On Wed 13-01-21 13:22:07, Muchun Song wrote:
> There is a race between dissolve_free_huge_page() and put_page().
> Theoretically, we should return -EBUSY when we encounter this race.
> In fact, we have a chance to successfully dissolve the page if we
> do a retry. Because the race window is quite small. If we seize
> this opportunity, it is an optimization for increasing the success
> rate of dissolving page.
>
> Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> ---
> mm/hugetlb.c | 20 ++++++++++++++++++--
> 1 file changed, 18 insertions(+), 2 deletions(-)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 4a9011e12175..898e4ea43e13 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1772,6 +1772,7 @@ int dissolve_free_huge_page(struct page *page)
> {
> int rc = -EBUSY;
>
> +retry:
> /* Not to disrupt normal path by vainly holding hugetlb_lock */
> if (!PageHuge(page))
> return 0;
> @@ -1793,8 +1794,23 @@ 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)))
> - goto out;
> + if (unlikely(!PageHugeFreed(head))) {
> + spin_unlock(&hugetlb_lock);
> +
> + /*
> + * Theoretically, we should return -EBUSY when we
> + * encounter this race. In fact, we have a chance
> + * to successfully dissolve the page if we do a
> + * retry. Because the race window is quite small.
> + * If we seize this opportunity, it is an optimization
> + * for increasing the success rate of dissolving page.
> + */
> + while (PageHeadHuge(head) && !PageHugeFreed(head)) {
> + cond_resched();
> + cpu_relax();
> + }
> + goto retry;
OK, so you have done the retry here. Please fold it into the previous
patch. Also do we need cpu_relax on top of cond_resched as well?
> + }
>
> /*
> * Move PageHWPoison flag from head page to the raw error page,
> --
> 2.11.0
--
Michal Hocko
SUSE Labs
