linux-kernel - Re: [PATCH 4/6] mm: hugetlb: add return -EAGAIN for dissolve_free_huge

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Date:   Mon, 4 Jan 2021 17:32:54 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Muchun Song <songmuchun@...edance.com>, akpm@...ux-foundation.org
Cc:     hillf.zj@...baba-inc.com, n-horiguchi@...jp.nec.com,
        ak@...ux.intel.com, yongjun_wei@...ndmicro.com.cn, mhocko@...e.cz,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/6] mm: hugetlb: add return -EAGAIN for
 dissolve_free_huge_page

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@...edance.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;
>  	}
>