mm, compaction: restrict async compaction to pageblocks of same migratetype

The migrate scanner in async compaction is currently limited to
MIGRATE_MOVABLE pageblocks.  This is a heuristic intended to reduce
latency, based on the assumption that non-MOVABLE pageblocks are
unlikely to contain movable pages.

However, with the exception of THP's, most high-order allocations are
not movable.  Should the async compaction succeed, this increases the
chance that the non-MOVABLE allocations will fallback to a MOVABLE
pageblock, making the long-term fragmentation worse.

This patch attempts to help the situation by changing async direct
compaction so that the migrate scanner only scans the pageblocks of the
requested migratetype.  If it's a non-MOVABLE type and there are such
pageblocks that do contain movable pages, chances are that the
allocation can succeed within one of such pageblocks, removing the need
for a fallback.  If that fails, the subsequent sync attempt will ignore
this restriction.

In testing based on 4.9 kernel with stress-highalloc from mmtests
configured for order-4 GFP_KERNEL allocations, this patch has reduced
the number of unmovable allocations falling back to movable pageblocks
by 30%.  The number of movable allocations falling back is reduced by
12%.

Link: http://lkml.kernel.org/r/20170307131545.28577-8-vbabka@suse.cz
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 13 insertions, 7 deletions

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index d90792addeb9..e7486afa7fa7 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3665,6 +3665,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
                                                 struct alloc_context *ac)
 {
         bool can_direct_reclaim = gfp_mask & __GFP_DIRECT_RECLAIM;
+        const bool costly_order = order > PAGE_ALLOC_COSTLY_ORDER;
         struct page *page = NULL;
         unsigned int alloc_flags;
         unsigned long did_some_progress;
@@ -3732,12 +3733,17 @@ retry_cpuset:
 
         /*
          * For costly allocations, try direct compaction first, as it's likely
-         * that we have enough base pages and don't need to reclaim. Don't try
-         * that for allocations that are allowed to ignore watermarks, as the
-         * ALLOC_NO_WATERMARKS attempt didn't yet happen.
+         * that we have enough base pages and don't need to reclaim. For non-
+         * movable high-order allocations, do that as well, as compaction will
+         * try prevent permanent fragmentation by migrating from blocks of the
+         * same migratetype.
+         * Don't try this for allocations that are allowed to ignore
+         * watermarks, as the ALLOC_NO_WATERMARKS attempt didn't yet happen.
          */
-        if (can_direct_reclaim && order > PAGE_ALLOC_COSTLY_ORDER &&
-                !gfp_pfmemalloc_allowed(gfp_mask)) {
+        if (can_direct_reclaim &&
+                        (costly_order ||
+                           (order > 0 && ac->migratetype != MIGRATE_MOVABLE))
+                        && !gfp_pfmemalloc_allowed(gfp_mask)) {
                 page = __alloc_pages_direct_compact(gfp_mask, order,
                                                 alloc_flags, ac,
                                                 INIT_COMPACT_PRIORITY,
@@ -3749,7 +3755,7 @@ retry_cpuset:
                  * Checks for costly allocations with __GFP_NORETRY, which
                  * includes THP page fault allocations
                  */
-                if (gfp_mask & __GFP_NORETRY) {
+                if (costly_order && (gfp_mask & __GFP_NORETRY)) {
                         /*
                          * If compaction is deferred for high-order allocations,
                          * it is because sync compaction recently failed. If
@@ -3830,7 +3836,7 @@ retry:
          * Do not retry costly high order allocations unless they are
          * __GFP_REPEAT
          */
-        if (order > PAGE_ALLOC_COSTLY_ORDER && !(gfp_mask & __GFP_REPEAT))
+        if (costly_order && !(gfp_mask & __GFP_REPEAT))
                 goto nopage;
 
         if (should_reclaim_retry(gfp_mask, order, ac, alloc_flags,