[PATCH 4/5] mm: fix check_move_unevictable_pages() on THP

[Hugh Dickins]

check_move_unevictable_pages() is used in making unevictable shmem pages
evictable: by shmem_unlock_mapping(), drm_gem_check_release_pagevec() and
i915/gem check_release_pagevec().  Those may pass down subpages of a huge
page, when /sys/kernel/mm/transparent_hugepage/shmem_enabled is "force".

That does not crash or warn at present, but the accounting of vmstats
unevictable_pgs_scanned and unevictable_pgs_rescued is inconsistent:
scanned being incremented on each subpage, rescued only on the head
(since tails already appear evictable once the head has been updated).

5.8 commit 5d91f31faf8e ("mm: swap: fix vmstats for huge page") has
established that vm_events in general (and unevictable_pgs_rescued in
particular) should count every subpage: so follow that precedent here.

Do this in such a way that if mem_cgroup_page_lruvec() is made stricter
(to check page->mem_cgroup is always set), no problem: skip the tails
before calling it, and add thp_nr_pages() to vmstats on the head.

Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
---
Nothing here worth going to stable, since it's just a testing config
that is fixed, whose event numbers are not very important; but this
will be needed before Alex Shi's warning, and might as well go in now.

The callers of check_move_unevictable_pages() could be optimized,
to skip over tails: but Matthew Wilcox has other changes in flight
there, so let's skip the optimization for now.

 mm/vmscan.c |   10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

--- 5.9-rc2/mm/vmscan.c	2020-08-16 17:32:50.721507348 -0700
+++ linux/mm/vmscan.c	2020-08-28 17:47:10.595580876 -0700
@@ -4260,8 +4260,14 @@ void check_move_unevictable_pages(struct
 	for (i = 0; i < pvec->nr; i++) {
 		struct page *page = pvec->pages[i];
 		struct pglist_data *pagepgdat = page_pgdat(page);
+		int nr_pages;
+
+		if (PageTransTail(page))
+			continue;
+
+		nr_pages = thp_nr_pages(page);
+		pgscanned += nr_pages;
 
-		pgscanned++;
 		if (pagepgdat != pgdat) {
 			if (pgdat)
 				spin_unlock_irq(&pgdat->lru_lock);
@@ -4280,7 +4286,7 @@ void check_move_unevictable_pages(struct
 			ClearPageUnevictable(page);
 			del_page_from_lru_list(page, lruvec, LRU_UNEVICTABLE);
 			add_page_to_lru_list(page, lruvec, lru);
-			pgrescued++;
+			pgrescued += nr_pages;
 		}
 	}