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    SubjectRe: [PATCH] Fix bug in try_to_free_pages and balance_pgdat when they fail to reclaim pages
    Martin Bligh wrote:

    > The same bug is contained in both try_to_free_pages and balance_pgdat.
    > On reclaiming the requisite number of pages we correctly set
    > prev_priority back to DEF_PRIORITY.


    AFAIKS, we set prev_priority to the priority at which the zone was
    deemed to require no more reclaiming, not DEF_PRIORITY.

    > However, we ALSO do this even
    > if we loop over all priorities and fail to reclaim.


    If that happens, shouldn't prev_priority be set to 0?

    I don't agree the patch is correct.

    >
    > Setting prev_priority artificially high causes reclaimers to set
    > distress artificially low, and fail to reclaim mapped pages, when
    > they are, in fact, under severe memory pressure (their priority
    > may be as low as 0). This causes the OOM killer to fire incorrectly.
    >
    > This patch changes that to set prev_priority to 0 instead, if we
    > fail to reclaim.


    We saw problems with this before releasing SLES10 too. See
    zone_is_near_oom and other changesets from around that era. I would
    like to know what workload was prevented from going OOM with these
    changes, but zone_is_near_oom didn't help -- it must have been very
    marginal (or there may indeed be a bug somewhere).

    Nick
    --

    >
    > Signed-off-by: Martin J. Bligh <mbligh@google.com>

    >
    >
    >------------------------------------------------------------------------
    >
    >diff -aurpN -X /home/mbligh/.diff.exclude linux-2.6.18/mm/vmscan.c 2.6.18-prev_reset/mm/vmscan.c
    >--- linux-2.6.18/mm/vmscan.c 2006-09-20 12:24:42.000000000 -0700
    >+++ 2.6.18-prev_reset/mm/vmscan.c 2006-10-16 17:23:48.000000000 -0700
    >@@ -962,7 +962,6 @@ static unsigned long shrink_zones(int pr
    > unsigned long try_to_free_pages(struct zone **zones, gfp_t gfp_mask)
    > {
    > int priority;
    >- int ret = 0;
    > unsigned long total_scanned = 0;
    > unsigned long nr_reclaimed = 0;
    > struct reclaim_state *reclaim_state = current->reclaim_state;
    >@@ -1000,8 +999,15 @@ unsigned long try_to_free_pages(struct z
    > }
    > total_scanned += sc.nr_scanned;
    > if (nr_reclaimed >= sc.swap_cluster_max) {
    >- ret = 1;
    >- goto out;
    >+ for (i = 0; zones[i] != 0; i++) {
    >+ struct zone *zone = zones[i];
    >+
    >+ if (!cpuset_zone_allowed(zone, __GFP_HARDWALL))
    >+ continue;
    >+
    >+ zone->prev_priority = zone->temp_priority;
    >+ }
    >+ return 1;
    > }
    >
    > /*
    >@@ -1021,16 +1027,15 @@ unsigned long try_to_free_pages(struct z
    > if (sc.nr_scanned && priority < DEF_PRIORITY - 2)
    > blk_congestion_wait(WRITE, HZ/10);
    > }
    >-out:
    > for (i = 0; zones[i] != 0; i++) {
    > struct zone *zone = zones[i];
    >
    > if (!cpuset_zone_allowed(zone, __GFP_HARDWALL))
    > continue;
    >
    >- zone->prev_priority = zone->temp_priority;
    >+ zone->prev_priority = 0;
    > }
    >- return ret;
    >+ return 0;
    > }
    >
    > /*
    >@@ -1186,7 +1191,10 @@ out:
    > for (i = 0; i < pgdat->nr_zones; i++) {
    > struct zone *zone = pgdat->node_zones + i;
    >
    >- zone->prev_priority = zone->temp_priority;
    >+ if (priority < 0) /* we failed to reclaim */
    >+ zone->prev_priority = 0;
    >+ else
    >+ zone->prev_priority = zone->temp_priority;
    > }
    > if (!all_zones_ok) {
    > cond_resched();
    >

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