Re: zone_reclaimable() leads to livelock in __alloc_pages_slowpath()

[Oleg Nesterov <oleg@redhat.com>]

sorry for delay,

On 05/25, Michal Hocko wrote:
>
> On Wed 25-05-16 00:43:41, Oleg Nesterov wrote:
> >
> > But. It _seems to me_ that the kernel "leaks" some pages in LRU_INACTIVE_FILE
> > list because inactive_file_is_low() returns the wrong value. And do not even
> > ask me why I think so, unlikely I will be able to explain ;) to remind, I never
> > tried to read vmscan.c before.

No, this is not because of inactive_file_is_low(), but

> >
> > But. if I change lruvec_lru_size()
> >
> > 	-       return zone_page_state(lruvec_zone(lruvec), NR_LRU_BASE + lru);
> > 	+       return zone_page_state_snapshot(lruvec_zone(lruvec), NR_LRU_BASE + lru);
> >
> > the problem goes away too.

Yes,

> This is a bit surprising but my testing shows that the result shouldn't
> make much difference. I can see some discrepancies between lru_vec size
> and zone_reclaimable_pages but they are too small to actually matter.

Yes, the difference is small but it does matter.

I do not pretend I understand this all, but finally it seems I understand
whats going on on my system when it hangs. At least, why the change in
lruvec_lru_size() or calculate_normal_threshold() makes a difference.

This single change in get_scan_count() under for_each_evictable_lru() loop

	-	size = lruvec_lru_size(lruvec, lru);
	+	size = zone_page_state_snapshot(lruvec_zone(lruvec), NR_LRU_BASE + lru);

fixes the problem too.

Without this change shrink*() continues to scan the LRU_ACTIVE_FILE list
while it is empty. LRU_INACTIVE_FILE is not empty (just a few pages) but
we do not even try to scan it, lruvec_lru_size() returns zero.

Then later we recheck zone_reclaimable() and it notices the INACTIVE_FILE
counter because it uses the _snapshot variant, this leads to livelock.

I guess this doesn't really matter, but in my particular case these
ACTIVE/INACTIVE counters were screwed by the recent putback_inactive_pages()
logic. The pages we "leak" in INACTIVE list were recently moved from ACTIVE
to INACTIVE list, and this updated only the per-cpu ->vm_stat_diff[] counters,
so the "non snapshot" lruvec_lru_size() in get_scan_count() sees the "old"
numbers.

I even added more printk's, and yes when the system hangs I have something
like, say,

	->vm_stat[ACTIVE] 	 = NR;		// small number
	->vm_stat_diff[ACTIVE]	 = -NR;		// so it is actually zero but
						// get_scan_count() sees NR

	->vm_stat[INACTIVE]	 = 0;		// this is what get_scan_count() sees
	->vm_stat_diff[INACTIVE] = NR;		// and this is what zone_reclaimable()

Oleg.

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