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SubjectRe: [PATCH v2 1/9] mm/hugetlb: remove checking hstate_is_gigantic() in return_unused_surplus_pages()
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On Tue, Jun 28, 2022 at 08:38:08AM +0000, HORIGUCHI NAOYA(堀口 直也) wrote:
> On Mon, Jun 27, 2022 at 10:25:13AM -0700, Mike Kravetz wrote:
> > On 06/27/22 06:02, HORIGUCHI NAOYA(堀口 直也) wrote:
> > > On Fri, Jun 24, 2022 at 12:11:07PM -0700, Mike Kravetz wrote:
> > > > On 06/24/22 08:34, HORIGUCHI NAOYA(堀口 直也) wrote:
> > > > > On Fri, Jun 24, 2022 at 04:15:19PM +0800, Miaohe Lin wrote:
> > > > > > On 2022/6/24 16:03, Muchun Song wrote:
> > > > > > > On Fri, Jun 24, 2022 at 10:25:48AM +0800, Miaohe Lin wrote:
> > > > > > >> On 2022/6/24 7:51, Naoya Horiguchi wrote:
> > > > > > >>> From: Naoya Horiguchi <naoya.horiguchi@nec.com>
> > > > > > >>
> > > > > > >> IIUC it might be better to do the below check:
> > > > > > >> /*
> > > > > > >> * Cannot return gigantic pages currently if runtime gigantic page
> > > > > > >> * allocation is not supported.
> > > > > > >> */
> > > > > > >> if (hstate_is_gigantic(h) && !gigantic_page_runtime_supported())
> > > > > > >> goto out;
> > > > > > >>
> > > > > > >
> > > > > > > The change looks good to me. However, the comments above is unnecessary
> > > > > > > since gigantic_page_runtime_supported() is straightforward.
> > > > > >
> > > > > > Agree. The comments can be removed.
> > > > >
> > > > > Thank you, both. Adding !gigantic_page_runtime_supported without comment
> > > > > makes sense to me.
> > > >
> > > > The change above makes sense to me. However, ...
> > > >
> > > > If we make the change above, will we have the same strange situation described
> > > > in the commit message when !gigantic_page_runtime_supported() is true?
> > > >
> > > > IIUC, !gigantic_page_runtime_supported implies that gigantic hugetlb
> > > > pages can not be allocated or freed at run time. They can only be
> > > > allocated at boot time. So, there should NEVER be surplus gigantic
> > > > pages if !gigantic_page_runtime_supported().
> > >
> > > I have the same understanding as the above.
> > >
> > > > To avoid this situation,
> > > > perhaps we should change set_max_huge_pages as follows (not tested)?
> > >
> > > The suggested diff looks clearer about what it does, so I'd like to take it
> > > in the next version. Then, what do we do on the "if (hstate_if_gigantic())"
> > > check in return_unused_surplus_pages in the original suggestion? Should it
> > > be kept as is, or removed, or checked with !gigantic_page_runtime_supported()?
> > > I guess that the new checks prevent calling return_unused_surplus_pages()
> > > during pool shrinking, so the check seems not necessary any more.
> >
> > My first thought was to keep the check in return_unused_surplus_pages() as it
> > is called in other code paths. However, it SHOULD only try to free surplus
> > hugetlb pages. With the modifications to set_max_huge_pages we will not
> > have any surplus gigantic pages if !gigantic_page_runtime_supported, so
> > the check can be removed.
> >
> > Also note that we never try to dynamically allocate surplus gigantic pages.
> > This also is left over from the time when we could not easily allocate a
> > gigantic page at runtime. It would not surprise me if someone found a use
> > case to ease this restriction in the future. Especially so if 1G THP support
> > is ever added. If this happens, the check would be necessary and I would
> > guess that it would not be added.
> >
> > Sorry for thinking our loud!!! Although not necessary, it 'might' be a good
> > idea to leave the check because it would be overlooked if dynamic allocation
> > of gigantic surplus pages is ever added. I do not have a strong opinion.
>
> OK, so let's keep the check.

Sorry, I found that keeping the check doesn't fix the problem.
I'll update the check with !gigantic_page_runtime_supported().

- Naoya Horiguchi
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