mm, compaction: more reliably increase direct compaction priority

[cascardo/linux.git] / mm / compaction.c

diff --git a/mm/compaction.c b/mm/compaction.c
index bbf41ee..29f6c49 100644 (file)
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1392,11 +1392,23 @@ static enum compact_result __compaction_suitable(struct zone *zone, int order,
 
        /*
         * Watermarks for order-0 must be met for compaction to be able to
-        * isolate free pages for migration targets.
+        * isolate free pages for migration targets. This means that the
+        * watermark and alloc_flags have to match, or be more pessimistic than
+        * the check in __isolate_free_page(). We don't use the direct
+        * compactor's alloc_flags, as they are not relevant for freepage
+        * isolation. We however do use the direct compactor's classzone_idx to
+        * skip over zones where lowmem reserves would prevent allocation even
+        * if compaction succeeds.
+        * For costly orders, we require low watermark instead of min for
+        * compaction to proceed to increase its chances.
+        * ALLOC_CMA is used, as pages in CMA pageblocks are considered
+        * suitable migration targets
         */
-       watermark = low_wmark_pages(zone) + compact_gap(order);
+       watermark = (order > PAGE_ALLOC_COSTLY_ORDER) ?
+                               low_wmark_pages(zone) : min_wmark_pages(zone);
+       watermark += compact_gap(order);
        if (!__zone_watermark_ok(zone, 0, watermark, classzone_idx,
-                                alloc_flags, wmark_target))
+                                               ALLOC_CMA, wmark_target))
                return COMPACT_SKIPPED;
 
        /*