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

[Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>]

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

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