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

From: Oleg Nesterov
Date: Tue May 31 2016 - 19:56:35 EST

On 05/31, Michal Hocko wrote:
> On Sun 29-05-16 23:25:40, Oleg Nesterov wrote:
> >
> > 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.
> OK, you seem to be really seeing a different issue than me.

quite possibly, but

> My debugging
> patch was showing when nothing was really isolated from the LRU lists
> (both for shrink_{in}active_list.

in my debugging session too. LRU_ACTIVE_FILE was empty, so there is nothing to
isolate even if shrink_active_list() is (wrongly called) with nr_to_scan != 0.
LRU_INACTIVE_FILE is not empty but it is not scanned because nr_to_scan == 0.

But I am afraid I misunderstood you, and you meant something else.

> > 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.
> Hmm. I am not really sure we can use the _snapshot version in lruvec_lru_size.

Yes, yes, I understand,

> But I am thinking whether we should simply revert 0db2cb8da89d ("mm,
> vmscan: make zone_reclaimable_pages more precise") in 4.6 stable tree.
> Does that help as well?

I'll test this tomorrow, but even if it helps I am not sure... Yes, this
way zone_reclaimable() and get_scan_count() will see the same numbers, but
how this can help to make zone_reclaimable() == F at the end?

Again, suppose that (say) ACTIVE list is empty but zone->vm_stat != 0
because there is something in per-cpu counter (so that _snapshot == 0).
This means that we sill continue to try to scan this list for no reason.

But Michal, let me repeat that I do not understand this code, so I can
be easily wrong.