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SubjectRe: [PATCH v2 1/9] mm/hugetlb: remove checking hstate_is_gigantic() in return_unused_surplus_pages()
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>
> >
> > 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 surprizing (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
> > it seems to me that this check is no longer unnecessary. Let's remove it.
>
> s/unnecessary/necessary/
>
> >
> > Signed-off-by: Naoya Horiguchi <naoya.horiguchi@nec.com>
> > ---
> > mm/hugetlb.c | 4 ----
> > 1 file changed, 4 deletions(-)
> >
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index a57e1be41401..c538278170a2 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -2432,10 +2432,6 @@ static void return_unused_surplus_pages(struct hstate *h,
> > /* Uncommit the reservation */
> > h->resv_huge_pages -= unused_resv_pages;
> >
> > - /* Cannot return gigantic pages currently */
> > - if (hstate_is_gigantic(h))
> > - goto out;
> > -
>
> 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.

Thanks.

> But I might be miss something.
>
> Thanks.
>
> > /*
> > * Part (or even all) of the reservation could have been backed
> > * by pre-allocated pages. Only free surplus pages.
> >
>
>

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