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

Luis Chamberlain
2025-04-10T01:49:51Z

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