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    SubjectRe: [PATCH v2 2/2] vmscan: shrink_slab() require number of lru_pages, not page order
    On Tue, 13 Jul 2010 14:41:28 +0900 (JST)
    KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> wrote:

    > Now, shrink_slab() has following scanning equation.
    >
    > lru_scanned max_pass
    > basic_scan_objects = 4 x ------------- x -----------------------------
    > lru_pages shrinker->seeks (default:2)
    >
    > scan_objects = min(basic_scan_objects, max_pass * 2)
    >
    > Then, If we pass very small value as lru_pages instead real number of
    > lru pages, shrink_slab() drop much objects rather than necessary. and
    > now, __zone_reclaim() pass 'order' as lru_pages by mistake. that makes
    > bad result.
    >
    > Example, If we receive very low memory pressure (scan = 32, order = 0),
    > shrink_slab() via zone_reclaim() always drop _all_ icache/dcache
    > objects. (see above equation, very small lru_pages make very big
    > scan_objects result)
    >
    > This patch fixes it.
    >
    > Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
    > Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
    > Acked-by: Christoph Lameter <cl@linux-foundation.org>
    > Acked-by: Rik van Riel <riel@redhat.com>
    > ---
    > mm/vmscan.c | 4 +++-
    > 1 files changed, 3 insertions(+), 1 deletions(-)
    >
    > diff --git a/mm/vmscan.c b/mm/vmscan.c
    > index 6ff51c0..1bf9f72 100644
    > --- a/mm/vmscan.c
    > +++ b/mm/vmscan.c
    > @@ -2612,6 +2612,8 @@ static int __zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
    >
    > nr_slab_pages0 = zone_page_state(zone, NR_SLAB_RECLAIMABLE);
    > if (nr_slab_pages0 > zone->min_slab_pages) {
    > + unsigned long lru_pages = zone_reclaimable_pages(zone);
    > +
    > /*
    > * shrink_slab() does not currently allow us to determine how
    > * many pages were freed in this zone. So we take the current
    > @@ -2622,7 +2624,7 @@ static int __zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
    > * Note that shrink_slab will free memory on all zones and may
    > * take a long time.
    > */
    > - while (shrink_slab(sc.nr_scanned, gfp_mask, order) &&
    > + while (shrink_slab(sc.nr_scanned, gfp_mask, lru_pages) &&
    > (zone_page_state(zone, NR_SLAB_RECLAIMABLE) + nr_pages >
    > nr_slab_pages0))
    > ;

    Wouldn't it be better to recalculate zone_reclaimable_pages() each time
    around the loop? For example, shrink_icache_memory()->prune_icache()
    will remove pagecache from an inode if it hits the tail of the list.
    This can change the number of reclaimable pages by squigabytes, but
    this code thinks nothing changed?




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