linux-kernel - Re: [RFC PATCH 3/5] mm, hugetlb: do not rely on overcommit limit during migration

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Date:   Thu, 14 Dec 2017 08:40:53 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     linux-mm@...ck.org, Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 3/5] mm, hugetlb: do not rely on overcommit limit
 during migration

On Wed 13-12-17 15:35:33, Mike Kravetz wrote:
> On 12/04/2017 06:01 AM, Michal Hocko wrote:
[...]
> > Before migration
> > /sys/devices/system/node/node0/hugepages/hugepages-2048kB/free_hugepages:0
> > /sys/devices/system/node/node0/hugepages/hugepages-2048kB/nr_hugepages:1
> > /sys/devices/system/node/node0/hugepages/hugepages-2048kB/surplus_hugepages:0
> > /sys/devices/system/node/node1/hugepages/hugepages-2048kB/free_hugepages:0
> > /sys/devices/system/node/node1/hugepages/hugepages-2048kB/nr_hugepages:0
> > /sys/devices/system/node/node1/hugepages/hugepages-2048kB/surplus_hugepages:0
> > 
> > After
> > 
> > /sys/devices/system/node/node0/hugepages/hugepages-2048kB/free_hugepages:0
> > /sys/devices/system/node/node0/hugepages/hugepages-2048kB/nr_hugepages:0
> > /sys/devices/system/node/node0/hugepages/hugepages-2048kB/surplus_hugepages:0
> > /sys/devices/system/node/node1/hugepages/hugepages-2048kB/free_hugepages:0
> > /sys/devices/system/node/node1/hugepages/hugepages-2048kB/nr_hugepages:1
> > /sys/devices/system/node/node1/hugepages/hugepages-2048kB/surplus_hugepages:0
> > 
> > with the previous implementation, both nodes would have nr_hugepages:1
> > until the page is freed.
> 
> With the previous implementation, the migration would have failed unless
> nr_overcommit_hugepages was explicitly set.  Correct?

yes

[...]

> In the previous version of this patch, I asked about handling of 'free' huge
> pages.  I did a little digging and IIUC, we do not attempt migration of
> free huge pages.  The routine isolate_huge_page() has this check:
> 
>         if (!page_huge_active(page) || !get_page_unless_zero(page)) {
>                 ret = false;
>                 goto unlock;
>         }
> 
> I believe one of your motivations for this effort was memory offlining.
> So, this implies that a memory area can not be offlined if it contains
> a free (not in use) huge page?

do_migrate_range will ignore this free huge page and then we will free
it up in dissolve_free_huge_pages

> Just FYI and may be something we want to address later.

Maybe yes. The free pool might be reserved which would make
dissolve_free_huge_pages to fail. Maybe we can be more clever and
allocate a new huge page in that case.
 
> My other issues were addressed.
> 
> Reviewed-by: Mike Kravetz <mike.kravetz@...cle.com>

Thanks!
-- 
Michal Hocko
SUSE Labs