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Subject[RFC 4/4] 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/internal.h | 1 +
mm/page_alloc.c | 14 ++++++++++----
2 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/mm/internal.h b/mm/internal.h
index 1fee63010dcc..a46eab383e8d 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -466,6 +466,7 @@ 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 0c00beec9336..8a8ef9ebeb4d 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2163,7 +2163,7 @@ __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;

@@ -2172,7 +2172,7 @@ static struct page *__rmqueue(struct zone *zone, unsigned int order,
if (migratetype == MIGRATE_MOVABLE)
page = __rmqueue_cma_fallback(zone, order);

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

@@ -2193,7 +2193,7 @@ 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;

@@ -2626,7 +2626,10 @@ 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)
@@ -3583,6 +3586,9 @@ __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)
--
2.10.0
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