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Message-ID: <1452843551-4464-1-git-send-email-junil0814.lee@lge.com>
Date: Fri, 15 Jan 2016 16:39:11 +0900
From: Junil Lee <junil0814.lee@....com>
To: <minchan@...nel.org>, <ngupta@...are.org>
CC: sergey.senozhatsky.work@...il.com, akpm@...ux-foundation.org,
linux-mm@...ck.org, linux-kernel@...r.kernel.org,
Junil Lee <junil0814.lee@....com>
Subject: [PATCH v2] zsmalloc: fix migrate_zspage-zs_free race condition
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.
Signed-off-by: Junil Lee <junil0814.lee@....com>
---
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() */
+ free_obj |= BIT(HANDLE_PIN_BIT);
record_obj(handle, free_obj);
unpin_tag(handle);
obj_free(pool, class, used_obj);
--
2.6.2
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