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    SubjectRe: [PATCH]vmscan: handle underflow for get_scan_ratio
    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]) {
    >
    >


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