[PATCH -v3 5/9] migrate_pages: batch _unmap and _move

[Huang Ying]

In this patch the _unmap and _move stage of the folio migration is
batched.  That for, previously, it is,

  for each folio
    _unmap()
    _move()

Now, it is,

  for each folio
    _unmap()
  for each folio
    _move()

Based on this, we can batch the TLB flushing and use some hardware
accelerator to copy folios between batched _unmap and batched _move
stages.

Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
Cc: Zi Yan <ziy@xxxxxxxxxx>
Cc: Yang Shi <shy828301@xxxxxxxxx>
Cc: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
Cc: Oscar Salvador <osalvador@xxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Bharata B Rao <bharata@xxxxxxx>
Cc: Alistair Popple <apopple@xxxxxxxxxx>
Cc: haoxin <xhao@xxxxxxxxxxxxxxxxx>
Cc: Minchan Kim <minchan@xxxxxxxxxx>
---
 mm/migrate.c | 207 +++++++++++++++++++++++++++++++++++++++++++++------
 1 file changed, 183 insertions(+), 24 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index 0428449149f4..143d96775b4d 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1033,6 +1033,33 @@ static void __migrate_folio_extract(struct folio *dst,
 	dst->private = NULL;
 }
 
+/* Restore the source folio to the original state upon failure */
+static void migrate_folio_undo_src(struct folio *src,
+				   int page_was_mapped,
+				   struct anon_vma *anon_vma,
+				   struct list_head *ret)
+{
+	if (page_was_mapped)
+		remove_migration_ptes(src, src, false);
+	/* Drop an anon_vma reference if we took one */
+	if (anon_vma)
+		put_anon_vma(anon_vma);
+	folio_unlock(src);
+	list_move_tail(&src->lru, ret);
+}
+
+/* Restore the destination folio to the original state upon failure */
+static void migrate_folio_undo_dst(struct folio *dst,
+				   free_page_t put_new_page,
+				   unsigned long private)
+{
+	folio_unlock(dst);
+	if (put_new_page)
+		put_new_page(&dst->page, private);
+	else
+		folio_put(dst);
+}
+
 /* Cleanup src folio upon migration success */
 static void migrate_folio_done(struct folio *src,
 			       enum migrate_reason reason)
@@ -1052,7 +1079,7 @@ static void migrate_folio_done(struct folio *src,
 }
 
 static int __migrate_folio_unmap(struct folio *src, struct folio *dst,
-				int force, enum migrate_mode mode)
+				 int force, bool force_lock, enum migrate_mode mode)
 {
 	int rc = -EAGAIN;
 	int page_was_mapped = 0;
@@ -1079,6 +1106,17 @@ static int __migrate_folio_unmap(struct folio *src, struct folio *dst,
 		if (current->flags & PF_MEMALLOC)
 			goto out;
 
+		/*
+		 * We have locked some folios, to avoid deadlock, we cannot
+		 * lock the folio synchronously.  Go out to process (and
+		 * unlock) all the locked folios.  Then we can lock the folio
+		 * synchronously.
+		 */
+		if (!force_lock) {
+			rc = -EDEADLOCK;
+			goto out;
+		}
+
 		folio_lock(src);
 	}
 
@@ -1191,6 +1229,10 @@ static int __migrate_folio_move(struct folio *src, struct folio *dst,
 	__migrate_folio_extract(dst, &page_was_mapped, &anon_vma);
 
 	rc = move_to_new_folio(dst, src, mode);
+
+	if (rc != -EAGAIN)
+		list_del(&dst->lru);
+
 	if (unlikely(!is_lru))
 		goto out_unlock_both;
 
@@ -1209,6 +1251,11 @@ static int __migrate_folio_move(struct folio *src, struct folio *dst,
 			lru_add_drain();
 	}
 
+	if (rc == -EAGAIN) {
+		__migrate_folio_record(dst, page_was_mapped, anon_vma);
+		return rc;
+	}
+
 	if (page_was_mapped)
 		remove_migration_ptes(src,
 			rc == MIGRATEPAGE_SUCCESS ? dst : src, false);
@@ -1233,7 +1280,7 @@ static int __migrate_folio_move(struct folio *src, struct folio *dst,
 /* Obtain the lock on page, remove all ptes. */
 static int migrate_folio_unmap(new_page_t get_new_page, free_page_t put_new_page,
 			       unsigned long private, struct folio *src,
-			       struct folio **dstp, int force,
+			       struct folio **dstp, int force, bool force_lock,
 			       enum migrate_mode mode, enum migrate_reason reason,
 			       struct list_head *ret)
 {
@@ -1261,7 +1308,7 @@ static int migrate_folio_unmap(new_page_t get_new_page, free_page_t put_new_page
 	*dstp = dst;
 
 	dst->private = NULL;
-	rc = __migrate_folio_unmap(src, dst, force, mode);
+	rc = __migrate_folio_unmap(src, dst, force, force_lock, mode);
 	if (rc == MIGRATEPAGE_UNMAP)
 		return rc;
 
