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authorSergey Senozhatsky <sergey.senozhatsky@gmail.com>2016-05-09 16:28:49 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2016-05-09 17:40:59 -0700
commit44f43e99fe70833058482d183e99fdfd11220996 (patch)
treef682bfc09f7c223790914e0f6b8778ae107516b9
parent1e92a61c4c7ed85c1bec037c046e92d6dc762f32 (diff)
downloadlinux-stericsson-44f43e99fe70833058482d183e99fdfd11220996.tar.gz
zsmalloc: fix zs_can_compact() integer overflow
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.com Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com> Cc: Minchan Kim <minchan@kernel.org> Cc: <stable@vger.kernel.org> [4.3+] Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
-rw-r--r--mm/zsmalloc.c7
1 files changed, 5 insertions, 2 deletions
diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index e72efb109fde..fe47fbba995a 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -1735,10 +1735,13 @@ static struct page *isolate_source_page(struct size_class *class)
static unsigned long zs_can_compact(struct size_class *class)
{
unsigned long obj_wasted;
+ unsigned long obj_allocated = zs_stat_get(class, OBJ_ALLOCATED);
+ unsigned long obj_used = zs_stat_get(class, OBJ_USED);
- obj_wasted = zs_stat_get(class, OBJ_ALLOCATED) -
- zs_stat_get(class, OBJ_USED);
+ if (obj_allocated <= obj_used)
+ return 0;
+ obj_wasted = obj_allocated - obj_used;
obj_wasted /= get_maxobj_per_zspage(class->size,
class->pages_per_zspage);