Re: [PATCH]vmscan: handle underflow for get_scan_ratio

Wu Fengguang
2010-03-31T05:51:13Z

KOSAKI-san,

On Wed, Mar 31, 2010 at 01:38:12PM +0800, KOSAKI Motohiro wrote:
> > On Tue, Mar 30, 2010 at 02:08:53PM +0800, KOSAKI Motohiro wrote:
> > > Hi
> > >
> > > > Commit 84b18490d1f1bc7ed5095c929f78bc002eb70f26 introduces a regression.
> > > > With it, our tmpfs test always oom. The test has a lot of rotated anon
> > > > pages and cause percent[0] zero. Actually the percent[0] is a very small
> > > > value, but our calculation round it to zero. The commit makes vmscan
> > > > completely skip anon pages and cause oops.
> > > > An option is if percent[x] is zero in get_scan_ratio(), forces it
> > > > to 1. See below patch.
> > > > But the offending commit still changes behavior. Without the commit, we scan
> > > > all pages if priority is zero, below patch doesn't fix this. Don't know if
> > > > It's required to fix this too.
> > >
> > > Can you please post your /proc/meminfo and reproduce program? I'll digg it.
> > >
> > > Very unfortunately, this patch isn't acceptable. In past time, vmscan
> > > had similar logic, but 1% swap-out made lots bug reports.
> > if 1% is still big, how about below patch?
>
> This patch makes a lot of sense than previous. however I think <1% anon ratio
> shouldn't happen anyway because file lru doesn't have reclaimable pages.
> <1% seems no good reclaim rate.
>
> perhaps I'll take your patch for stable tree. but we need to attack the root
> cause. iow, I guess we need to fix scan ratio equation itself.

I tend to regard this patch as a general improvement for both
.33-stable and .34.

I do agree with you that it's desirable to do more test&analyze and
check further for possibly hidden problems.

