linux-kernel - Re: [RFC 2/8] mm/hugetlb: remove useless set_page

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Date:   Tue, 26 Oct 2021 11:44:17 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Pasha Tatashin <pasha.tatashin@...een.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-m68k@...ts.linux-m68k.org, anshuman.khandual@....com,
        willy@...radead.org, akpm@...ux-foundation.org,
        william.kucharski@...cle.com, vbabka@...e.cz, geert@...ux-m68k.org,
        schmitzmic@...il.com, rostedt@...dmis.org, mingo@...hat.com,
        hannes@...xchg.org, guro@...com, songmuchun@...edance.com,
        weixugc@...gle.com, gthelen@...gle.com
Subject: Re: [RFC 2/8] mm/hugetlb: remove useless set_page_count()

On 10/26/21 10:38 AM, Pasha Tatashin wrote:
> prep_compound_gigantic_page() calls set_page_count(0, p), but it is not
> needed because page_ref_freeze(p, 1) already sets refcount to 0.
> 
> Using, set_page_count() is dangerous, because it unconditionally resets
> refcount from the current value to unrestrained value, and therefore
> should be minimized.
> 
> Signed-off-by: Pasha Tatashin <pasha.tatashin@...een.com>

Thanks!

My bad for not removing the set_page_count when adding the page_ref_freeze.

FYI, there have been additional changes to this routine in Andrew's
tree.  Not really sure if we want/need the VM_BUG_ON_PAGE as that would
only check if there was a 'bug' in page_ref_freeze.

-- 
Mike Kravetz

> ---
>  mm/hugetlb.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 95dc7b83381f..7e3996c8b696 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1707,7 +1707,7 @@ static bool prep_compound_gigantic_page(struct page *page, unsigned int order)
>  			pr_warn("HugeTLB page can not be used due to unexpected inflated ref count\n");
>  			goto out_error;
>  		}
> -		set_page_count(p, 0);
> +		VM_BUG_ON_PAGE(page_count(p), p);
>  		set_compound_head(p, page);
>  	}
>  	atomic_set(compound_mapcount_ptr(page), -1);
>