[PATCH v2 7/8] mm/migrate: enable noref migration for jbd2

[Luis Chamberlain <mcgrof@kernel.org>]

From: Davidlohr Bueso <dave@stgolabs.net>

Add semantics to enable future optimizations for buffer head noref jbd2
migration. This adds a new BH_Migrate flag which ensures we can bail
on the lookup path. This should enable jbd2 to get semantics of when
a buffer head is under folio migration, and should yield to it and to
eventually remove the buffer_meta() check skipping current jbd2 folio
migration.

Suggested-by: Jan Kara <jack@suse.cz>
Co-developed-by: Luis Chamberlain <mcgrof@kernel.org>
Signed-off-by: Davidlohr Bueso <dave@stgolabs.net>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
---
 fs/buffer.c                 | 12 +++++++++++-
 fs/ext4/ialloc.c            |  3 ++-
 include/linux/buffer_head.h |  1 +
 mm/migrate.c                |  4 ++++
 4 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/fs/buffer.c b/fs/buffer.c
index 07ec57ec100e..753fc45667da 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -211,6 +211,15 @@ __find_get_block_slow(struct block_device *bdev, sector_t block, bool atomic)
 	head = folio_buffers(folio);
 	if (!head)
 		goto out_unlock;
+	/*
+	 * Upon a noref migration, the folio lock serializes here;
+	 * otherwise bail.
+	 */
+	if (test_bit_acquire(BH_Migrate, &head->b_state)) {
+		WARN_ON(folio_locked);
+		goto out_unlock;
+	}
+
 	bh = head;
 	do {
 		if (!buffer_mapped(bh))
@@ -1393,7 +1402,8 @@ lookup_bh_lru(struct block_device *bdev, sector_t block, unsigned size)
 /*
  * Perform a pagecache lookup for the matching buffer.  If it's there, refresh
  * it in the LRU and mark it as accessed.  If it is not present then return
- * NULL
+ * NULL. Atomic context callers may also return NULL if the buffer is being
+ * migrated; similarly the page is not marked accessed either.
  */
 static struct buffer_head *
 find_get_block_common(struct block_device *bdev, sector_t block,
diff --git a/fs/ext4/ialloc.c b/fs/ext4/ialloc.c
index 38bc8d74f4cc..e7ecc7c8a729 100644
--- a/fs/ext4/ialloc.c
+++ b/fs/ext4/ialloc.c
@@ -691,7 +691,8 @@ static int recently_deleted(struct super_block *sb, ext4_group_t group, int ino)
 	if (!bh || !buffer_uptodate(bh))
 		/*
 		 * If the block is not in the buffer cache, then it
-		 * must have been written out.
+		 * must have been written out, or, most unlikely, is
+		 * being migrated - false failure should be OK here.
 		 */
 		goto out;
 
diff --git a/include/linux/buffer_head.h b/include/linux/buffer_head.h
index 8db10ca288fc..5559b906b1de 100644
--- a/include/linux/buffer_head.h
+++ b/include/linux/buffer_head.h
@@ -34,6 +34,7 @@ enum bh_state_bits {
 	BH_Meta,	/* Buffer contains metadata */
 	BH_Prio,	/* Buffer should be submitted with REQ_PRIO */
 	BH_Defer_Completion, /* Defer AIO completion to workqueue */
+	BH_Migrate,     /* Buffer is being migrated (norefs) */
 
 	BH_PrivateStart,/* not a state bit, but the first bit available
 			 * for private allocation by other entities
diff --git a/mm/migrate.c b/mm/migrate.c
index 32fa72ba10b4..8fed2655f2e8 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -851,6 +851,8 @@ static int __buffer_migrate_folio(struct address_space *mapping,
 		bool busy;
 		bool invalidated = false;
 
+		VM_WARN_ON_ONCE(test_and_set_bit_lock(BH_Migrate,
+						      &head->b_state));
 recheck_buffers:
 		busy = false;
 		spin_lock(&mapping->i_private_lock);
@@ -884,6 +886,8 @@ static int __buffer_migrate_folio(struct address_space *mapping,
 		bh = bh->b_this_page;
 	} while (bh != head);
 unlock_buffers:
+	if (check_refs)
+		clear_bit_unlock(BH_Migrate, &head->b_state);
 	bh = head;
 	do {
 		unlock_buffer(bh);
-- 
2.47.2