vma().  For hugepage allocations, if the allocating 
> > > process's node is in the set of allowed nodes, allocate with 
> > > __GFP_THISNODE for that node (for MPOL_PREFERRED, use that node with 
> > > __GFP_THISNODE instead).
> > 
> > Why is it wrong to fallback to an explicitly configured mbind mask?
> > 
> 
> The new_page() case is similar to the shmem_alloc_hugepage() case.  Prior 
> to 89c83fb539f9 ("mm, thp: consolidate THP gfp handling into 
> alloc_hugepage_direct_gfpmask"), shmem_alloc_hugepage() did 
> alloc_pages_vma() with hugepage == true, which effected a different 
> allocation policy: if the node current is running on is allowed by the 
> policy, use __GFP_THISNODE (considering ac5b2c18911ff is reverted, which 
> it is in Linus's tree).
> 
> After 89c83fb539f9, we lose that and can fallback to remote memory.  Since 
> the discussion is on-going wrt the NUMA aspects of hugepage allocations, 
> it's better to have a stable 4.20 tree while that is being worked out and 
> likely deserves separate patches for both new_page() and 
> shmem_alloc_hugepage().  For the latter specifically, I assume it would be 
> nice to get an Acked-by by Kirill who implemented shmem_alloc_hugepage() 
> with hugepage == true back in 4.8 that also had the __GFP_THISNODE 
> behavior before the allocation policy is suddenly changed.

This should be a part of the changelog.

-- 
Michal Hocko
SUSE Labs