Re: [PATCH 4/6] mm: hugetlb: add return -EAGAIN for dissolve_free_huge_page

Mike Kravetz
2021-01-05T01:33:11Z

On 1/3/21 10:58 PM, Muchun Song wrote:
> When dissolve_free_huge_page() races with __free_huge_page(), we can
> do a retry. Because the race window is small.

In general, I agree that the race window is small. However, worst case
would be if the freeing of the page is put on a work queue. Is it acceptable
to keep retrying in that case? In addition, the 'Free some vmemmap' series
may slow the free_huge_page path even more.

In these worst case scenarios, I am not sure we want to just spin retrying.

--
Mike Kravetz

>
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> ---
> mm/hugetlb.c | 16 +++++++++++-----
> 1 file changed, 11 insertions(+), 5 deletions(-)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 72608008f8b4..db00ae375d2a 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1763,10 +1763,11 @@ static int free_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed,
> * nothing for in-use hugepages and non-hugepages.
> * This function returns values like below:
> *
> - * -EBUSY: failed to dissolved free hugepages or the hugepage is in-use
> - * (allocated or reserved.)
> - * 0: successfully dissolved free hugepages or the page is not a
> - * hugepage (considered as already dissolved)
> + * -EAGAIN: race with __free_huge_page() and can do a retry
> + * -EBUSY: failed to dissolved free hugepages or the hugepage is in-use
> + * (allocated or reserved.)
> + * 0: successfully dissolved free hugepages or the page is not a
> + * hugepage (considered as already dissolved)
> */
> int dissolve_free_huge_page(struct page *page)
> {
> @@ -1815,8 +1816,10 @@ int dissolve_free_huge_page(struct page *page)
> * We should make sure that the page is already on the free list
> * when it is dissolved.
> */
> - if (unlikely(!PageHugeFreed(head)))
> + if (unlikely(!PageHugeFreed(head))) {
> + rc = -EAGAIN;
> goto out;
> + }
>
> /*
> * Move PageHWPoison flag from head page to the raw error page,
> @@ -1857,7 +1860,10 @@ int dissolve_free_huge_pages(unsigned long start_pfn, unsigned long end_pfn)
>
> for (pfn = start_pfn; pfn < end_pfn; pfn += 1 << minimum_order) {
> page = pfn_to_page(pfn);
> +retry:
> rc = dissolve_free_huge_page(page);
> + if (rc == -EAGAIN)
> + goto retry;
> if (rc)
> break;
> }
>