[PATCH v7 0/3] [PATCH v7 0/3] mm/swap: use swap_ops to register swap device's methods

[PATCH v7 0/3] [PATCH v7 0/3] mm/swap: use swap_ops to register swap device's methods

[Baoquan He]

This can simplify the code logic and benefit any new type of swap device
added later.

Patch 2 renames the page_io functions to a consistent
swap_<backend>_<op>_<folio> naming convention. Patch 3 fixes a
leaky abstraction where FS swap unplug bypassed swap_ops, adding
an .unplug callback and dispatcher.

Changlog:
===
v7:
  - Drop the old patch 1 "[PATCH v6 1/3] mm/swap: rename mm/page_io.c
    to mm/swap_io.c" as Christoph suggested.
  - Add the .unplug callback to address Christoph's review about
    the leaky abstraction.
  - Minor cleanups and fixes per review feedback.

-v6:
  - Fix a code bug Kairui found out when reviewing patch in mm/swapfile.c
    in patch 2/3. Has fixed it by moving it to appropriate place and add
    comment to explain.

-v5:
  - Change the return value of init_swap_ops() as -EINVAL as per Chris's
    suggestion and adjust its invocation in swapon() accordingly.
  - Add Chris and Usama's Ack tags.

-v4:
  - Fix a typo opeations -> operations
  - Fix a code bug inside init_swap_ops(). I was taking a change at the
    time, thought the change is trivial, so I only compiled but didn't
    run kernel to test in v3. Now fix it and test passed.
    Thanaks to Usama for catching the above two issues.

-v3:
  - Rename setup_swap_ops() to init_swap_ops() which reflect the function
    behaviour a little better
  - Check if sis->ops, sis->ops->read_folio and sis->ops->write_folio is
    NULL in init_swap_ops(), but not spread them where they are called.
    And once the checking failed, fail swapon immediately. This is
    suggested by Chris.
  - Call init_swap_ops() before setup_swap_extents() invocation. This
    doesn't harm anything and can benefit later adding sis->ops->swap_activate
    method.

-v2:
  - Lots of cleanup for patch 2/3: renaming, moving data
    structures, and using const properly
  - Collected tags from Kairui, Nhat and Barry

-v1:
  - https://lore.kernel.org/linux-mm/20260302104016.163542-1-bhe@redhat.com/

Baoquan He (3):
  mm/swap: use swap_ops to register swap device's methods
  mm/page_io.c: rename page_io functions to consistent naming
  mm/swap: add unplug callback to swap_ops to fix leaky abstraction

 include/linux/swap.h |   3 ++
 mm/page_io.c         | 115 ++++++++++++++++++++++++++-----------------
 mm/swap.h            |  11 ++++-
 mm/swapfile.c        |   9 ++++
 mm/zswap.c           |   2 +-
 5 files changed, 94 insertions(+), 46 deletions(-)

-- 
2.52.0

[PATCH v7 1/3] mm/swap: use swap_ops to register swap device's methods

[Baoquan He]

This simplifies codes and makes logic clearer. And also makes later any
new swap device type being added easier to handle.

Currently there are three types of swap devices: bdev_fs, bdev_sync
and bdev_async, and only operations read_folio and write_folio are
included. In the future, there could be more swap device types added
and more appropriate operations adapted into swap_ops.

Suggested-by: Chris Li <chrisl@kernel.org>
Signed-off-by: Baoquan He <baoquan.he@linux.dev>
---
 include/linux/swap.h |   3 ++
 mm/page_io.c         | 104 +++++++++++++++++++++++++------------------
 mm/swap.h            |  10 ++++-
 mm/swapfile.c        |   9 ++++
 mm/zswap.c           |   2 +-
 5 files changed, 83 insertions(+), 45 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 7a09df6977a5..0d045fc8ec35 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -20,6 +20,8 @@ struct notifier_block;
 
 struct bio;
 
+struct swap_ops;
+
 #define SWAP_FLAG_PREFER	0x8000	/* set if swap priority specified */
 #define SWAP_FLAG_PRIO_MASK	0x7fff
 #define SWAP_FLAG_DISCARD	0x10000 /* enable discard for swap */
@@ -282,6 +284,7 @@ struct swap_info_struct {
 	struct work_struct reclaim_work; /* reclaim worker */
 	struct list_head discard_clusters; /* discard clusters list */
 	struct plist_node avail_list;   /* entry in swap_avail_head */
+	const struct swap_ops *ops;
 };
 
