[PATCH v2 2/3] mm: page_alloc: make the high-order watermark check honour ALLOC_NOFRAGMENT

["Kiryl Shutsemau (Meta)" <kas@kernel.org>]

With defrag_mode, every slowpath allocation carries ALLOC_NOFRAGMENT.  A
request below pageblock_order may take a page of its own migratetype or
claim a whole free block, but not steal from another migratetype's block.
The high-order part of the watermark check still accepts a free page of
any migratetype.

Under memory pressure, movable allocations take free movable pages as
soon as reclaim frees them.  Free space in unmovable and reclaimable
pageblocks is off-limits to them under ALLOC_NOFRAGMENT, and builds up.
The free pages of order 4 and up end up mostly in those pageblocks.

A 64K mTHP fault cannot take them, but direct compaction's up-front check
counts them and returns COMPACT_SUCCESS without compacting anything.  The
allocation then fails.  Because compaction "succeeded", it is never
deferred, and every fault stalls in it again before falling back to small
pages.

Dev Jain's benchmark faults 64K mTHPs under swap pressure, in a 7G VM
with only the 64K size enabled.  With defrag_mode, those faults stall in
compaction 18 times as often and get about a third of the folios
(medians of 15 runs):

                              defrag_mode=0  defrag_mode=1  defrag_mode=1
                                                            + this patch
    compact_stall                      1690          30669          2607
    compact_fail                        621          30531           906
    64K faults allocated              20750           7391         11538
    64K faults falling back to 4K       725          13832          9945

Make __zone_watermark_ok_mt() skip other migratetypes' free lists below
pageblock_order for an ALLOC_NOFRAGMENT request.  Give
__compact_finished() the same rule for its fallback test, so that
compaction, once it runs, stops on a page the request can take.

zone_watermark_fast() calls __zone_watermark_ok_mt() too.  With
defrag_mode, an allocation now wakes kswapd and starts reclaim or
compaction as soon as the free lists hold nothing it can take.  For the
orders the per-cpu lists serve, that is earlier than before: the check
does not count their pages, so the allocation no longer uses up the local
list first.  A request that may not wake kswapd or reclaim, such as an
mTHP fault with THP defrag=never, falls back to a smaller folio instead.

Fixes: e3aa7df331bc ("mm: page_alloc: defrag_mode")
Reported-by: Dev Jain <dev.jain@arm.com>
Closes: https://lore.kernel.org/all/4aa14b41-9b89-494c-8825-55fffd5a541c@arm.com/
Tested-by: Dev Jain <dev.jain@arm.com>
Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
---
 mm/compaction.c |  5 +++++
 mm/page_alloc.c | 21 ++++++++++++++++++---
 2 files changed, 23 insertions(+), 3 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index 6c553d0e90f8..7b2267bbf115 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -2365,6 +2365,11 @@ static enum compact_result __compact_finished(struct compact_control *cc)
 			!free_area_empty(area, MIGRATE_CMA))
 			return COMPACT_SUCCESS;
 #endif
+		/* Under ALLOC_NOFRAGMENT it may only claim a whole block */
+		if ((cc->alloc_flags & ALLOC_NOFRAGMENT) &&
+		    order < pageblock_order)
+			continue;
+
 		/*
 		 * Job done if allocation would steal freepages from
 		 * other migratetype buddy lists.
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 7730660bdd09..d53fb2c67ae1 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3586,6 +3586,11 @@ static inline long __zone_watermark_unusable_free(struct zone *z,
  * will return true of the order-0 watermark is reached and there is at least
  * one free page of a suitable size. Checking now avoids taking the zone lock
  * to check in the allocation paths if no pages are free.
+ *
+ * Under ALLOC_NOFRAGMENT, a request below pageblock_order can take another
+ * migratetype's page only by claiming the whole block, so only free pages of
+ * @migratetype count below pageblock_order.  Without ALLOC_NOFRAGMENT,
+ * @migratetype is not used.
  */
 bool __zone_watermark_ok_mt(struct zone *z, unsigned int order,
 			    unsigned long mark, int highest_zoneidx,
@@ -3648,6 +3653,9 @@ bool __zone_watermark_ok_mt(struct zone *z, unsigned int order,
 			continue;
 
 		for (mt = 0; mt < MIGRATE_PCPTYPES; mt++) {
+			if ((alloc_flags & ALLOC_NOFRAGMENT) &&
+			    mt != migratetype && o < pageblock_order)
+				continue;
 			if (!free_area_empty(area, mt))
 				return true;
 		}
@@ -3677,8 +3685,12 @@ bool __zone_watermark_ok(struct zone *z, unsigned int order, unsigned long mark,
 			 int highest_zoneidx, unsigned int alloc_flags,
 			 long free_pages)
 {
+	/* Serving an ALLOC_NOFRAGMENT request needs __zone_watermark_ok_mt() */
+	VM_WARN_ON_ONCE(order && order < pageblock_order &&
+			(alloc_flags & ALLOC_NOFRAGMENT));
 	return __zone_watermark_ok_mt(z, order, mark, highest_zoneidx,
-				      alloc_flags, free_pages, MIGRATE_MOVABLE);
+				      alloc_flags & ~ALLOC_NOFRAGMENT,
+				      free_pages, MIGRATE_MOVABLE);
 }
 
 bool zone_watermark_ok(struct zone *z, unsigned int order, unsigned long mark,
@@ -4685,10 +4697,13 @@ should_reclaim_retry(gfp_t gfp_mask, unsigned order,
 
 		/*
 		 * Would the allocation succeed if we reclaimed all
-		 * reclaimable pages?
+		 * reclaimable pages?  Count free pages of every migratetype:
+		 * reclaim and compaction work across migratetypes to free a
+		 * whole pageblock, which the allocation can then claim.
 		 */
 		wmark = __zone_watermark_ok(zone, order, min_wmark,
-				ac->highest_zoneidx, alloc_flags, available);
+				ac->highest_zoneidx,
+				alloc_flags & ~ALLOC_NOFRAGMENT, available);
 		trace_reclaim_retry_zone(z, order, reclaimable,
 				available, min_wmark, *no_progress_loops, wmark);
 		if (wmark) {
-- 
2.54.0