[V2] mm: compaction: skip memory compaction when there are not enough migratable pages

Commit Message

Ge Yang Jan. 8, 2025, 8:37 a.m. UTC

From: yangge <[e-mail]>

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.

During the start-up of the virtual machine, it will call
pin_user_pages_remote(..., FOLL_LONGTERM, ...) to allocate memory.
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.
However, if there aren't enough migratable pages available, performing
memory compaction is also meaningless. Besides checking whether
the order-0 watermark is met, __compaction_suitable() also needs
to determine whether there are sufficient migratable pages available
for memory compaction.

For costly allocations, because __compaction_suitable() always
returns true, __alloc_pages_slowpath() can't exit at the appropriate
place, 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

When the 16G of non-CMA memory on a single node is exhausted, we will
fallback to allocating memory on other nodes. In order to quickly
fallback to remote nodes, we should skip memory compaction when
migratable pages are insufficient. After this fix, it only takes a
few tens of seconds to start a 32GB virtual machine with device
passthrough functionality.

Signed-off-by: yangge <[e-mail]>
---

V2:
- consider unevictable folios 

 mm/compaction.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

Patch

diff --git a/mm/compaction.c b/mm/compaction.c
index 07bd227..1630abd 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -2383,7 +2383,27 @@  static bool __compaction_suitable(struct zone *zone, int order,
 				  int highest_zoneidx,
 				  unsigned long wmark_target)
 {
+	struct pglist_data *pgdat = zone->zone_pgdat;
+	unsigned long sum, nr_pinned;
 	unsigned long watermark;
+
+	sum = node_page_state(pgdat, NR_INACTIVE_FILE) +
+		node_page_state(pgdat, NR_INACTIVE_ANON) +
+		node_page_state(pgdat, NR_ACTIVE_FILE) +
+		node_page_state(pgdat, NR_ACTIVE_ANON) +
+		node_page_state(pgdat, NR_UNEVICTABLE);
+
+	nr_pinned = node_page_state(pgdat, NR_FOLL_PIN_ACQUIRED) -
+		node_page_state(pgdat, NR_FOLL_PIN_RELEASED);
+
+	/*
+	 * Gup-pinned pages are non-migratable. After subtracting these pages,
+	 * we need to check if the remaining pages are sufficient for memory
+	 * compaction.
+	 */
+	if ((sum - nr_pinned) < (1 << order))
+		return false;
+
 	/*
 	 * Watermarks for order-0 must be met for compaction to be able to
 	 * isolate free pages for migration targets. This means that the