Re: [mm-unstable PATCH v4 1/9] mm/hugetlb: check gigantic_page_runtime_supported() in return_unused_surplus_pages()

["HORIGUCHI NAOYA(堀口 直也)" <naoya.horiguchi@nec.com>]

On Wed, Jul 06, 2022 at 02:51:00PM -0700, Mike Kravetz wrote:
> On 07/04/22 10:33, Naoya Horiguchi wrote:
> > From: Naoya Horiguchi <naoya.horiguchi@nec.com>
> > 
> > I found a weird state of 1GB hugepage pool, caused by the following
> > procedure:
> > 
> >   - run a process reserving all free 1GB hugepages,
> >   - shrink free 1GB hugepage pool to zero (i.e. writing 0 to
> >     /sys/kernel/mm/hugepages/hugepages-1048576kB/nr_hugepages), then
> >   - kill the reserving process.
> > 
> > , then all the hugepages are free *and* surplus at the same time.
> > 
> >   $ cat /sys/kernel/mm/hugepages/hugepages-1048576kB/nr_hugepages
> >   3
> >   $ cat /sys/kernel/mm/hugepages/hugepages-1048576kB/free_hugepages
> >   3
> >   $ cat /sys/kernel/mm/hugepages/hugepages-1048576kB/resv_hugepages
> >   0
> >   $ cat /sys/kernel/mm/hugepages/hugepages-1048576kB/surplus_hugepages
> >   3
> > 
> > This state is resolved by reserving and allocating the pages then
> > freeing them again, so this seems not to result in serious problem.
> > But it's a little surprising (shrinking pool suddenly fails).
> > 
> > This behavior is caused by hstate_is_gigantic() check in
> > return_unused_surplus_pages(). This was introduced so long ago in 2008
> > by commit aa888a74977a ("hugetlb: support larger than MAX_ORDER"), and
> > at that time the gigantic pages were not supposed to be allocated/freed
> > at run-time.  Now kernel can support runtime allocation/free, so let's
> > check gigantic_page_runtime_supported() together.
> > 
> > Signed-off-by: Naoya Horiguchi <naoya.horiguchi@nec.com>
> > ---
> > v2 -> v3:
> > - Fixed typo in patch description,
> > - add !gigantic_page_runtime_supported() check instead of removing
> >   hstate_is_gigantic() check (suggested by Miaohe and Muchun)
> > - add a few more !gigantic_page_runtime_supported() check in
> >   set_max_huge_pages() (by Mike).
> 
> Hi Naoya,
> 
> My apologies for suggesting the above checks in set_max_huge_pages().
> set_max_huge_pages is only called from __nr_hugepages_store_common.
> At the very beginning of __nr_hugepages_store_common is this:
> 
> 	if (hstate_is_gigantic(h) && !gigantic_page_runtime_supported())
> 		return -EINVAL;
> 
> So, those extra checks in set_max_huge_pages are unnecessary.  Sorry!

OK, so I'll drop both checks, thank you.

- Naoya Horiguchi