@@ -1270,7 +1317,7 @@ static int migrate_folio_unmap(new_page_t get_new_page, free_page_t put_new_page
 	 * references and be restored.
 	 */
 	/* restore the folio to right list. */
-	if (rc != -EAGAIN)
+	if (rc != -EAGAIN && rc != -EDEADLOCK)
 		list_move_tail(&src->lru, ret);
 
 	if (put_new_page)
@@ -1309,9 +1356,8 @@ static int migrate_folio_move(free_page_t put_new_page, unsigned long private,
 	 */
 	if (rc == MIGRATEPAGE_SUCCESS) {
 		migrate_folio_done(src, reason);
-	} else {
-		if (rc != -EAGAIN)
-			list_add_tail(&src->lru, ret);
+	} else if (rc != -EAGAIN) {
+		list_add_tail(&src->lru, ret);
 
 		if (put_new_page)
 			put_new_page(&dst->page, private);
@@ -1591,7 +1637,7 @@ static int migrate_pages_batch(struct list_head *from, new_page_t get_new_page,
 		enum migrate_mode mode, int reason, struct list_head *ret_folios,
 		struct migrate_pages_stats *stats)
 {
-	int retry = 1;
+	int retry;
 	int large_retry = 1;
 	int thp_retry = 1;
 	int nr_failed = 0;
@@ -1600,13 +1646,19 @@ static int migrate_pages_batch(struct list_head *from, new_page_t get_new_page,
 	int pass = 0;
 	bool is_large = false;
 	bool is_thp = false;
-	struct folio *folio, *folio2, *dst = NULL;
-	int rc, nr_pages;
+	struct folio *folio, *folio2, *dst = NULL, *dst2;
+	int rc, rc_saved, nr_pages;
 	LIST_HEAD(split_folios);
+	LIST_HEAD(unmap_folios);
+	LIST_HEAD(dst_folios);
 	bool nosplit = (reason == MR_NUMA_MISPLACED);
 	bool no_split_folio_counting = false;
+	bool force_lock;
 
-split_folio_migration:
+retry:
+	rc_saved = 0;
+	force_lock = true;
+	retry = 1;
 	for (pass = 0;
 	     pass < NR_MAX_MIGRATE_PAGES_RETRY && (retry || large_retry);
 	     pass++) {
@@ -1628,16 +1680,15 @@ static int migrate_pages_batch(struct list_head *from, new_page_t get_new_page,
 			cond_resched();
 
