Re: [PATCH] zsmalloc: fix zs_can_compact() integer overflow

Sergey Senozhatsky
2016-05-09T08:40:25Z

Hello,

On (05/09/16 17:07), Minchan Kim wrote:
[..]
> > Depending on the circumstances, OBJ_ALLOCATED can become less
> > than OBJ_USED, which can result in either very high or negative
> > `total_scan' value calculated in do_shrink_slab().
>
> So, do you see pr_err("shrink_slab: %pF negative objects xxxx)
> in vmscan.c and skip shrinking?

yes

: 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
: 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
: 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
: 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

> It would be better to explain what's the result without this patch
> and end-user effect for going -stable.

it seems that not every overflowed value returned from zs_can_compact()
is getting detected in do_shrink_slab():

freeable = shrinker->count_objects(shrinker, shrinkctl);
if (freeable == 0)
return 0;

/*
* copy the current shrinker scan count into a local variable
* and zero it so that other concurrent shrinker invocations
* don't also do this scanning work.
*/
nr = atomic_long_xchg(&shrinker->nr_deferred[nid], 0);

total_scan = nr;
delta = (4 * nr_scanned) / shrinker->seeks;
delta *= freeable;
do_div(delta, nr_eligible + 1);
total_scan += delta;
if (total_scan < 0) {
pr_err("shrink_slab: %pF negative objects to delete nr=%ld\n",
shrinker->scan_objects, total_scan);
total_scan = freeable;
}

this calculation can hide the shrinker->count_objects() error. I added
some debugging code (on x86_64), and the output was:

[ 59.041959] vmscan: >> OVERFLOW: shrinker->count_objects() == -1 [18446744073709551615]
[ 59.041963] vmscan: >> but total_scan > 0: 92679974445502
[ 59.041964] vmscan: >> resulting total_scan: 92679974445502

[ 59.192734] vmscan: >> OVERFLOW: shrinker->count_objects() == -1 [18446744073709551615]
[ 59.192737] vmscan: >> but total_scan > 0: 5830197242006811
[ 59.192738] vmscan: >> resulting total_scan: 5830197242006811

[ 59.259805] vmscan: >> OVERFLOW: shrinker->count_objects() == -1 [18446744073709551615]
[ 59.259809] vmscan: >> but total_scan > 0: 23649671889371219
[ 59.259810] vmscan: >> resulting total_scan: 23649671889371219

[ 76.279767] vmscan: >> OVERFLOW: shrinker->count_objects() == -1 [18446744073709551615]
[ 76.279770] vmscan: >> but total_scan > 0: 895907920044174
[ 76.279771] vmscan: >> resulting total_scan: 895907920044174

[ 84.807837] vmscan: >> OVERFLOW: shrinker->count_objects() == -1 [18446744073709551615]
[ 84.807841] vmscan: >> but total_scan > 0: 22634041808232578
[ 84.807842] vmscan: >> resulting total_scan: 22634041808232578

so we can end up with insanely huge total_scan values.

[..]
> > @@ -2262,10 +2262,13 @@ static void SetZsPageMovable(struct zs_pool *pool, struct zspage *zspage)
>
> It seems this patch is based on my old page migration work?
> It's not go to the mainline yet but your patch which fixes the bug should
> be supposed to go to the -stable. So, I hope this patch first.

oops... my fat fingers! good catch, thanks! I have two versions: for -next and
-mmots (with your LRU rework applied, indeed). somehow I managed to cd to the
wrong dir. sorry, will resend.

-ss