[PATCH 2/2] mm: hugetlb: support gigantic surplus pages

[Huang Shijie]

On Mon, Nov 07, 2016 at 04:25:04PM +0100, Gerald Schaefer wrote:
> On Thu, 3 Nov 2016 10:51:38 +0800
> Huang Shijie <shijie.huang at arm.com> wrote:
> 
> > When testing the gigantic page whose order is too large for the buddy
> > allocator, the libhugetlbfs test case "counter.sh" will fail.
> > 
> > The failure is caused by:
> >  1) kernel fails to allocate a gigantic page for the surplus case.
> >     And the gather_surplus_pages() will return NULL in the end.
> > 
> >  2) The condition checks for "over-commit" is wrong.
> > 
> > This patch adds code to allocate the gigantic page in the
> > __alloc_huge_page(). After this patch, gather_surplus_pages()
> > can return a gigantic page for the surplus case.
> > 
> > This patch also changes the condition checks for:
> >      return_unused_surplus_pages()
> >      nr_overcommit_hugepages_store()
> > 
> > After this patch, the counter.sh can pass for the gigantic page.
> > 
> > Acked-by: Steve Capper <steve.capper at arm.com>
> > Signed-off-by: Huang Shijie <shijie.huang at arm.com>
> > ---
> >  mm/hugetlb.c | 15 ++++++++++-----
> >  1 file changed, 10 insertions(+), 5 deletions(-)
> > 
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index 0bf4444..2b67aff 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -1574,7 +1574,7 @@ static struct page *__alloc_huge_page(struct hstate *h,
> >  	struct page *page;
> >  	unsigned int r_nid;
> > 
> > -	if (hstate_is_gigantic(h))
> > +	if (hstate_is_gigantic(h) && !gigantic_page_supported())
> >  		return NULL;
> 
> Is it really possible to stumble over gigantic pages w/o having
> gigantic_page_supported()?
> 
> Also, I've just tried this on s390 and counter.sh still fails after these
> patches, and it should fail on all archs as long as you use the gigantic
I guess the failure you met is caused by the libhugetlbfs itself, there are
several bugs in the libhugetlbfs. I have a patch set for the
libhugetlbfs too. I will send it as soon as possible.

> hugepage size as default hugepage size. This is because you only changed
> nr_overcommit_hugepages_store(), which handles nr_overcommit_hugepages
> in sysfs, and missed hugetlb_overcommit_handler() which handles
> /proc/sys/vm/nr_overcommit_hugepages for the default sized hugepages.
This is wrong. :)

I did have an extra patch to fix the hugetlb_overcommit_handler().
but the counters.sh does not use the /proc/sys/vm/nr_overcommit_hugepages.
Please grep it in the code.

Thanks
Huang Shijie