On 06/09/2016 08:04 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@xxxxxxxxxxxxxxxxxxx>
---
mm/vmscan.c | 72 ++++++++++++++++++++++++++++++++++++++++---------------------
1 file changed, 47 insertions(+), 25 deletions(-)
diff --git a/mm/vmscan.c b/mm/vmscan.c
index f87a5a0f8793..ab1b28e7e20a 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -84,6 +84,9 @@ struct scan_control {
/* Scan (total_size >> priority) pages at once */
int priority;
+ /* The highest zone to isolate pages for reclaim from */
+ enum zone_type reclaim_idx;
+
unsigned int may_writepage:1;
/* Can mapped pages be reclaimed? */
@@ -1369,6 +1372,7 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
struct list_head *src = &lruvec->lists[lru];
unsigned long nr_taken = 0;
unsigned long scan;
+ LIST_HEAD(pages_skipped);
for (scan = 0; scan < nr_to_scan && nr_taken < nr_to_scan &&
!list_empty(src); scan++) {
@@ -1379,6 +1383,11 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
VM_BUG_ON_PAGE(!PageLRU(page), page);
+ if (page_zonenum(page) > sc->reclaim_idx) {
+ list_move(&page->lru, &pages_skipped);
+ continue;
+ }
+
switch (__isolate_lru_page(page, mode)) {
case 0:
nr_taken += hpage_nr_pages(page);
@@ -1395,6 +1404,15 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
}
}
+ /*
+ * Splice any skipped pages to the start of the LRU list. Note that
+ * this disrupts the LRU order when reclaiming for lower zones but
+ * we cannot splice to the tail. If we did then the SWAP_CLUSTER_MAX
+ * scanning would soon rescan the same pages to skip and put the
+ * system at risk of premature OOM.
+ */
+ if (!list_empty(&pages_skipped))
+ list_splice(&pages_skipped, src);
Hmm, that's unfortunate. But probably better than reclaiming the pages
in the name of LRU order, even though it wouldn't help the allocation at
hand.
[...]
@@ -2516,14 +2535,14 @@ 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
@@ -2536,15 +2555,15 @@ 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) {
- enum zone_type classzone_idx;
-
if (!populated_zone(zone))
continue;
- classzone_idx = requested_highidx;
while (!populated_zone(zone->zone_pgdat->node_zones +
- classzone_idx))
+ classzone_idx)) {
+ sc->reclaim_idx--;
classzone_idx--;
+ continue;
Isn't this wrong to do this across whole zonelist which will contain
multiple nodes? Example: a small node 0 without Normal zone will get us
sc->reclaim_idx == classzone_idx == dma32. Node 1 won't have dma/dma32
zones so we won't see classzone_idx populated, and the while loop will
lead to underflow?
And sc->reclaim_idx seems to be unitialized when called via
try_to_free_pages() -> do_try_to_free_pages() -> shrink_zones() ?
Which means it's zero and we underflow immediately?
@@ -3207,15 +3228,14 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx)
sc.may_writepage = 1;
/*
- * Now scan the zone in the dma->highmem direction, stopping
- * at the last zone which needs scanning.
- *
- * We do this because the page allocator works in the opposite
- * direction. This prevents the page allocator from allocating
- * pages behind kswapd's direction of progress, which would
- * cause too much scanning of the lower zones.
+ * Continue scanning in the highmem->dma direction stopping at
+ * the last zone which needs scanning. This may reclaim lowmem
+ * pages that are not necessary for zone balancing but it
+ * preserves LRU ordering. It is assumed that the bulk of
+ * allocation requests can use arbitrary zones with the
+ * possible exception of big highmem:lowmem configurations.
*/
- for (i = 0; i <= end_zone; i++) {
+ for (i = end_zone; i >= end_zone; i--) {
i >= 0 ?
--
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