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    SubjectRe: [PATCH 04/27] mm, vmscan: Begin reclaiming pages on a per-node basis
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    On 06/22/2016 04:04 PM, Vlastimil Babka wrote:
    > On 06/21/2016 04:15 PM, Mel Gorman wrote:
    >> This patch makes reclaim decisions on a per-node basis. A reclaimer knows
    >> what zone is required by the allocation request and skips pages from
    >> higher zones. In many cases this will be ok because it's a GFP_HIGHMEM
    >> request of some description. On 64-bit, ZONE_DMA32 requests will cause
    >> some problems but 32-bit devices on 64-bit platforms are increasingly
    >> rare. Historically it would have been a major problem on 32-bit with big
    >> Highmem:Lowmem ratios but such configurations are also now rare and even
    >> where they exist, they are not encouraged. If it really becomes a problem,
    >> it'll manifest as very low reclaim efficiencies.
    >>
    >> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
    >
    > [...]
    >
    >> @@ -2540,14 +2559,14 @@ static inline bool compaction_ready(struct zone *zone, int order, int classzone_
    >> * If a zone is deemed to be full of pinned pages then just give it a light
    >> * scan then give up on it.
    >> */
    >> -static void shrink_zones(struct zonelist *zonelist, struct scan_control *sc)
    >> +static void shrink_zones(struct zonelist *zonelist, struct scan_control *sc,
    >> + enum zone_type classzone_idx)
    >> {
    >> struct zoneref *z;
    >> struct zone *zone;
    >> unsigned long nr_soft_reclaimed;
    >> unsigned long nr_soft_scanned;
    >> gfp_t orig_mask;
    >> - enum zone_type requested_highidx = gfp_zone(sc->gfp_mask);
    >>
    >> /*
    >> * If the number of buffer_heads in the machine exceeds the maximum
    >> @@ -2560,15 +2579,20 @@ static void shrink_zones(struct zonelist *zonelist, struct scan_control *sc)
    >>
    >> for_each_zone_zonelist_nodemask(zone, z, zonelist,
    >> gfp_zone(sc->gfp_mask), sc->nodemask) {
    >
    > Using sc->reclaim_idx could be faster/nicer here than gfp_zone()?
    > Although after "mm, vmscan: Update classzone_idx if buffer_heads_over_limit"
    > there would need to be a variable for the highmem adjusted value - maybe reuse
    > "requested_highidx"? Not important though.
    >
    >> - enum zone_type classzone_idx;
    >> -
    >> if (!populated_zone(zone))
    >> continue;
    >>
    >> - classzone_idx = requested_highidx;
    >> + /*
    >> + * Note that reclaim_idx does not change as it is the highest
    >> + * zone reclaimed from which for empty zones is a no-op but
    >> + * classzone_idx is used by shrink_node to test if the slabs
    >> + * should be shrunk on a given node.
    >> + */
    >> while (!populated_zone(zone->zone_pgdat->node_zones +
    >> - classzone_idx))
    >> + classzone_idx)) {
    >> classzone_idx--;
    >> + continue;

    Oh and Michal's comment on Patch 20 made me realize that my objection to v6
    about possible underflow of sc->reclaim_idx and classzone_idx seems to still
    apply here for classzone_idx? Updated example: Normal zone allocation. A small
    node 0 without Normal zone will get us classzone_idx == dma32. Node 1 next in
    zonelist won't have dma/dma32 zones so we won't see node_zones + classzone_idx
    populated, and the while loop will lead to underflow of classzone_idx.
    I may be missing something, but I don't really see another way around it than
    resetting classzone_idx to sc->reclaim_idx before the while loop.

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