 static inline swp_entry_t page_swap_entry(struct page *page)
diff --git a/mm/page_io.c b/mm/page_io.c
index 70cea9e24d2f..2b520af88376 100644
--- a/mm/page_io.c
+++ b/mm/page_io.c
@@ -240,6 +240,9 @@ static void swap_zeromap_folio_clear(struct folio *folio)
 int swap_writeout(struct folio *folio, struct swap_iocb **swap_plug)
 {
 	int ret = 0;
+	struct swap_info_struct *sis = __swap_entry_to_info(folio->swap);
+
+	VM_WARN_ON_FOLIO(!folio_test_swapcache(folio), folio);
 
 	if (folio_free_swap(folio))
 		goto out_unlock;
@@ -285,7 +288,7 @@ int swap_writeout(struct folio *folio, struct swap_iocb **swap_plug)
 	}
 	rcu_read_unlock();
 
-	__swap_writepage(folio, swap_plug);
+	sis->ops->write_folio(sis, folio, swap_plug);
 	return 0;
 out_unlock:
 	folio_unlock(folio);
@@ -375,10 +378,11 @@ static void sio_write_complete(struct kiocb *iocb, long ret)
 	mempool_free(sio, sio_pool);
 }
 
-static void swap_writepage_fs(struct folio *folio, struct swap_iocb **swap_plug)
+static void swap_writepage_fs(struct swap_info_struct *sis,
+			      struct folio *folio,
+			      struct swap_iocb **swap_plug)
 {
 	struct swap_iocb *sio = swap_plug ? *swap_plug : NULL;
-	struct swap_info_struct *sis = __swap_entry_to_info(folio->swap);
 	struct file *swap_file = sis->swap_file;
 	loff_t pos = swap_dev_pos(folio->swap);
 
@@ -411,8 +415,9 @@ static void swap_writepage_fs(struct folio *folio, struct swap_iocb **swap_plug)
 		*swap_plug = sio;
 }
 
-static void swap_writepage_bdev_sync(struct folio *folio,
-		struct swap_info_struct *sis)
+static void swap_writepage_bdev_sync(struct swap_info_struct *sis,
+				     struct folio *folio,
+				     struct swap_iocb **plug)
 {
 	struct bio_vec bv;
 	struct bio bio;
@@ -431,8 +436,9 @@ static void swap_writepage_bdev_sync(struct folio *folio,
 	__end_swap_bio_write(&bio);
 }
 
-static void swap_writepage_bdev_async(struct folio *folio,
-		struct swap_info_struct *sis)
+static void swap_writepage_bdev_async(struct swap_info_struct *sis,
+				      struct folio *folio,
+				      struct swap_iocb **plug)
 {
 	struct bio *bio;
 
@@ -448,29 +454,6 @@ static void swap_writepage_bdev_async(struct folio *folio,
 	submit_bio(bio);
 }
 
-void __swap_writepage(struct folio *folio, struct swap_iocb **swap_plug)
-{
-	struct swap_info_struct *sis = __swap_entry_to_info(folio->swap);
-
-	VM_BUG_ON_FOLIO(!folio_test_swapcache(folio), folio);
-	/*
-	 * ->flags can be updated non-atomically,
-	 * but that will never affect SWP_FS_OPS, so the data_race
-	 * is safe.
-	 */
-	if (data_race(sis->flags & SWP_FS_OPS))
-		swap_writepage_fs(folio, swap_plug);
-	/*
-	 * ->flags can be updated non-atomically,
-	 * but that will never affect SWP_SYNCHRONOUS_IO, so the data_race
-	 * is safe.
-	 */
-	else if (data_race(sis->flags & SWP_SYNCHRONOUS_IO))
-		swap_writepage_bdev_sync(folio, sis);
-	else
-		swap_writepage_bdev_async(folio, sis);
-}
-
 void swap_write_unplug(struct swap_iocb *sio)
 {
 	struct iov_iter from;
@@ -539,9 +522,10 @@ static bool swap_read_folio_zeromap(struct folio *folio)
 	return true;
 }
 
-static void swap_read_folio_fs(struct folio *folio, struct swap_iocb **plug)
+static void swap_read_folio_fs(struct swap_info_struct *sis,
+			       struct folio *folio,
+			       struct swap_iocb **plug)
 {
-	struct swap_info_struct *sis = __swap_entry_to_info(folio->swap);
 	struct swap_iocb *sio = NULL;
 	loff_t pos = swap_dev_pos(folio->swap);
 
@@ -573,8 +557,9 @@ static void swap_read_folio_fs(struct folio *folio, struct swap_iocb **plug)
 		*plug = sio;
 }
 
