linux-kernel - Re: [PATCH 1/1] userfaultfd: remove WRITE_ONCE when setting folio->index during UFFDIO

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Date: Mon, 15 Apr 2024 09:37:56 +0200
From: David Hildenbrand <david@...hat.com>
To: Suren Baghdasaryan <surenb@...gle.com>, akpm@...ux-foundation.org
Cc: willy@...radead.org, peterx@...hat.com, lokeshgidra@...gle.com,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] userfaultfd: remove WRITE_ONCE when setting
 folio->index during UFFDIO_MOVE

On 15.04.24 04:08, Suren Baghdasaryan wrote:
> When folio is moved with UFFDIO_MOVE it gets locked before the rmap and
> index are modified. Due to the folio lock being already held, WRITE_ONCE()
> is not needed when setting the folio index. Remove it.
> 
> Reported-by: Matthew Wilcox <willy@...radead.org>
> Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> Cc: David Hildenbrand <david@...hat.com>
> Cc: Peter Xu <peterx@...hat.com>
> Cc: Lokesh Gidra <lokeshgidra@...gle.com>
> ---
>   mm/huge_memory.c | 2 +-
>   mm/userfaultfd.c | 2 +-
>   2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 58f2c4745d80..ee12726291f1 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2200,7 +2200,7 @@ int move_pages_huge_pmd(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd, pm
>   		}
>   
>   		folio_move_anon_rmap(src_folio, dst_vma);
> -		WRITE_ONCE(src_folio->index, linear_page_index(dst_vma, dst_addr));
> +		src_folio->index = linear_page_index(dst_vma, dst_addr);
>   
>   		_dst_pmd = mk_huge_pmd(&src_folio->page, dst_vma->vm_page_prot);
>   		/* Follow mremap() behavior and treat the entry dirty after the move */
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index b70618e8dcd2..575ccf90325a 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -1026,7 +1026,7 @@ static int move_present_pte(struct mm_struct *mm,
>   	}
>   
>   	folio_move_anon_rmap(src_folio, dst_vma);
> -	WRITE_ONCE(src_folio->index, linear_page_index(dst_vma, dst_addr));
> +	src_folio->index = linear_page_index(dst_vma, dst_addr);
>   
>   	orig_dst_pte = mk_pte(&src_folio->page, dst_vma->vm_page_prot);
>   	/* Follow mremap() behavior and treat the entry dirty after the move */
> 
> base-commit: 3aec6b2b34e219898883d1e9ea7e911b4d3762a9

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

-- 
Cheers,

David / dhildenb