On 5/13/19 1:09 AM, Michal Hocko wrote:
On Sat 11-05-19 00:23:40, Yang Shi wrote:
Since commit bd4c82c22c36 ("mm, THP, swap: delay splitting THP after
swapped out"), THP can be swapped out in a whole. But, nr_reclaimed
still gets inc'ed by one even though a whole THP (512 pages) gets
swapped out.
This doesn't make too much sense to memory reclaim. For example, direct
reclaim may just need reclaim SWAP_CLUSTER_MAX pages, reclaiming one THP
could fulfill it. But, if nr_reclaimed is not increased correctly,
direct reclaim may just waste time to reclaim more pages,
SWAP_CLUSTER_MAX * 512 pages in worst case.
You are technically right here. This has been a known issue for a while.
I am wondering whether somebody actually noticed some misbehavior.
Swapping out is a rare event and you have to have a considerable number
of THPs to notice.
The commit bd4c82c22c36 ("mm, THP, swap: delay splitting THP after
swapped out") was added in 4.14, it might be not used widely yet. And we
need swap + THPs to trigger it, furthermore the misbehavior might be
accumulative over time. I don't expect it will have any obvious
misbehavior for a single shot.
This change may result in more reclaimed pages than scanned pages showed
by /proc/vmstat since scanning one head page would reclaim 512 base pages.
This is quite nasty and confusing. I am worried that having those two
unsynced begs for subtle issues. Can we account THP as scanning 512 base
pages as well?
Actually, such unsync has been there for a while. The
isolate_lru_pages() returns nr_taken which has THP accounted as 512 base
pages, but nr_scanned just shows 1. nr_taken is not showed via
/proc/vmstat, but it is showed via trace point.
I think we can just account 512 base pages for nr_scanned for
isolate_lru_pages() to make the counters sane since PGSCAN_KSWAPD/DIRECT
just use it.
And, sc->nr_scanned should be accounted as 512 base pages too otherwise
we may have nr_scanned < nr_to_reclaim all the time to result in
false-negative for priority raise and something else wrong (e.g. wrong
vmpressure).
Cc: "Huang, Ying" <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: "Kirill A . Shutemov" <[email protected]>
Cc: Hugh Dickins <[email protected]>
Reviewed-by: Shakeel Butt <[email protected]>
Signed-off-by: Yang Shi <[email protected]>
---
v2: Added Shakeel's Reviewed-by
Use hpage_nr_pages instead of compound_order per Huang Ying and William
Kucharski
mm/vmscan.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)
diff --git a/mm/vmscan.c b/mm/vmscan.c
index fd9de50..4226d6b 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1446,7 +1446,11 @@ static unsigned long shrink_page_list(struct list_head
*page_list,
unlock_page(page);
free_it:
- nr_reclaimed++;
+ /*
+ * THP may get swapped out in a whole, need account
+ * all base pages.
+ */
+ nr_reclaimed += hpage_nr_pages(page);
/*
* Is there need to periodically free_page_list? It would
--
1.8.3.1