-static void swap_read_folio_bdev_sync(struct folio *folio,
-		struct swap_info_struct *sis)
+static void swap_read_folio_bdev_sync(struct swap_info_struct *sis,
+				      struct folio *folio,
+				      struct swap_iocb **plug)
 {
 	struct bio_vec bv;
 	struct bio bio;
@@ -595,8 +580,9 @@ static void swap_read_folio_bdev_sync(struct folio *folio,
 	put_task_struct(current);
 }
 
-static void swap_read_folio_bdev_async(struct folio *folio,
-		struct swap_info_struct *sis)
+static void swap_read_folio_bdev_async(struct swap_info_struct *sis,
+				       struct folio *folio,
+				       struct swap_iocb **plug)
 {
 	struct bio *bio;
 
@@ -610,6 +596,44 @@ static void swap_read_folio_bdev_async(struct folio *folio,
 	submit_bio(bio);
 }
 
+static const struct swap_ops bdev_fs_swap_ops = {
+	.read_folio = swap_read_folio_fs,
+	.write_folio = swap_writepage_fs,
+};
+
+static const struct swap_ops bdev_sync_swap_ops = {
+	.read_folio = swap_read_folio_bdev_sync,
+	.write_folio = swap_writepage_bdev_sync,
+};
+
+static const struct swap_ops bdev_async_swap_ops = {
+	.read_folio = swap_read_folio_bdev_async,
+	.write_folio = swap_writepage_bdev_async,
+};
+
+int init_swap_ops(struct swap_info_struct *sis)
+{
+	/*
+	 * ->flags can be updated non-atomically, but that will
+	 * never affect SWP_FS_OPS, so the data_race is safe.
+	 */
+	if (data_race(sis->flags & SWP_FS_OPS))
+		sis->ops = &bdev_fs_swap_ops;
+	/*
+	 * ->flags can be updated non-atomically, but that will
+	 * never affect SWP_SYNCHRONOUS_IO, so the data_race is safe.
+	 */
+	else if (data_race(sis->flags & SWP_SYNCHRONOUS_IO))
+		sis->ops = &bdev_sync_swap_ops;
+	else
+		sis->ops = &bdev_async_swap_ops;
+
+	if (!sis->ops || !sis->ops->read_folio || !sis->ops->write_folio)
+		return -EINVAL;
+
+	return 0;
+}
+
 void swap_read_folio(struct folio *folio, struct swap_iocb **plug)
 {
 	struct swap_info_struct *sis = __swap_entry_to_info(folio->swap);
@@ -644,13 +668,7 @@ void swap_read_folio(struct folio *folio, struct swap_iocb **plug)
 	/* We have to read from slower devices. Increase zswap protection. */
 	zswap_folio_swapin(folio);
 
-	if (data_race(sis->flags & SWP_FS_OPS)) {
-		swap_read_folio_fs(folio, plug);
-	} else if (synchronous) {
-		swap_read_folio_bdev_sync(folio, sis);
-	} else {
-		swap_read_folio_bdev_async(folio, sis);
-	}
+	sis->ops->read_folio(sis, folio, plug);
 
 finish:
 	if (workingset) {
diff --git a/mm/swap.h b/mm/swap.h
index a77016f2423b..8d4375f91632 100644
--- a/mm/swap.h
+++ b/mm/swap.h
@@ -217,6 +217,15 @@ extern void __swap_cluster_free_entries(struct swap_info_struct *si,
 /* linux/mm/page_io.c */
 int sio_pool_init(void);
 struct swap_iocb;
+struct swap_ops {
+	void (*read_folio)(struct swap_info_struct *sis,
+			struct folio *folio,
+			struct swap_iocb **plug);
+	void (*write_folio)(struct swap_info_struct *sis,
+			struct folio *folio,
+			struct swap_iocb **plug);
+};
+int init_swap_ops(struct swap_info_struct *sis);
 void swap_read_folio(struct folio *folio, struct swap_iocb **plug);
 void __swap_read_unplug(struct swap_iocb *plug);
 static inline void swap_read_unplug(struct swap_iocb *plug)
@@ -226,7 +235,6 @@ static inline void swap_read_unplug(struct swap_iocb *plug)
 }
 void swap_write_unplug(struct swap_iocb *sio);
 int swap_writeout(struct folio *folio, struct swap_iocb **swap_plug);
-void __swap_writepage(struct folio *folio, struct swap_iocb **swap_plug);
 
 /* linux/mm/swap_state.c */
 extern struct address_space swap_space __read_mostly;
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 9174f1eeffb0..29ae79d0fa2e 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -3597,6 +3597,15 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 		}
 	}
 
