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Subject[PATCH] mm, vmscan: do not count freed pages as PGDEACTIVATE
Date
From: Michal Hocko <mhocko@suse.com>

PGDEACTIVATE represents the number of pages moved from the active list
to the inactive list. At least this sounds like the original motivation
of the counter. move_active_pages_to_lru, however, counts pages which
got freed in the mean time as deactivated as well. This is a very rare
event and counting them as deactivation in itself is not harmful but it
makes the code more convoluted than necessary - we have to count both
all pages and those which are freed which is a bit confusing.

After this patch the PGDEACTIVATE should have a slightly more clear
semantic and only count those pages which are moved from the active to
the inactive list which is a plus.

Suggested-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Michal Hocko <mhocko@suse.com>
---
Hi,
Vlastimil has pointed out [1] that move_active_pages_to_lru is more
confusing than necessary because we count two things, pgmoved and
nr_moved. I believe that counting freed pages as PGDEACTIVATE is more
confusing than helpful. I doubt that this patch will make any real
difference in the real life but it at least makes the code easier which
is a plus so I think this is more a cleanup than any bug fix.

[1] http://lkml.kernel.org/r/646c3551-e794-611c-5247-490bd89133db@suse.cz

mm/vmscan.c | 4 +---
1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index cf940af609fd..7e1c3cd91fab 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1878,7 +1878,6 @@ static unsigned move_active_pages_to_lru(struct lruvec *lruvec,
enum lru_list lru)
{
struct pglist_data *pgdat = lruvec_pgdat(lruvec);
- unsigned long pgmoved = 0;
struct page *page;
int nr_pages;
int nr_moved = 0;
@@ -1893,7 +1892,6 @@ static unsigned move_active_pages_to_lru(struct lruvec *lruvec,
nr_pages = hpage_nr_pages(page);
update_lru_size(lruvec, lru, page_zonenum(page), nr_pages);
list_move(&page->lru, &lruvec->lists[lru]);
- pgmoved += nr_pages;

if (put_page_testzero(page)) {
__ClearPageLRU(page);
@@ -1913,7 +1911,7 @@ static unsigned move_active_pages_to_lru(struct lruvec *lruvec,
}

if (!is_active_lru(lru))
- __count_vm_events(PGDEACTIVATE, pgmoved);
+ __count_vm_events(PGDEACTIVATE, nr_moved);

return nr_moved;
}
--
2.11.0
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