linux-kernel - Re: gigantic hugepages vs. movable zones

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Message-ID: <20170727081236.GK20970@dhcp22.suse.cz>
Date:   Thu, 27 Jul 2017 10:12:36 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Cc:     Luiz Capitulino <lcapitulino@...hat.com>,
        Mike Kravetz <mike.kravetz@...cle.com>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: gigantic hugepages vs. movable zones

On Thu 27-07-17 13:30:31, Aneesh Kumar K.V wrote:
> 
> 
> On 07/27/2017 12:58 PM, Michal Hocko wrote:
> >On Thu 27-07-17 07:52:08, Aneesh Kumar K.V wrote:
> >>Michal Hocko <mhocko@...nel.org> writes:
> >>
> >>>Hi,
> >>>I've just noticed that alloc_gigantic_page ignores movability of the
> >>>gigantic page and it uses any existing zone. Considering that
> >>>hugepage_migration_supported only supports 2MB and pgd level hugepages
> >>>then 1GB pages are not migratable and as such allocating them from a
> >>>movable zone will break the basic expectation of this zone. Standard
> >>>hugetlb allocations try to avoid that by using htlb_alloc_mask and I
> >>>believe we should do the same for gigantic pages as well.
> >>>
> >>>I suspect this behavior is not intentional. What do you think about the
> >>>following untested patch?
> >>
> >>
> >>I also noticed an unrelated issue with the usage of
> >>start_isolate_page_range. On error we set the migrate type to
> >>MIGRATE_MOVABLE.
> >
> >Why that should be a problem? I think it is perfectly OK to have
> >MIGRATE_MOVABLE pageblocks inside kernel zones.
> >
> 
> we can pick pages with migrate type movable and if we fail to isolate won't
> we set the migrate type of that pages to MOVABLE ?

I do not see an immediate problem. GFP_KERNEL allocations can fallback
to movable migrate pageblocks AFAIR. But I am not very much familiar
with migratetypes. Vlastimil, could you have a look please?
-- 
Michal Hocko
SUSE Labs