linux-kernel - Re: [PATCH] userfaultfd: change src_folio after ensuring it's unpinned in UFFDIO

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Date: Thu, 4 Apr 2024 22:17:08 +0200
From: David Hildenbrand <david@...hat.com>
To: Lokesh Gidra <lokeshgidra@...gle.com>, akpm@...ux-foundation.org
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, surenb@...gle.com,
 kernel-team@...roid.com, aarcange@...hat.com, peterx@...hat.com,
 zhengqi.arch@...edance.com, kaleshsingh@...gle.com, ngeoffray@...gle.com
Subject: Re: [PATCH] userfaultfd: change src_folio after ensuring it's
 unpinned in UFFDIO_MOVE

On 04.04.24 19:17, Lokesh Gidra wrote:
> Commit d7a08838ab74 ("mm: userfaultfd: fix unexpected change to src_folio
> when UFFDIO_MOVE fails") moved the src_folio->{mapping, index} changing
> to after clearing the page-table and ensuring that it's not pinned. This
> avoids failure of swapout+migration and possibly memory corruption.
> 
> However, the commit missed fixing it in the huge-page case.
> 
> Fixes: adef440691ba ("userfaultfd: UFFDIO_MOVE uABI")
> Signed-off-by: Lokesh Gidra <lokeshgidra@...gle.com>
> ---
>   mm/huge_memory.c | 6 +++---
>   1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 9859aa4f7553..89f58c7603b2 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2259,9 +2259,6 @@ int move_pages_huge_pmd(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd, pm
>   			goto unlock_ptls;
>   		}
>   
> -		folio_move_anon_rmap(src_folio, dst_vma);
> -		WRITE_ONCE(src_folio->index, linear_page_index(dst_vma, dst_addr));
> -
>   		src_pmdval = pmdp_huge_clear_flush(src_vma, src_addr, src_pmd);
>   		/* Folio got pinned from under us. Put it back and fail the move. */
>   		if (folio_maybe_dma_pinned(src_folio)) {
> @@ -2270,6 +2267,9 @@ int move_pages_huge_pmd(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd, pm
>   			goto unlock_ptls;
>   		}
>   
> +		folio_move_anon_rmap(src_folio, dst_vma);
> +		WRITE_ONCE(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 */
>   		_dst_pmd = pmd_mkwrite(pmd_mkdirty(_dst_pmd), dst_vma);

Ouch

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

-- 
Cheers,

David / dhildenb