linux-kernel - Re: [PATCH v4 06/11] mm/migrate: make a standard migration target allocation function

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Date:   Thu, 9 Jul 2020 12:28:03 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Joonsoo Kim <js1304@...il.com>
Cc:     Vlastimil Babka <vbabka@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>, kernel-team@....com,
        Christoph Hellwig <hch@...radead.org>,
        Roman Gushchin <guro@...com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [PATCH v4 06/11] mm/migrate: make a standard migration target
 allocation function

On Thu 09-07-20 16:15:07, Joonsoo Kim wrote:
> 2020년 7월 8일 (수) 오전 4:00, Michal Hocko <mhocko@...nel.org>님이 작성:
> >
> > On Tue 07-07-20 16:49:51, Vlastimil Babka wrote:
> > > On 7/7/20 9:44 AM, js1304@...il.com wrote:
> > > > From: Joonsoo Kim <iamjoonsoo.kim@....com>
> > > >
> > > > There are some similar functions for migration target allocation.  Since
> > > > there is no fundamental difference, it's better to keep just one rather
> > > > than keeping all variants.  This patch implements base migration target
> > > > allocation function.  In the following patches, variants will be converted
> > > > to use this function.
> > > >
> > > > Changes should be mechanical but there are some differences. First, Some
> > > > callers' nodemask is assgined to NULL since NULL nodemask will be
> > > > considered as all available nodes, that is, &node_states[N_MEMORY].
> > > > Second, for hugetlb page allocation, gfp_mask is ORed since a user could
> > > > provide a gfp_mask from now on.
> > >
> > > I think that's wrong. See how htlb_alloc_mask() determines between
> > > GFP_HIGHUSER_MOVABLE and GFP_HIGHUSER, but then you OR it with __GFP_MOVABLE so
> > > it's always GFP_HIGHUSER_MOVABLE.
> 
> Indeed.
> 
> > Right you are! Not that it would make any real difference because only
> > migrateable hugetlb pages will get __GFP_MOVABLE and so we shouldn't
> > really end up here for !movable pages in the first place (not sure about
> > soft offlining at this moment). But yeah it would be simply better to
> > override gfp mask for hugetlb which we have been doing anyway.
> 
> Override gfp mask doesn't work since some users will call this function with
> __GFP_THISNODE.

> I will use hugepage_movable_supported() here and
> clear __GFP_MOVABLE if needed.

hugepage_movable_supported is really an implementation detail, do not
use it here. I think it would be better to add

gfp_t htlb_modify_alloc_mask(struct hstate *h, gfp_t mask)
{
	gfp_t default_mask = htlb_alloc_mask(h);

	/* Some callers might want to enforce node */
	return default_mask | (mask & __GFP_THISNODE);
}

If we need to special case others, eg reclaim restrictions there would
be a single place to do so.
-- 
Michal Hocko
SUSE Labs