linux-kernel - Re: [PATCH] userfaultfd/hugetlbfs: Fix minor fault page leak

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Date:   Mon, 22 Mar 2021 13:19:50 -0700
From:   Axel Rasmussen <axelrasmussen@...gle.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     Linux MM <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>
Subject: Re: [PATCH] userfaultfd/hugetlbfs: Fix minor fault page leak

On Mon, Mar 22, 2021 at 10:51 AM Peter Xu <peterx@...hat.com> wrote:
>
> When uffd-minor enabled, we need to put the page cache before handling the
> userfault in hugetlb_no_page(), otherwise the page refcount got leaked.
>
> This can be reproduced by running userfaultfd selftest with hugetlb_shared
> mode, then cat /proc/meminfo.
>
> Cc: Axel Rasmussen <axelrasmussen@...gle.com>
> Cc: Andrea Arcangeli <aarcange@...hat.com>
> Cc: Mike Kravetz <mike.kravetz@...cle.com>
> Cc: Mike Rapoport <rppt@...ux.vnet.ibm.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Fixes: f2bf15fb0969 ("userfaultfd: add minor fault registration mode")
> Signed-off-by: Peter Xu <peterx@...hat.com>
> ---
>  mm/hugetlb.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 408dbc08298a..56b78a206913 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -4449,6 +4449,7 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
>                 /* Check for page in userfault range. */
>                 if (userfaultfd_minor(vma)) {
>                         unlock_page(page);
> +                       put_page(page);
>                         ret = hugetlb_handle_userfault(vma, mapping, idx,
>                                                        flags, haddr,
>                                                        VM_UFFD_MINOR);
> --
> 2.26.2
>

Thanks for the fix, Peter! I applied the patch and verified it does
indeed fix the issue.

Reviewed-by: Axel Rasmussen <axelrasmussen@...gle.com>