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  1. 21 5月, 2016 4 次提交
  3. 10 5月, 2016 1 次提交
    • S
      zsmalloc: fix zs_can_compact() integer overflow · 44f43e99
      由 Sergey Senozhatsky 提交于 
      zs_can_compact() has two race conditions in its core calculation:

unsigned long obj_wasted = zs_stat_get(class, OBJ_ALLOCATED) -
				zs_stat_get(class, OBJ_USED);

1) classes are not locked, so the numbers of allocated and used
   objects can change by the concurrent ops happening on other CPUs
2) shrinker invokes it from preemptible context

Depending on the circumstances, thus, OBJ_ALLOCATED can become
less than OBJ_USED, which can result in either very high or
negative `total_scan' value calculated later in do_shrink_slab().

do_shrink_slab() has some logic to prevent those cases:

 vmscan: shrink_slab: zs_shrinker_scan+0x0/0x28 [zsmalloc] negative objects to delete nr=-62
 vmscan: shrink_slab: zs_shrinker_scan+0x0/0x28 [zsmalloc] negative objects to delete nr=-62
 vmscan: shrink_slab: zs_shrinker_scan+0x0/0x28 [zsmalloc] negative objects to delete nr=-64
 vmscan: shrink_slab: zs_shrinker_scan+0x0/0x28 [zsmalloc] negative objects to delete nr=-62
 vmscan: shrink_slab: zs_shrinker_scan+0x0/0x28 [zsmalloc] negative objects to delete nr=-62
 vmscan: shrink_slab: zs_shrinker_scan+0x0/0x28 [zsmalloc] negative objects to delete nr=-62

However, due to the way `total_scan' is calculated, not every
shrinker->count_objects() overflow can be spotted and handled.
To demonstrate the latter, I added some debugging code to do_shrink_slab()
(x86_64) and the results were:

 vmscan: OVERFLOW: shrinker->count_objects() == -1 [18446744073709551615]
 vmscan: but total_scan > 0: 92679974445502
 vmscan: resulting total_scan: 92679974445502
[..]
 vmscan: OVERFLOW: shrinker->count_objects() == -1 [18446744073709551615]
 vmscan: but total_scan > 0: 22634041808232578
 vmscan: resulting total_scan: 22634041808232578

Even though shrinker->count_objects() has returned an overflowed value,
the resulting `total_scan' is positive, and, what is more worrisome, it
is insanely huge. This value is getting used later on in
shrinker->scan_objects() loop:

        while (total_scan >= batch_size ||
               total_scan >= freeable) {
                unsigned long ret;
                unsigned long nr_to_scan = min(batch_size, total_scan);

                shrinkctl->nr_to_scan = nr_to_scan;
                ret = shrinker->scan_objects(shrinker, shrinkctl);
                if (ret == SHRINK_STOP)
                        break;
                freed += ret;

                count_vm_events(SLABS_SCANNED, nr_to_scan);
                total_scan -= nr_to_scan;

                cond_resched();
        }

`total_scan >= batch_size' is true for a very-very long time and
'total_scan >= freeable' is also true for quite some time, because
`freeable < 0' and `total_scan' is large enough, for example,
22634041808232578. The only break condition, in the given scheme of
things, is shrinker->scan_objects() == SHRINK_STOP test, which is a
bit too weak to rely on, especially in heavy zsmalloc-usage scenarios.

To fix the issue, take a pool stat snapshot and use it instead of
racy zs_stat_get() calls.

Link: http://lkml.kernel.org/r/20160509140052.3389-1-sergey.senozhatsky@gmail.comSigned-off-by: NSergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: <stable@vger.kernel.org>        [4.3+]
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
      44f43e99
  5. 18 3月, 2016 2 次提交
  7. 21 1月, 2016 1 次提交
    • J
      zsmalloc: fix migrate_zspage-zs_free race condition · c102f07c
      由 Junil Lee 提交于 
      record_obj() in migrate_zspage() does not preserve handle's
HANDLE_PIN_BIT, set by find_aloced_obj()->trypin_tag(), and implicitly
(accidentally) un-pins the handle, while migrate_zspage() still performs
an explicit unpin_tag() on the that handle.  This additional explicit
unpin_tag() introduces a race condition with zs_free(), which can pin
that handle by this time, so the handle becomes un-pinned.

Schematically, it goes like this:

  CPU0                                        CPU1
  migrate_zspage
    find_alloced_obj
      trypin_tag
        set HANDLE_PIN_BIT                    zs_free()
                                                pin_tag()
  obj_malloc() -- new object, no tag
  record_obj() -- remove HANDLE_PIN_BIT           set HANDLE_PIN_BIT
  unpin_tag()  -- remove zs_free's HANDLE_PIN_BIT

The race condition may result in a NULL pointer dereference:

  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:
     get_zspage_mapping+0x0/0x24
     zs_free+0x88/0x114
     zram_free_page+0x64/0xcc
     zram_slot_free_notify+0x90/0x108
     swap_entry_free+0x278/0x294
     free_swap_and_cache+0x38/0x11c
     unmap_single_vma+0x480/0x5c8
     unmap_vmas+0x44/0x60
     exit_mmap+0x50/0x110
     mmput+0x58/0xe0
     do_exit+0x320/0x8dc
     do_group_exit+0x44/0xa8
     get_signal+0x538/0x580
     do_signal+0x98/0x4b8
     do_notify_resume+0x14/0x5c

This patch keeps the lock bit in migration path and update value
atomically.
Signed-off-by: NJunil Lee <junil0814.lee@lge.com>
Signed-off-by: NMinchan Kim <minchan@kernel.org>
Acked-by: NVlastimil Babka <vbabka@suse.cz>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc: <stable@vger.kernel.org> [4.1+]
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
      c102f07c
  9. 16 1月, 2016 1 次提交
  11. 07 11月, 2015 8 次提交
  13. 09 9月, 2015 13 次提交
  15. 26 6月, 2015 2 次提交
  17. 11 6月, 2015 1 次提交
  19. 16 4月, 2015 7 次提交