linux-kernel - Re: [PATCH 1/6] mm/thp: Simplify copying of huge zero page pmd when fork

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Date:   Thu, 29 Apr 2021 10:03:57 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Peter Xu <peterx@...hat.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Hugh Dickins <hughd@...gle.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jerome Glisse <jglisse@...hat.com>
Subject: Re: [PATCH 1/6] mm/thp: Simplify copying of huge zero page pmd when
 fork

On 29.04.21 00:50, Peter Xu wrote:
> Huge zero page is handled in a special path in copy_huge_pmd(), however it
> should share most codes with a normal thp page.  Trying to share more code with
> it by removing the special path.  The only leftover so far is the huge zero
> page refcounting (mm_get_huge_zero_page()), because that's separately done with
> a global counter.
> 
> This prepares for a future patch to modify the huge pmd to be installed, so
> that we don't need to duplicate it explicitly into huge zero page case too.
> 
> Cc: Kirill A. Shutemov <kirill@...temov.name>
> Signed-off-by: Peter Xu <peterx@...hat.com>
> ---
>   mm/huge_memory.c | 9 +++------
>   1 file changed, 3 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 98456017744d6..22bf2d0fff79b 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1076,17 +1076,13 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
>   	 * a page table.
>   	 */
>   	if (is_huge_zero_pmd(pmd)) {
> -		struct page *zero_page;
>   		/*
>   		 * get_huge_zero_page() will never allocate a new page here,
>   		 * since we already have a zero page to copy. It just takes a
>   		 * reference.
>   		 */
> -		zero_page = mm_get_huge_zero_page(dst_mm);
> -		set_huge_zero_page(pgtable, dst_mm, vma, addr, dst_pmd,
> -				zero_page);
> -		ret = 0;
> -		goto out_unlock;
> +		mm_get_huge_zero_page(dst_mm);
> +		goto out_zero_page;
>   	}
>   
>   	src_page = pmd_page(pmd);
> @@ -1110,6 +1106,7 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
>   	get_page(src_page);
>   	page_dup_rmap(src_page, true);
>   	add_mm_counter(dst_mm, MM_ANONPAGES, HPAGE_PMD_NR);
> +out_zero_page:
>   	mm_inc_nr_ptes(dst_mm);
>   	pgtable_trans_huge_deposit(dst_mm, dst_pmd, pgtable);
>   
> 

AFAIKs, the only change is that we're now doing an additional

pmdp_set_wrprotect(src_mm, addr, src_pmd)
pmd = pmd_mkold(pmd_wrprotect(pmd));

But as we are copying the zeropage, it should already be 
write-protected, so no effective change.

LGTM

-- 
Thanks,

David / dhildenb