+	/*
+	 * init_swap_ops() sets si->ops based on flags. It does not need
+	 * swapon_mutex, and must complete before enable_swap_info()
+	 * exposes the device.
+	 */
+	error = init_swap_ops(si);
+	if (error)
+		goto bad_swap_unlock_inode;
+
 	error = zswap_swapon(si->type, maxpages);
 	if (error)
 		goto bad_swap_unlock_inode;
diff --git a/mm/zswap.c b/mm/zswap.c
index 4b5149173b0e..192401f46de4 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -1054,7 +1054,7 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
 	folio_set_reclaim(folio);
 
 	/* start writeback */
-	__swap_writepage(folio, NULL);
+	si->ops->write_folio(si, folio, NULL);
 
 out:
 	if (ret && ret != -EEXIST) {
-- 
2.52.0

[PATCH v7 2/3] mm/page_io.c: rename page_io functions to consistent naming

[Baoquan He]

Rename the swap I/O functions to use a consistent
swap_<backend>_<op>_<folio> naming scheme, so the backend type
immediately follows the swap_ prefix. The new names align with
swap_ops callbacks: .write_folio and .read_folio.

  swap_writepage_fs          ->  swap_fs_write_folio
  swap_writepage_bdev_sync   ->  swap_bdev_sync_write_folio
  swap_writepage_bdev_async  ->  swap_bdev_async_write_folio
  swap_read_folio_fs         ->  swap_fs_read_folio
  swap_read_folio_bdev_sync  ->  swap_bdev_sync_read_folio
  swap_read_folio_bdev_async ->  swap_bdev_async_read_folio

Signed-off-by: Baoquan He <baoquan.he@linux.dev>
---
 mm/page_io.c | 24 ++++++++++++------------
 1 file changed, 12 insertions(+), 12 deletions(-)

diff --git a/mm/page_io.c b/mm/page_io.c
index 2b520af88376..38b94c560c37 100644
--- a/mm/page_io.c
+++ b/mm/page_io.c
@@ -378,7 +378,7 @@ static void sio_write_complete(struct kiocb *iocb, long ret)
 	mempool_free(sio, sio_pool);
 }
 
-static void swap_writepage_fs(struct swap_info_struct *sis,
+static void swap_fs_write_folio(struct swap_info_struct *sis,
 			      struct folio *folio,
 			      struct swap_iocb **swap_plug)
 {
@@ -415,7 +415,7 @@ static void swap_writepage_fs(struct swap_info_struct *sis,
 		*swap_plug = sio;
 }
 
-static void swap_writepage_bdev_sync(struct swap_info_struct *sis,
+static void swap_bdev_sync_write_folio(struct swap_info_struct *sis,
 				     struct folio *folio,
 				     struct swap_iocb **plug)
 {
@@ -436,7 +436,7 @@ static void swap_writepage_bdev_sync(struct swap_info_struct *sis,
 	__end_swap_bio_write(&bio);
 }
 
-static void swap_writepage_bdev_async(struct swap_info_struct *sis,
+static void swap_bdev_async_write_folio(struct swap_info_struct *sis,
 				      struct folio *folio,
 				      struct swap_iocb **plug)
 {
@@ -522,7 +522,7 @@ static bool swap_read_folio_zeromap(struct folio *folio)
 	return true;
 }
 
-static void swap_read_folio_fs(struct swap_info_struct *sis,
+static void swap_fs_read_folio(struct swap_info_struct *sis,
 			       struct folio *folio,
 			       struct swap_iocb **plug)
 {
@@ -557,7 +557,7 @@ static void swap_read_folio_fs(struct swap_info_struct *sis,
 		*plug = sio;
 }
 
-static void swap_read_folio_bdev_sync(struct swap_info_struct *sis,
+static void swap_bdev_sync_read_folio(struct swap_info_struct *sis,
 				      struct folio *folio,
 				      struct swap_iocb **plug)
 {
@@ -580,7 +580,7 @@ static void swap_read_folio_bdev_sync(struct swap_info_struct *sis,
 	put_task_struct(current);
 }
 