 			rc = migrate_folio_unmap(get_new_page, put_new_page, private,
-						 folio, &dst, pass > 2, mode,
-						 reason, ret_folios);
-			if (rc == MIGRATEPAGE_UNMAP)
-				rc = migrate_folio_move(put_new_page, private,
-							folio, dst, mode,
-							reason, ret_folios);
+						 folio, &dst, pass > 2, force_lock,
+						 mode, reason, ret_folios);
 			/*
 			 * The rules are:
 			 *	Success: folio will be freed
+			 *	Unmap: folio will be put on unmap_folios list,
+			 *	       dst folio put on dst_folios list
 			 *	-EAGAIN: stay on the from list
+			 *	-EDEADLOCK: stay on the from list
 			 *	-ENOMEM: stay on the from list
 			 *	-ENOSYS: stay on the from list
 			 *	Other errno: put on ret_folios list
@@ -1672,7 +1723,7 @@ static int migrate_pages_batch(struct list_head *from, new_page_t get_new_page,
 			case -ENOMEM:
 				/*
 				 * When memory is low, don't bother to try to migrate
-				 * other folios, just exit.
+				 * other folios, move unmapped folios, then exit.
 				 */
 				if (is_large) {
 					nr_large_failed++;
@@ -1711,7 +1762,19 @@ static int migrate_pages_batch(struct list_head *from, new_page_t get_new_page,
 				/* nr_failed isn't updated for not used */
 				nr_large_failed += large_retry;
 				stats->nr_thp_failed += thp_retry;
-				goto out;
+				rc_saved = rc;
+				if (list_empty(&unmap_folios))
+					goto out;
+				else
+					goto move;
+			case -EDEADLOCK:
+				/*
+				 * The folio cannot be locked for potential deadlock.
+				 * Go move (and unlock) all locked folios.  Then we can
+				 * try again.
+				 */
+				rc_saved = rc;
+				goto move;
 			case -EAGAIN:
 				if (is_large) {
 					large_retry++;
@@ -1725,6 +1788,15 @@ static int migrate_pages_batch(struct list_head *from, new_page_t get_new_page,
 				stats->nr_succeeded += nr_pages;
 				stats->nr_thp_succeeded += is_thp;
 				break;
+			case MIGRATEPAGE_UNMAP:
+				/*
+				 * We have locked some folios, don't force lock
+				 * to avoid deadlock.
+				 */
+				force_lock = false;
+				list_move_tail(&folio->lru, &unmap_folios);
+				list_add_tail(&dst->lru, &dst_folios);
+				break;
 			default:
 				/*
 				 * Permanent failure (-EBUSY, etc.):
@@ -1748,12 +1820,95 @@ static int migrate_pages_batch(struct list_head *from, new_page_t get_new_page,
 	nr_large_failed += large_retry;
 	stats->nr_thp_failed += thp_retry;
 	stats->nr_failed_pages += nr_retry_pages;
+move:
+	retry = 1;
+	for (pass = 0;
+	     pass < NR_MAX_MIGRATE_PAGES_RETRY && (retry || large_retry);
+	     pass++) {
+		retry = 0;
+		large_retry = 0;
+		thp_retry = 0;
+		nr_retry_pages = 0;
+
+		dst = list_first_entry(&dst_folios, struct folio, lru);
+		dst2 = list_next_entry(dst, lru);
+		list_for_each_entry_safe(folio, folio2, &unmap_folios, lru) {
+			is_large = folio_test_large(folio);
+			is_thp = is_large && folio_test_pmd_mappable(folio);
+			nr_pages = folio_nr_pages(folio);
+
+			cond_resched();
+
+			rc = migrate_folio_move(put_new_page, private,
+						folio, dst, mode,
+						reason, ret_folios);
+			/*
+			 * The rules are:
+			 *	Success: folio will be freed
+			 *	-EAGAIN: stay on the unmap_folios list
+			 *	Other errno: put on ret_folios list
+			 */
+			switch(rc) {
+			case -EAGAIN:
+				if (is_large) {
+					large_retry++;
+					thp_retry += is_thp;
+				} else if (!no_split_folio_counting) {
+					retry++;
+				}
+				nr_retry_pages += nr_pages;
+				break;
+			case MIGRATEPAGE_SUCCESS:
+				stats->nr_succeeded += nr_pages;
+				stats->nr_thp_succeeded += is_thp;
+				break;
+			default:
+				if (is_large) {
+					nr_large_failed++;
+					stats->nr_thp_failed += is_thp;
+				} else if (!no_split_folio_counting) {
+					nr_failed++;
+				}
+
+				stats->nr_failed_pages += nr_pages;
+				break;
+			}
+			dst = dst2;
+			dst2 = list_next_entry(dst, lru);
+		}
+	}
+	nr_failed += retry;
+	nr_large_failed += large_retry;
+	stats->nr_thp_failed += thp_retry;
+	stats->nr_failed_pages += nr_retry_pages;
+
+	if (rc_saved)
+		rc = rc_saved;
+	else
+		rc = nr_failed + nr_large_failed;
+out:
+	/* Cleanup remaining folios */
+	dst = list_first_entry(&dst_folios, struct folio, lru);
+	dst2 = list_next_entry(dst, lru);
+	list_for_each_entry_safe(folio, folio2, &unmap_folios, lru) {
+		int page_was_mapped = 0;
+		struct anon_vma *anon_vma = NULL;
+
+		__migrate_folio_extract(dst, &page_was_mapped, &anon_vma);
+		migrate_folio_undo_src(folio, page_was_mapped, anon_vma,
+				       ret_folios);
+		list_del(&dst->lru);
+		migrate_folio_undo_dst(dst, put_new_page, private);
+		dst = dst2;
+		dst2 = list_next_entry(dst, lru);
+	}
+
 	/*
 	 * Try to migrate split folios of fail-to-migrate large folios, no
 	 * nr_failed counting in this round, since all split folios of a
 	 * large folio is counted as 1 failure in the first round.
 	 */
-	if (!list_empty(&split_folios)) {
+	if (rc >= 0 && !list_empty(&split_folios)) {
 		/*
 		 * Move non-migrated folios (after NR_MAX_MIGRATE_PAGES_RETRY
 		 * retries) to ret_folios to avoid migrating them again.
@@ -1761,12 +1916,16 @@ static int migrate_pages_batch(struct list_head *from, new_page_t get_new_page,
 		list_splice_init(from, ret_folios);
 		list_splice_init(&split_folios, from);
 		no_split_folio_counting = true;
-		retry = 1;
-		goto split_folio_migration;
+		goto retry;
 	}
 
-	rc = nr_failed + nr_large_failed;
-out:
+	/*
+	 * We have unlocked all locked folios, so we can force lock now, let's
+	 * try again.
+	 */
+	if (rc == -EDEADLOCK)
+		goto retry;
+
 	return rc;
 }
 
-- 
2.35.1