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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 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>
> >>>
> >>> 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/

Thanks, I fixed it.

> >>
> >>>
> >>> 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.
>
> Agree. The comments can be removed.

Thank you, both. Adding !gigantic_page_runtime_supported without comment
makes sense to me.

Thanks,
Naoya Horiguchi
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