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Subject[RFC v2 09/10] mm, page_alloc: disallow migratetype fallback in fastpath
Date
The previous patch has adjusted async compaction so that it helps against
longterm fragmentation when compacting for a non-MOVABLE high-order allocation.
The goal of this patch is to force such allocations go through compaction
once before being allowed to fallback to a pageblock of different migratetype
(e.g. MOVABLE). In contexts where compaction is not allowed (and for order-0
allocations), this delayed fallback possibility can still help by trying a
different zone where fallback might not be needed and potentially waking up
kswapd earlier.

Not-yet-signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
mm/compaction.c | 22 +++++++++++++++++-----
mm/internal.h | 2 ++
mm/page_alloc.c | 15 +++++++++++----
3 files changed, 30 insertions(+), 9 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index cef77a5fffea..bb18d21c6a56 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1357,9 +1357,11 @@ static enum compact_result __compact_finished(struct zone *zone,
#endif
/*
* Job done if allocation would steal freepages from
- * other migratetype buddy lists.
+ * other migratetype buddy lists. This is not allowed
+ * for async direct compaction.
*/
- if (find_suitable_fallback(area, order, migratetype,
+ if (!cc->prevent_fallback &&
+ find_suitable_fallback(area, order, migratetype,
true, &can_steal) != -1) {

/* movable pages are OK in any pageblock */
@@ -1530,8 +1532,17 @@ static enum compact_result compact_zone(struct zone *zone, struct compact_contro
cc->migratetype = gfpflags_to_migratetype(cc->gfp_mask);
ret = compaction_suitable(zone, cc->order, cc->alloc_flags,
cc->classzone_idx);
- /* Compaction is likely to fail */
- if (ret == COMPACT_SUCCESS || ret == COMPACT_SKIPPED)
+ /*
+ * Compaction should not be needed. If we don't allow stealing from
+ * pageblocks of different migratetype, the watermark checks cannot
+ * distinguish that, so assume we would need to steal, and leave the
+ * thorough check to compact_finished().
+ */
+ if (ret == COMPACT_SUCCESS && !cc->prevent_fallback)
+ return ret;
+
+ /* Compaction is likely to fail due to insufficient free pages */
+ if (ret == COMPACT_SKIPPED)
return ret;

/* huh, compaction_suitable is returning something unexpected */
@@ -1699,7 +1710,8 @@ static enum compact_result compact_zone_order(struct zone *zone, int order,
.direct_compaction = true,
.whole_zone = (prio == MIN_COMPACT_PRIORITY),
.ignore_skip_hint = (prio == MIN_COMPACT_PRIORITY),
- .ignore_block_suitable = (prio == MIN_COMPACT_PRIORITY)
+ .ignore_block_suitable = (prio == MIN_COMPACT_PRIORITY),
+ .prevent_fallback = (prio == COMPACT_PRIO_ASYNC)
};
INIT_LIST_HEAD(&cc.freepages);
INIT_LIST_HEAD(&cc.migratepages);
diff --git a/mm/internal.h b/mm/internal.h
index cdb33c957906..1b7a89a9a9d7 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -189,6 +189,7 @@ struct compact_control {
bool whole_zone; /* Whole zone should/has been scanned */
bool contended; /* Signal lock or sched contention */
bool finishing_block; /* Finishing current pageblock */
+ bool prevent_fallback; /* Stealing migratetypes not allowed */
};

unsigned long
@@ -467,6 +468,7 @@ unsigned long reclaim_clean_pages_from_list(struct zone *zone,
#define ALLOC_HIGH 0x20 /* __GFP_HIGH set */
#define ALLOC_CPUSET 0x40 /* check for correct cpuset */
#define ALLOC_CMA 0x80 /* allow allocations from CMA areas */
+#define ALLOC_FALLBACK 0x100 /* allow fallback of migratetype */

enum ttu_flags;
struct tlbflush_unmap_batch;
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 6d9ba640a12d..5270be8325fd 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2197,7 +2197,7 @@ __rmqueue_fallback(struct zone *zone, unsigned int order, int start_migratetype)
* Call me with the zone->lock already held.
*/
static struct page *__rmqueue(struct zone *zone, unsigned int order,
- int migratetype)
+ int migratetype, bool allow_fallback)
{
struct page *page;

@@ -2207,7 +2207,8 @@ static struct page *__rmqueue(struct zone *zone, unsigned int order,
if (migratetype == MIGRATE_MOVABLE)
page = __rmqueue_cma_fallback(zone, order);

- if (!page && __rmqueue_fallback(zone, order, migratetype))
+ if (!page && allow_fallback &&
+ __rmqueue_fallback(zone, order, migratetype))
goto retry;
}

@@ -2228,7 +2229,7 @@ static int rmqueue_bulk(struct zone *zone, unsigned int order,

spin_lock(&zone->lock);
for (i = 0; i < count; ++i) {
- struct page *page = __rmqueue(zone, order, migratetype);
+ struct page *page = __rmqueue(zone, order, migratetype, true);
if (unlikely(page == NULL))
break;

@@ -2661,7 +2662,10 @@ struct page *buffered_rmqueue(struct zone *preferred_zone,
trace_mm_page_alloc_zone_locked(page, order, migratetype);
}
if (!page)
- page = __rmqueue(zone, order, migratetype);
+ page = __rmqueue(zone, order, migratetype,
+ alloc_flags &
+ (ALLOC_FALLBACK |
+ ALLOC_NO_WATERMARKS));
} while (page && check_new_pages(page, order));
spin_unlock(&zone->lock);
if (!page)
@@ -3616,6 +3620,9 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
}
}

+ /* async direct compaction didn't help, now allow fallback */
+ alloc_flags |= ALLOC_FALLBACK;
+
retry:
/* Ensure kswapd doesn't accidentally go to sleep as long as we loop */
if (gfp_mask & __GFP_KSWAPD_RECLAIM)
--
2.11.0
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