[RFC PATCH v4 3/6] mm/migrate: add batch-copy path in migrate_pages_batch

[Shivank Garg <shivankg@amd.com>]

Split unmapped folios into batch-eligible (src_batch/dst_batch) and
standard (src_std/dst_std) lists, gated by the migrate_offload_enabled
which is off by default. So, when no offload driver is active, the
branch is never taken and everything goes through the standard path.

After TLB flush, batch copy the eligible folios via folios_mc_copy()
and pass already_copied=true into migrate_folios_move() so
__migrate_folio() skips the per-folio copy.

On batch copy failure, already_copied flag stays false and each folio
fall back to individual copy.

Signed-off-by: Shivank Garg <shivankg@amd.com>
---
 mm/migrate.c | 55 +++++++++++++++++++++++++++++++++++++++++-----------
 1 file changed, 44 insertions(+), 11 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index 1d8c1fb627c9..69daa16f9cf3 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -43,6 +43,7 @@
 #include <linux/sched/sysctl.h>
 #include <linux/memory-tiers.h>
 #include <linux/pagewalk.h>
+#include <linux/jump_label.h>
 
 #include <asm/tlbflush.h>
 
@@ -51,6 +52,8 @@
 #include "internal.h"
 #include "swap.h"
 
+DEFINE_STATIC_KEY_FALSE(migrate_offload_enabled);
+
 static const struct movable_operations *offline_movable_ops;
 static const struct movable_operations *zsmalloc_movable_ops;
 
@@ -1706,6 +1709,12 @@ static int migrate_hugetlbs(struct list_head *from, new_folio_t get_new_folio,
 	return nr_failed;
 }
 
+/* movable_ops folios have their own migrate path */
+static bool folio_supports_batch_copy(struct folio *folio)
+{
+	return likely(!page_has_movable_ops(&folio->page));
+}
+
 static void migrate_folios_move(struct list_head *src_folios,
 		struct list_head *dst_folios,
 		free_folio_t put_new_folio, unsigned long private,
@@ -1805,8 +1814,12 @@ static int migrate_pages_batch(struct list_head *from,
 	bool is_large = false;
 	struct folio *folio, *folio2, *dst = NULL;
 	int rc, rc_saved = 0, nr_pages;
-	LIST_HEAD(unmap_folios);
-	LIST_HEAD(dst_folios);
+	unsigned int nr_batch = 0;
+	bool batch_copied = false;
+	LIST_HEAD(src_batch);
+	LIST_HEAD(dst_batch);
+	LIST_HEAD(src_std);
+	LIST_HEAD(dst_std);
 	bool nosplit = (reason == MR_NUMA_MISPLACED);
 
 	VM_WARN_ON_ONCE(mode != MIGRATE_ASYNC &&
@@ -1943,7 +1956,7 @@ static int migrate_pages_batch(struct list_head *from,
 				/* nr_failed isn't updated for not used */
 				stats->nr_thp_failed += thp_retry;
 				rc_saved = rc;
-				if (list_empty(&unmap_folios))
+				if (list_empty(&src_batch) && list_empty(&src_std))
 					goto out;
 				else
 					goto move;
@@ -1953,8 +1966,15 @@ static int migrate_pages_batch(struct list_head *from,
 				nr_retry_pages += nr_pages;
 				break;
 			case 0:
-				list_move_tail(&folio->lru, &unmap_folios);
-				list_add_tail(&dst->lru, &dst_folios);
+				if (static_branch_unlikely(&migrate_offload_enabled) &&
+				    folio_supports_batch_copy(folio)) {
+					list_move_tail(&folio->lru, &src_batch);
+					list_add_tail(&dst->lru, &dst_batch);
+					nr_batch++;
+				} else {
+					list_move_tail(&folio->lru, &src_std);
+					list_add_tail(&dst->lru, &dst_std);
+				}
 				break;
 			default:
 				/*
@@ -1977,17 +1997,28 @@ static int migrate_pages_batch(struct list_head *from,
 	/* Flush TLBs for all unmapped folios */
 	try_to_unmap_flush();
 
+	/* Batch-copy eligible folios before the move phase */
+	if (!list_empty(&src_batch)) {
+		rc = folios_mc_copy(&dst_batch, &src_batch, nr_batch);
+		batch_copied = (rc == 0);
+	}
+
 	retry = 1;
 	for (pass = 0; pass < nr_pass && retry; pass++) {
 		retry = 0;
 		thp_retry = 0;
 		nr_retry_pages = 0;
 
-		/* Move the unmapped folios */
-		migrate_folios_move(&unmap_folios, &dst_folios,
-				put_new_folio, private, mode, reason,
-				ret_folios, stats, &retry, &thp_retry,
-				&nr_failed, &nr_retry_pages, false);
+		if (!list_empty(&src_batch))
+			migrate_folios_move(&src_batch, &dst_batch, put_new_folio,
+					private, mode, reason, ret_folios, stats,
+					&retry, &thp_retry, &nr_failed,
+					&nr_retry_pages, batch_copied);
+		if (!list_empty(&src_std))
+			migrate_folios_move(&src_std, &dst_std,	put_new_folio,
+					private, mode, reason, ret_folios, stats,
+					&retry, &thp_retry, &nr_failed,
+					&nr_retry_pages, false);
 	}
 	nr_failed += retry;
 	stats->nr_thp_failed += thp_retry;
@@ -1996,7 +2027,9 @@ static int migrate_pages_batch(struct list_head *from,
 	rc = rc_saved ? : nr_failed;
 out:
 	/* Cleanup remaining folios */
-	migrate_folios_undo(&unmap_folios, &dst_folios,
+	migrate_folios_undo(&src_batch, &dst_batch,
+			put_new_folio, private, ret_folios);
+	migrate_folios_undo(&src_std, &dst_std,
 			put_new_folio, private, ret_folios);
 
 	return rc;
-- 
2.43.0