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SubjectRe: [PATCH 2/3] mm: shrink_inactive_lis() nr_scan accounting fix fix
On Thu, Jul 16, 2009 at 08:53:42AM +0800, KOSAKI Motohiro wrote:
> Subject: [PATCH] mm: shrink_inactive_lis() nr_scan accounting fix fix
>
> If sc->isolate_pages() return 0, we don't need to call shrink_page_list().
> In past days, shrink_inactive_list() handled it properly.
>
> But commit fb8d14e1 (three years ago commit!) breaked it. current shrink_inactive_list()
> always call shrink_page_list() although isolate_pages() return 0.
>
> This patch restore proper return value check.

Patch looks good, but there is another minor problem..

>
> Requirements:
> o "nr_taken == 0" condition should stay before calling shrink_page_list().
> o "nr_taken == 0" condition should stay after nr_scan related statistics
> modification.
>
>
> Cc: Wu Fengguang <fengguang.wu@intel.com>
> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> ---
> mm/vmscan.c | 30 ++++++++++++++++++------------
> 1 file changed, 18 insertions(+), 12 deletions(-)
>
> Index: b/mm/vmscan.c
> ===================================================================
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1071,6 +1071,20 @@ static unsigned long shrink_inactive_lis
> nr_taken = sc->isolate_pages(sc->swap_cluster_max,
> &page_list, &nr_scan, sc->order, mode,
> zone, sc->mem_cgroup, 0, file);
> +
> + if (scanning_global_lru(sc)) {
> + zone->pages_scanned += nr_scan;
> + if (current_is_kswapd())
> + __count_zone_vm_events(PGSCAN_KSWAPD, zone,
> + nr_scan);
> + else
> + __count_zone_vm_events(PGSCAN_DIRECT, zone,
> + nr_scan);
> + }
> +
> + if (nr_taken == 0)
> + goto done;

Not a newly introduced problem, but this early break might under scan
the list, if (max_scan > swap_cluster_max). Luckily the only two
callers all call with (max_scan <= swap_cluster_max).

What shall we do? The comprehensive solution may be to
- remove the big do-while loop
- replace sc->swap_cluster_max => max_scan
- take care in the callers to not passing small max_scan values

Or to simply make this function more robust like this?

---
mm/vmscan.c | 13 +++++++------
1 file changed, 7 insertions(+), 6 deletions(-)

--- linux.orig/mm/vmscan.c
+++ linux/mm/vmscan.c
@@ -1098,7 +1098,7 @@ static unsigned long shrink_inactive_lis

lru_add_drain();
spin_lock_irq(&zone->lru_lock);
- do {
+ while (nr_scanned < max_scan) {
struct page *page;
unsigned long nr_taken;
unsigned long nr_scan;
@@ -1112,6 +1112,7 @@ static unsigned long shrink_inactive_lis
nr_taken = sc->isolate_pages(sc->swap_cluster_max,
&page_list, &nr_scan, sc->order, mode,
zone, sc->mem_cgroup, 0, file);
+ nr_scanned += nr_scan;

if (scanning_global_lru(sc)) {
zone->pages_scanned += nr_scan;
@@ -1123,8 +1124,10 @@ static unsigned long shrink_inactive_lis
nr_scan);
}

- if (nr_taken == 0)
- goto done;
+ if (nr_taken == 0) {
+ cond_resched_lock(&zone->lru_lock);
+ continue;
+ }

nr_active = clear_active_flags(&page_list, count);
__count_vm_events(PGDEACTIVATE, nr_active);
@@ -1150,7 +1153,6 @@ static unsigned long shrink_inactive_lis

spin_unlock_irq(&zone->lru_lock);

- nr_scanned += nr_scan;
nr_freed = shrink_page_list(&page_list, sc, PAGEOUT_IO_ASYNC);

/*
@@ -1212,9 +1214,8 @@ static unsigned long shrink_inactive_lis
__mod_zone_page_state(zone, NR_ISOLATED_ANON, -nr_anon);
__mod_zone_page_state(zone, NR_ISOLATED_FILE, -nr_file);

- } while (nr_scanned < max_scan);
+ }

-done:
spin_unlock_irq(&zone->lru_lock);
pagevec_release(&pvec);
return nr_reclaimed;

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