-static void swap_read_folio_bdev_async(struct swap_info_struct *sis,
+static void swap_bdev_async_read_folio(struct swap_info_struct *sis,
 				       struct folio *folio,
 				       struct swap_iocb **plug)
 {
@@ -597,18 +597,18 @@ static void swap_read_folio_bdev_async(struct swap_info_struct *sis,
 }
 
 static const struct swap_ops bdev_fs_swap_ops = {
-	.read_folio = swap_read_folio_fs,
-	.write_folio = swap_writepage_fs,
+	.read_folio = swap_fs_read_folio,
+	.write_folio = swap_fs_write_folio,
 };
 
 static const struct swap_ops bdev_sync_swap_ops = {
-	.read_folio = swap_read_folio_bdev_sync,
-	.write_folio = swap_writepage_bdev_sync,
+	.read_folio = swap_bdev_sync_read_folio,
+	.write_folio = swap_bdev_sync_write_folio,
 };
 
 static const struct swap_ops bdev_async_swap_ops = {
-	.read_folio = swap_read_folio_bdev_async,
-	.write_folio = swap_writepage_bdev_async,
+	.read_folio = swap_bdev_async_read_folio,
+	.write_folio = swap_bdev_async_write_folio,
 };
 
 int init_swap_ops(struct swap_info_struct *sis)
-- 
2.52.0

[PATCH v7 3/3] mm/swap: add unplug callback to swap_ops to fix leaky abstraction

[Baoquan He]

When swap_ops was introduced, the FS-swap batch submission remained
as a standalone swap_write_unplug() that directly called
mapping->a_ops->swap_rw(). This meant callers still had implicit
knowledge of filesystem internals rather than going through the
swap_ops abstraction.

Fix this by adding an unplug callback to struct swap_ops.
Each ops table provides its own implementation:
 - bdev_fs_swap_ops uses the existing FS batch-submission logic
 - bdev_sync/bdev_async_swap_ops leave it NULL since block-layer
   plugging handles their I/O

The swap_iocb now carries a pointer to its ops table so that
swap_write_unplug() can dispatch through the callback without
the caller needing to know the swap device type.

Signed-off-by: Baoquan He <baoquan.he@linux.dev>
---
 mm/page_io.c | 11 ++++++++++-
 mm/swap.h    |  1 +
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/mm/page_io.c b/mm/page_io.c
index 38b94c560c37..2c36d261ad98 100644
--- a/mm/page_io.c
+++ b/mm/page_io.c
@@ -329,6 +329,7 @@ static void bio_associate_blkg_from_page(struct bio *bio, struct folio *folio)
 
 struct swap_iocb {
 	struct kiocb		iocb;
+	const struct swap_ops   *ops;
 	struct bio_vec		bvec[SWAP_CLUSTER_MAX];
 	int			pages;
 	int			len;
@@ -401,6 +402,7 @@ static void swap_fs_write_folio(struct swap_info_struct *sis,
 		init_sync_kiocb(&sio->iocb, swap_file);
 		sio->iocb.ki_complete = sio_write_complete;
 		sio->iocb.ki_pos = pos;
+		sio->ops = sis->ops;
 		sio->pages = 0;
 		sio->len = 0;
 	}
@@ -454,7 +456,7 @@ static void swap_bdev_async_write_folio(struct swap_info_struct *sis,
 	submit_bio(bio);
 }
 
-void swap_write_unplug(struct swap_iocb *sio)
+static void swap_fs_write_folio_unplug(struct swap_iocb *sio)
 {
 	struct iov_iter from;
 	struct address_space *mapping = sio->iocb.ki_filp->f_mapping;
@@ -466,6 +468,12 @@ void swap_write_unplug(struct swap_iocb *sio)
 		sio_write_complete(&sio->iocb, ret);
 }
 
+void swap_write_unplug(struct swap_iocb *sio)
+{
+	if (sio->ops && sio->ops->unplug)
+		sio->ops->unplug(sio);
+}
+
 static void sio_read_complete(struct kiocb *iocb, long ret)
 {
 	struct swap_iocb *sio = container_of(iocb, struct swap_iocb, iocb);
@@ -599,6 +607,7 @@ static void swap_bdev_async_read_folio(struct swap_info_struct *sis,
 static const struct swap_ops bdev_fs_swap_ops = {
 	.read_folio = swap_fs_read_folio,
 	.write_folio = swap_fs_write_folio,
+	.unplug = swap_fs_write_folio_unplug,
 };
 
 static const struct swap_ops bdev_sync_swap_ops = {
diff --git a/mm/swap.h b/mm/swap.h
index 8d4375f91632..67e3b1617146 100644
--- a/mm/swap.h
+++ b/mm/swap.h
@@ -224,6 +224,7 @@ struct swap_ops {
 	void (*write_folio)(struct swap_info_struct *sis,
 			struct folio *folio,
 			struct swap_iocb **plug);
+	void (*unplug)(struct swap_iocb *sio);
 };
 int init_swap_ops(struct swap_info_struct *sis);
 void swap_read_folio(struct folio *folio, struct swap_iocb **plug);
-- 
2.52.0