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Message-ID: <20160115143434.GA25332@blaptop.local>
Date: Fri, 15 Jan 2016 23:34:34 +0900
From: Minchan Kim <minchan@...nel.org>
To: Junil Lee <junil0814.lee@....com>
Cc: ngupta@...are.org, sergey.senozhatsky.work@...il.com,
akpm@...ux-foundation.org, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] zsmalloc: fix migrate_zspage-zs_free race condition
On Fri, Jan 15, 2016 at 04:39:11PM +0900, Junil Lee wrote:
> To prevent unlock at the not correct situation, tagging the new obj to
> assure lock in migrate_zspage() before right unlock path.
>
> Two functions are in race condition by tag which set 1 on last bit of
> obj, however unlock succrently when update new obj to handle before call
> unpin_tag() which is right unlock path.
>
> summarize this problem by call flow as below:
>
> CPU0 CPU1
> migrate_zspage
> find_alloced_obj()
> trypin_tag() -- obj |= HANDLE_PIN_BIT
> obj_malloc() -- new obj is not set zs_free
> record_obj() -- unlock and break sync pin_tag() -- get lock
> unpin_tag()
>
> Before code make crash as below:
> Unable to handle kernel NULL pointer dereference at virtual address 00000000
> CPU: 0 PID: 19001 Comm: CookieMonsterCl Tainted:
> PC is at get_zspage_mapping+0x0/0x24
> LR is at obj_free.isra.22+0x64/0x128
> Call trace:
> [<ffffffc0001a3aa8>] get_zspage_mapping+0x0/0x24
> [<ffffffc0001a4918>] zs_free+0x88/0x114
> [<ffffffc00053ae54>] zram_free_page+0x64/0xcc
> [<ffffffc00053af4c>] zram_slot_free_notify+0x90/0x108
> [<ffffffc000196638>] swap_entry_free+0x278/0x294
> [<ffffffc000199008>] free_swap_and_cache+0x38/0x11c
> [<ffffffc0001837ac>] unmap_single_vma+0x480/0x5c8
> [<ffffffc000184350>] unmap_vmas+0x44/0x60
> [<ffffffc00018a53c>] exit_mmap+0x50/0x110
> [<ffffffc00009e408>] mmput+0x58/0xe0
> [<ffffffc0000a2854>] do_exit+0x320/0x8dc
> [<ffffffc0000a3cb4>] do_group_exit+0x44/0xa8
> [<ffffffc0000ae1bc>] get_signal+0x538/0x580
> [<ffffffc000087e44>] do_signal+0x98/0x4b8
> [<ffffffc00008843c>] do_notify_resume+0x14/0x5c
>
> and for test, print obj value after pin_tag() in zs_free().
> Sometimes obj is even number means break synchronization.
>
> After patched, crash is not occurred and obj is only odd number in same
> situation.
If you verified it solved your problem, we should mark this patch
as stable.
>
> Signed-off-by: Junil Lee <junil0814.lee@....com>
Acked-by: Minchan Kim <minchan@...nel.org>
Below comment.
> ---
> mm/zsmalloc.c | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index e7414ce..a24ccb1 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -1635,6 +1635,8 @@ static int migrate_zspage(struct zs_pool *pool, struct size_class *class,
> free_obj = obj_malloc(d_page, class, handle);
> zs_object_copy(free_obj, used_obj, class);
> index++;
> + /* Must not unlock before unpin_tag() */
I want to make comment more clear.
/*
* record_obj updates handle's value to free_obj and it will invalidate
* lock bit(ie, HANDLE_PIN_BIT) of handle, which breaks synchronization
* using pin_tag(e,g, zs_free) so let's keep the lock bit.
*/
Thanks.
> + free_obj |= BIT(HANDLE_PIN_BIT);
> record_obj(handle, free_obj);
> unpin_tag(handle);
> obj_free(pool, class, used_obj);
> --
> 2.6.2
>
--
Kind regards,
Minchan Kim
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