[PATCH V2] mm: compaction: use the actual allocation context to determine the watermarks for costly order during async memory compaction

[yangge1116@xxxxxxx]

From: yangge <yangge1116@xxxxxxx>

There are 4 NUMA nodes on my machine, and each NUMA node has 32GB
of memory. I have configured 16GB of CMA memory on each NUMA node,
and starting a 32GB virtual machine with device passthrough is
extremely slow, taking almost an hour.

Long term GUP cannot allocate memory from CMA area, so a maximum of
16 GB of no-CMA memory on a NUMA node can be used as virtual machine
memory. There is 16GB of free CMA memory on a NUMA node, which is
sufficient to pass the order-0 watermark check, causing the
__compaction_suitable() function to  consistently return true.

For costly allocations, if the __compaction_suitable() function always
returns true, it causes the __alloc_pages_slowpath() function to fail
to exit at the appropriate point. This prevents timely fallback to
allocating memory on other nodes, ultimately resulting in excessively
long virtual machine startup times.
Call trace:
__alloc_pages_slowpath
    if (compact_result == COMPACT_SKIPPED ||
        compact_result == COMPACT_DEFERRED)
        goto nopage; // should exit __alloc_pages_slowpath() from here

We could use the real unmovable allocation context to have
__zone_watermark_unusable_free() subtract CMA pages, and thus we won't
pass the order-0 check anymore once the non-CMA part is exhausted. There
is some risk that in some different scenario the compaction could in
fact migrate pages from the exhausted non-CMA part of the zone to the
CMA part and succeed, and we'll skip it instead. But only __GFP_NORETRY
allocations should be affected in the immediate "goto nopage" when
compaction is skipped, others will attempt with DEF_COMPACT_PRIORITY
anyway and won't fail without trying to compact-migrate the non-CMA
pageblocks into CMA pageblocks first, so it should be fine.

After this fix, it only takes a few tens of seconds to start a 32GB
virtual machine with device passthrough functionality.

Link: https://lore.kernel.org/lkml/1736335854-548-1-git-send-email-yangge1116@xxxxxxx/
Signed-off-by: yangge <yangge1116@xxxxxxx>
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
---

V2:
- update code and message suggested by Vlastimil

 mm/compaction.c | 29 +++++++++++++++++++++++++----
 1 file changed, 25 insertions(+), 4 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index 07bd227..3de7b67 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -2490,7 +2490,8 @@ bool compaction_zonelist_suitable(struct alloc_context *ac, int order,
  */
 static enum compact_result
 compaction_suit_allocation_order(struct zone *zone, unsigned int order,
-				 int highest_zoneidx, unsigned int alloc_flags)
+				 int highest_zoneidx, unsigned int alloc_flags,
+				 bool async)
 {
 	unsigned long watermark;
 
@@ -2499,6 +2500,23 @@ compaction_suit_allocation_order(struct zone *zone, unsigned int order,
 			      alloc_flags))
 		return COMPACT_SUCCESS;
 
+	/*
+	 * For unmovable allocations (without ALLOC_CMA), check if there is enough
+	 * free memory in the non-CMA pageblocks. Otherwise compaction could form
+	 * the high-order page in CMA pageblocks, which would not help the
+	 * allocation to succeed. However, limit the check to costly order async
+	 * compaction (such as opportunistic THP attempts) because there is the
+	 * possibility that compaction would migrate pages from non-CMA to CMA
+	 * pageblock.
+	 */
+	if (order > PAGE_ALLOC_COSTLY_ORDER && async &&
+	    !(alloc_flags & ALLOC_CMA)) {
+		watermark = low_wmark_pages(zone) + compact_gap(order);
+		if (!__zone_watermark_ok(zone, 0, watermark, highest_zoneidx,
+					   0, zone_page_state(zone, NR_FREE_PAGES)))
+			return COMPACT_SKIPPED;
+	}
+
 	if (!compaction_suitable(zone, order, highest_zoneidx))
 		return COMPACT_SKIPPED;
 
@@ -2534,7 +2552,8 @@ compact_zone(struct compact_control *cc, struct capture_control *capc)
 	if (!is_via_compact_memory(cc->order)) {
 		ret = compaction_suit_allocation_order(cc->zone, cc->order,
 						       cc->highest_zoneidx,
-						       cc->alloc_flags);
+						       cc->alloc_flags,
+						       cc->mode == MIGRATE_ASYNC);
 		if (ret != COMPACT_CONTINUE)
 			return ret;
 	}
@@ -3037,7 +3056,8 @@ static bool kcompactd_node_suitable(pg_data_t *pgdat)
 
 		ret = compaction_suit_allocation_order(zone,
 				pgdat->kcompactd_max_order,
-				highest_zoneidx, ALLOC_WMARK_MIN);
+				highest_zoneidx, ALLOC_WMARK_MIN,
+				false);
 		if (ret == COMPACT_CONTINUE)
 			return true;
 	}
@@ -3078,7 +3098,8 @@ static void kcompactd_do_work(pg_data_t *pgdat)
 			continue;
 
 		ret = compaction_suit_allocation_order(zone,
-				cc.order, zoneid, ALLOC_WMARK_MIN);
+				cc.order, zoneid, ALLOC_WMARK_MIN,
+				false);
 		if (ret != COMPACT_CONTINUE)
 			continue;
 
-- 
2.7.4