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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 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;

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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