linux-kernel - Re: [PATCH v5 16/21] mm/hugetlb: Flush work when dissolving hugetlb page

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Date:   Fri, 20 Nov 2020 09:20:44 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     corbet@....net, mike.kravetz@...cle.com, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, x86@...nel.org, hpa@...or.com,
        dave.hansen@...ux.intel.com, luto@...nel.org, peterz@...radead.org,
        viro@...iv.linux.org.uk, akpm@...ux-foundation.org,
        paulmck@...nel.org, mchehab+huawei@...nel.org,
        pawan.kumar.gupta@...ux.intel.com, rdunlap@...radead.org,
        oneukum@...e.com, anshuman.khandual@....com, jroedel@...e.de,
        almasrymina@...gle.com, rientjes@...gle.com, willy@...radead.org,
        osalvador@...e.de, song.bao.hua@...ilicon.com,
        duanxiongchun@...edance.com, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v5 16/21] mm/hugetlb: Flush work when dissolving hugetlb
 page

On Fri 20-11-20 14:43:20, Muchun Song wrote:
> We should flush work when dissolving a hugetlb page to make sure that
> the hugetlb page is freed to the buddy.

Why? This explanation on its own doen't really help to understand what
is the point of the patch.

> 
> Signed-off-by: Muchun Song <songmuchun@...edance.com>
> ---
>  mm/hugetlb.c | 18 +++++++++++++++++-
>  1 file changed, 17 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index b853aacd5c16..9aad0b63d369 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1328,6 +1328,12 @@ static void update_hpage_vmemmap_workfn(struct work_struct *work)
>  }
>  static DECLARE_WORK(hpage_update_work, update_hpage_vmemmap_workfn);
>  
> +static inline void flush_hpage_update_work(struct hstate *h)
> +{
> +	if (free_vmemmap_pages_per_hpage(h))
> +		flush_work(&hpage_update_work);
> +}
> +
>  static inline void __update_and_free_page(struct hstate *h, struct page *page)
>  {
>  	/* No need to allocate vmemmap pages */
> @@ -1928,6 +1934,7 @@ static int free_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed,
>  int dissolve_free_huge_page(struct page *page)
>  {
>  	int rc = -EBUSY;
> +	struct hstate *h = NULL;
>  
>  	/* Not to disrupt normal path by vainly holding hugetlb_lock */
>  	if (!PageHuge(page))
> @@ -1941,8 +1948,9 @@ int dissolve_free_huge_page(struct page *page)
>  
>  	if (!page_count(page)) {
>  		struct page *head = compound_head(page);
> -		struct hstate *h = page_hstate(head);
>  		int nid = page_to_nid(head);
> +
> +		h = page_hstate(head);
>  		if (h->free_huge_pages - h->resv_huge_pages == 0)
>  			goto out;
>  
> @@ -1956,6 +1964,14 @@ int dissolve_free_huge_page(struct page *page)
>  	}
>  out:
>  	spin_unlock(&hugetlb_lock);
> +
> +	/*
> +	 * We should flush work before return to make sure that
> +	 * the HugeTLB page is freed to the buddy.
> +	 */
> +	if (!rc && h)
> +		flush_hpage_update_work(h);
> +
>  	return rc;
>  }
>  
> -- 
> 2.11.0

-- 
Michal Hocko
SUSE Labs