Thanks,
Fengguang

>
>
> > Commit 84b18490d1f1bc7ed5095c929f78bc002eb70f26 introduces a regression.
> > With it, our tmpfs test always oom. The test has a lot of rotated anon
> > pages and cause percent[0] zero. Actually the percent[0] is a very small
> > value, but our calculation round it to zero. The commit makes vmscan
> > completely skip anon pages and cause oops.
> > To avoid underflow, we don't use percentage, instead we directly calculate
> > how many pages should be scaned.
> >
> > Signed-off-by: Shaohua Li <shaohua.li@intel.com>
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 79c8098..80a7ed5 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -1519,27 +1519,50 @@ static unsigned long shrink_list(enum lru_list lru, unsigned long nr_to_scan,
> > }
> >
> > /*
> > + * Smallish @nr_to_scan's are deposited in @nr_saved_scan,
> > + * until we collected @swap_cluster_max pages to scan.
> > + */
> > +static unsigned long nr_scan_try_batch(unsigned long nr_to_scan,
> > + unsigned long *nr_saved_scan)
> > +{
> > + unsigned long nr;
> > +
> > + *nr_saved_scan += nr_to_scan;
> > + nr = *nr_saved_scan;
> > +
> > + if (nr >= SWAP_CLUSTER_MAX)
> > + *nr_saved_scan = 0;
> > + else
> > + nr = 0;
> > +
> > + return nr;
> > +}
> > +
> > +/*
> > * Determine how aggressively the anon and file LRU lists should be
> > * scanned. The relative value of each set of LRU lists is determined
> > * by looking at the fraction of the pages scanned we did rotate back
> > * onto the active list instead of evict.
> > *
> > - * percent[0] specifies how much pressure to put on ram/swap backed
> > - * memory, while percent[1] determines pressure on the file LRUs.
> > + * nr[x] specifies how many pages should be scaned
> > */
> > -static void get_scan_ratio(struct zone *zone, struct scan_control *sc,
> > - unsigned long *percent)
> > +static void get_scan_count(struct zone *zone, struct scan_control *sc,
> > + unsigned long *nr, int priority)
> > {
> > unsigned long anon, file, free;
> > unsigned long anon_prio, file_prio;
> > unsigned long ap, fp;
> > struct zone_reclaim_stat *reclaim_stat = get_reclaim_stat(zone, sc);
> > + unsigned long fraction[2], denominator[2];
> > + enum lru_list l;
> >
> > /* If we have no swap space, do not bother scanning anon pages. */
> > if (!sc->may_swap || (nr_swap_pages <= 0)) {
> > - percent[0] = 0;
> > - percent[1] = 100;
> > - return;
> > + fraction[0] = 0;
> > + denominator[0] = 1;
> > + fraction[1] = 1;
> > + denominator[1] = 1;
> > + goto out;
> > }
> >
> > anon = zone_nr_lru_pages(zone, sc, LRU_ACTIVE_ANON) +
> > @@ -1552,9 +1575,11 @@ static void get_scan_ratio(struct zone *zone, struct scan_control *sc,
> > /* If we have very few page cache pages,
> > force-scan anon pages. */
> > if (unlikely(file + free <= high_wmark_pages(zone))) {
> > - percent[0] = 100;
> > - percent[1] = 0;
> > - return;
> > + fraction[0] = 1;
> > + denominator[0] = 1;
> > + fraction[1] = 0;
> > + denominator[1] = 1;
> > + goto out;
> > }
> > }
> >
> > @@ -1601,29 +1626,29 @@ static void get_scan_ratio(struct zone *zone, struct scan_control *sc,
> > fp = (file_prio + 1) * (reclaim_stat->recent_scanned[1] + 1);
> > fp /= reclaim_stat->recent_rotated[1] + 1;
> >
> > - /* Normalize to percentages */
> > - percent[0] = 100 * ap / (ap + fp + 1);
> > - percent[1] = 100 - percent[0];
> > -}
> > -
> > -/*
> > - * Smallish @nr_to_scan's are deposited in @nr_saved_scan,
> > - * until we collected @swap_cluster_max pages to scan.
> > - */
> > -static unsigned long nr_scan_try_batch(unsigned long nr_to_scan,
> > - unsigned long *nr_saved_scan)
> > -{
> > - unsigned long nr;
> > + fraction[0] = ap;
> > + denominator[0] = ap + fp + 1;
> > + fraction[1] = fp;
> > + denominator[1] = ap + fp + 1;
> >
> > - *nr_saved_scan += nr_to_scan;
> > - nr = *nr_saved_scan;
> > +out:
> > + for_each_evictable_lru(l) {
> > + int file = is_file_lru(l);
> > + unsigned long scan;
> >
> > - if (nr >= SWAP_CLUSTER_MAX)
> > - *nr_saved_scan = 0;
> > - else
> > - nr = 0;
> > + if (fraction[file] == 0) {
> > + nr[l] = 0;
> > + continue;
> > + }
> >
> > - return nr;
> > + scan = zone_nr_lru_pages(zone, sc, l);
> > + if (priority) {
> > + scan >>= priority;
> > + scan = (scan * fraction[file] / denominator[file]);
> > + }
> > + nr[l] = nr_scan_try_batch(scan,
> > + &reclaim_stat->nr_saved_scan[l]);
> > + }
> > }
> >
> > /*
> > @@ -1634,31 +1659,11 @@ static void shrink_zone(int priority, struct zone *zone,
> > {
> > unsigned long nr[NR_LRU_LISTS];
> > unsigned long nr_to_scan;
> > - unsigned long percent[2]; /* anon @ 0; file @ 1 */
> > enum lru_list l;
> > unsigned long nr_reclaimed = sc->nr_reclaimed;
> > unsigned long nr_to_reclaim = sc->nr_to_reclaim;
> > - struct zone_reclaim_stat *reclaim_stat = get_reclaim_stat(zone, sc);
> > -
> > - get_scan_ratio(zone, sc, percent);
> >
> > - for_each_evictable_lru(l) {
> > - int file = is_file_lru(l);
> > - unsigned long scan;
> > -
> > - if (percent[file] == 0) {
> > - nr[l] = 0;
> > - continue;
> > - }
> > -
> > - scan = zone_nr_lru_pages(zone, sc, l);
> > - if (priority) {
> > - scan >>= priority;
> > - scan = (scan * percent[file]) / 100;
> > - }
> > - nr[l] = nr_scan_try_batch(scan,
> > - &reclaim_stat->nr_saved_scan[l]);
> > - }
> > + get_scan_count(zone, sc, nr, priority);
> >
> > while (nr[LRU_INACTIVE_ANON] || nr[LRU_ACTIVE_FILE] ||
> > nr[LRU_INACTIVE_FILE]) {
>
>