linux-kernel - Re: [PATCH v2 05/14] mm: khugepaged: __collapse_huge_page_swapin() use pte_offset_map_ro

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Message-ID: <0b7ca0c2-7a64-4d0e-aa18-2fc4257c199e@redhat.com>
Date: Mon, 26 Aug 2024 17:33:23 +0200
From: David Hildenbrand <david@...hat.com>
To: Qi Zheng <zhengqi.arch@...edance.com>, hughd@...gle.com,
 willy@...radead.org, muchun.song@...ux.dev, vbabka@...nel.org,
 akpm@...ux-foundation.org, rppt@...nel.org, vishal.moola@...il.com,
 peterx@...hat.com, ryan.roberts@....com, christophe.leroy2@...soprasteria.com
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 linux-arm-kernel@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2 05/14] mm: khugepaged: __collapse_huge_page_swapin()
 use pte_offset_map_ro_nolock()

On 22.08.24 09:13, Qi Zheng wrote:
> In __collapse_huge_page_swapin(), we just use the ptl for pte_same() check
> in do_swap_page(). In other places, we directly use pte_offset_map_lock(),
> so convert it to using pte_offset_map_ro_nolock().
> 
> Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
> ---
>   mm/khugepaged.c | 6 +++++-
>   1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 4a83c40d90538..53bfa7f4b7f82 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1011,7 +1011,11 @@ static int __collapse_huge_page_swapin(struct mm_struct *mm,
>   		};
>   
>   		if (!pte++) {
> -			pte = pte_offset_map_nolock(mm, pmd, address, &ptl);
> +			/*
> +			 * Here the ptl is only used to check pte_same() in
> +			 * do_swap_page(), so readonly version is enough.
> +			 */
> +			pte = pte_offset_map_ro_nolock(mm, pmd, address, &ptl);
>   			if (!pte) {
>   				mmap_read_unlock(mm);
>   				result = SCAN_PMD_NULL;

Suboptimal that the pteval comparison + unmap is buried in 
do_swap_page(). Moving that to the caller is also not significantly 
better ...

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers,

David / dhildenb