linux-kernel - Re: [PATCH v4 4/6] mm: hugetlb: retry dissolve page when hitting race

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Date:   Wed, 13 Jan 2021 10:33:31 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     mike.kravetz@...cle.com, akpm@...ux-foundation.org,
        n-horiguchi@...jp.nec.com, ak@...ux.intel.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 4/6] mm: hugetlb: retry dissolve page when hitting race

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@...edance.com>
> ---
>  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