linux-kernel - Re: [PATCH] mm/hugetlb: avoid corrupting page->mapping in hugetlb_mcopy_atomic

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Date:   Tue, 12 Jul 2022 10:39:20 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     akpm@...ux-foundation.org, songmuchun@...edance.com,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Peter Xu <peterx@...hat.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/hugetlb: avoid corrupting page->mapping in
 hugetlb_mcopy_atomic_pte

On 07/12/22 21:05, Miaohe Lin wrote:
> In MCOPY_ATOMIC_CONTINUE case with a non-shared VMA, pages in the page
> cache are installed in the ptes. But hugepage_add_new_anon_rmap is called
> for them mistakenly because they're not vm_shared. This will corrupt the
> page->mapping used by page cache code.
> 
> Fixes: f619147104c8 ("userfaultfd: add UFFDIO_CONTINUE ioctl")
> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> ---
>  mm/hugetlb.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

This looks correct to me.

Reviewed-by: Mike Kravetz <mike.kravetz@...cle.com>

However, I am having a hard time wrapping my head around how UFFDIO_CONTINUE
should work on non-anon private mappings.  For example, a private mapping of
a hugetlbfs file.  I think we just map the page in the file/cache and do not
set the write bit in the pte.  So, yes we would want page_dup_file_rmap()
in this case as shown below.

Adding Axel and Peter on Cc: as they were more involved in adding that code
and the design of UFFDIO_CONTINUE.
-- 
Mike Kravetz

> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 8d379e03f672..b232e1508e49 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -6038,7 +6038,7 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
>  	if (!huge_pte_none_mostly(huge_ptep_get(dst_pte)))
>  		goto out_release_unlock;
>  
> -	if (vm_shared) {
> +	if (page_in_pagecache) {
>  		page_dup_file_rmap(page, true);
>  	} else {
>  		ClearHPageRestoreReserve(page);
> -- 
> 2.23.0
>