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SubjectRe: [PATCH -mm] mm: Fix race between swapoff and mincore
On Tue 13-03-18 09:20:36, Huang, Ying wrote:
> From: Huang Ying <ying.huang@intel.com>
>
> >From commit 4b3ef9daa4fc ("mm/swap: split swap cache into 64MB
> trunks") on, after swapoff, the address_space associated with the swap
> device will be freed. So swap_address_space() users which touch the
> address_space need some kind of mechanism to prevent the address_space
> from being freed during accessing.
>
> When mincore process unmapped range for swapped shmem pages, it
> doesn't hold the lock to prevent swap device from being swapoff. So
> the following race is possible,
>
> CPU1 CPU2
> do_mincore() swapoff()
> walk_page_range()
> mincore_unmapped_range()
> __mincore_unmapped_range
> mincore_page
> as = swap_address_space()
> ... exit_swap_address_space()
> ... kvfree(spaces)
> find_get_page(as)
>
> The address space may be accessed after being freed.
>
> To fix the race, get_swap_device()/put_swap_device() is used to
> enclose find_get_page() to check whether the swap entry is valid and
> prevent the swap device from being swapoff during accessing.
>
> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>

Fixes: 4b3ef9daa4fc ("mm/swap: split swap cache into 64MB trunks")

> Cc: Minchan Kim <minchan@kernel.org>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Dave Hansen <dave.hansen@intel.com>
> Cc: Hugh Dickins <hughd@google.com>

Acked-by: Michal Hocko <mhocko@suse.com>

> ---
> mm/mincore.c | 12 ++++++++++--
> 1 file changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/mm/mincore.c b/mm/mincore.c
> index fc37afe226e6..a66f2052c7b1 100644
> --- a/mm/mincore.c
> +++ b/mm/mincore.c
> @@ -68,8 +68,16 @@ static unsigned char mincore_page(struct address_space *mapping, pgoff_t pgoff)
> */
> if (radix_tree_exceptional_entry(page)) {
> swp_entry_t swp = radix_to_swp_entry(page);
> - page = find_get_page(swap_address_space(swp),
> - swp_offset(swp));
> + struct swap_info_struct *si;
> +
> + /* Prevent swap device to being swapoff under us */
> + si = get_swap_device(swp);
> + if (si) {
> + page = find_get_page(swap_address_space(swp),
> + swp_offset(swp));
> + put_swap_device(si);
> + } else
> + page = NULL;
> }
> } else
> page = find_get_page(mapping, pgoff);
> --
> 2.15.1
>

--
Michal Hocko
SUSE Labs

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