linux-kernel - Re: [PATCH 2/2] mm, thp: consolidate THP gfp handling into alloc_hugepage_direct

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Date:   Fri, 19 Oct 2018 10:06:57 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     "Kirill A. Shutemov" <kirill@...temov.name>,
        Mel Gorman <mgorman@...e.de>, Vlastimil Babka <vbabka@...e.cz>,
        David Rientjes <rientjes@...gle.com>,
        Andrea Argangeli <andrea@...nel.org>,
        Zi Yan <zi.yan@...rutgers.edu>,
        Stefan Priebe - Profihost AG <s.priebe@...fihost.ag>,
        linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] mm, thp: consolidate THP gfp handling into
 alloc_hugepage_direct_gfpmask

On Thu 18-10-18 19:11:47, Andrew Morton wrote:
> On Wed, 26 Sep 2018 16:22:27 +0200 Michal Hocko <mhocko@...nel.org> wrote:
> 
> > > MPOL_PREFERRED is handled by policy_node() before we call __alloc_pages_nodemask.
> > > __GFP_THISNODE is applied only when we are not using
> > > __GFP_DIRECT_RECLAIM which is handled in alloc_hugepage_direct_gfpmask
> > > now.
> > > Lastly MPOL_BIND wasn't handled explicitly but in the end the removed
> > > late check would remove __GFP_THISNODE for it as well. So in the end we
> > > are doing the same thing unless I miss something
> > 
> > Forgot to add. One notable exception would be that the previous code
> > would allow to hit
> > 	WARN_ON_ONCE(policy->mode == MPOL_BIND && (gfp & __GFP_THISNODE));
> > in policy_node if the requested node (e.g. cpu local one) was outside of
> > the mbind nodemask. This is not possible now. We haven't heard about any
> > such warning yet so it is unlikely that it happens though.
> 
> Perhaps a changelog addition is needed to cover the above?

: THP allocation mode is quite complex and it depends on the defrag
: mode. This complexity is hidden in alloc_hugepage_direct_gfpmask from a
: large part currently. The NUMA special casing (namely __GFP_THISNODE) is
: however independent and placed in alloc_pages_vma currently. This both
: adds an unnecessary branch to all vma based page allocation requests and
: it makes the code more complex unnecessarily as well. Not to mention
: that e.g. shmem THP used to do the node reclaiming unconditionally
: regardless of the defrag mode until recently. This was not only
: unexpected behavior but it was also hardly a good default behavior and I
: strongly suspect it was just a side effect of the code sharing more than
: a deliberate decision which suggests that such a layering is wrong.
: 
: Moreover the oriinal code allowed to trigger
: 	WARN_ON_ONCE(policy->mode == MPOL_BIND && (gfp & __GFP_THISNODE));
: in policy_node if the requested node (e.g. cpu local one) was outside of
: the mbind nodemask. This is not possible now. We haven't heard about any
: such warning yet so it is unlikely that it happens but still a signal of
: a wrong code layering.
: 
: Get rid of the thp special casing from alloc_pages_vma and move the logic
: to alloc_hugepage_direct_gfpmask. __GFP_THISNODE is applied to
: the resulting gfp mask only when the direct reclaim is not requested and
: when there is no explicit numa binding to preserve the current logic.
: 
: This allows for removing alloc_hugepage_vma as well.

Better?

> I assume that David's mbind() concern has gone away.

Either I've misunderstood it or it was not really a real issue.
-- 
Michal Hocko
SUSE Labs