linux-kernel - Re: [PATCH v7 3/7] mm,hugetlb: Clear HPageFreed outside of the lock

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Message-ID: <YHWbPjgPpsLoqGvL@dhcp22.suse.cz>
Date:   Tue, 13 Apr 2021 15:23:10 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Oscar Salvador <osalvador@...e.de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        David Hildenbrand <david@...hat.com>,
        Muchun Song <songmuchun@...edance.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 3/7] mm,hugetlb: Clear HPageFreed outside of the lock

On Tue 13-04-21 12:47:43, Oscar Salvador wrote:
> Currently, the clearing of the flag is done under the lock, but this
> is unnecessary as we just allocated the page and we did not give it
> away yet, so no one should be messing with it.
> 
> Also, this helps making clear that here the lock is only protecting the
> counter.

While moving the flag clearing is ok I am wondering why do we need that
in the first place. I think it is just a leftover from 6c0371490140
("hugetlb: convert PageHugeFreed to HPageFreed flag"). Prior to that a tail
page as been used to keep track of the state but now all happens in the
head page and the flag uses page->private which is always initialized
when allocated by the allocator (post_alloc_hook).

Or do we need it for giga pages which are not allocated by the page
allocator? If yes then moving it to prep_compound_gigantic_page would be
better.

So should we just drop it here?

> Signed-off-by: Oscar Salvador <osalvador@...e.de>
> ---
>  mm/hugetlb.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 54d81d5947ed..e40d5fe5c63c 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1490,10 +1490,10 @@ static void prep_new_huge_page(struct hstate *h, struct page *page, int nid)
>  	hugetlb_set_page_subpool(page, NULL);
>  	set_hugetlb_cgroup(page, NULL);
>  	set_hugetlb_cgroup_rsvd(page, NULL);
> +	ClearHPageFreed(page);
>  	spin_lock_irq(&hugetlb_lock);
>  	h->nr_huge_pages++;
>  	h->nr_huge_pages_node[nid]++;
> -	ClearHPageFreed(page);
>  	spin_unlock_irq(&hugetlb_lock);
>  }
>  
> -- 
> 2.16.3

-- 
Michal Hocko
SUSE Labs