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@xxxxxxxxxx>
Thanks!
--
Michal Hocko
SUSE Labs
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