[PATCH v2 0/3] mm/swap: use swap_ops to register swap device's methods

[PATCH v2 0/3] mm/swap: use swap_ops to register swap device's methods

[Barry Song]

From: Barry Song <baohua@kernel.org>

-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/

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

And also do renaming in this patchset:
-------
   file renaming:
   ---
   mm/page_io.c to mm/swap_io.c

   function renaming:
   ---
   swap_writepage_* to swap_write_folio_* in file mm/swap_io.c

Baoquan He (3):
  mm/swap: rename mm/page_io.c to mm/swap_io.c
  mm/swap: use swap_ops to register swap device's methods
  mm/swap_io.c: rename swap_writepage_* to swap_write_folio_*

 MAINTAINERS                 |   2 +-
 include/linux/swap.h        |   2 +
 mm/Makefile                 |   2 +-
 mm/swap.h                   |  12 ++++-
 mm/{page_io.c => swap_io.c} | 101 ++++++++++++++++++++----------------
 mm/swapfile.c               |   1 +
 mm/zswap.c                  |   3 +-
 7 files changed, 73 insertions(+), 50 deletions(-)
 rename mm/{page_io.c => swap_io.c} (89%)

-- 
2.39.3 (Apple Git-146)

[PATCH v2 1/3] mm/swap: rename mm/page_io.c to mm/swap_io.c

[Barry Song]

From: Baoquan He <bhe@redhat.com>

Codes in mm/page_io.c are only related to swap io, it has
nothing to do with other page io.

Rename it to avoid confusion.

Reviewed-by: Nhat Pham <nphamcs@gmail.com>
Reviewed-by: Barry Song <baohua@kernel.org>
Acked-by: Kairui Song <kasong@tencent.com>
Signed-off-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Barry Song <baohua@kernel.org>
---
 MAINTAINERS                 | 2 +-
 mm/Makefile                 | 2 +-
 mm/swap.h                   | 2 +-
 mm/{page_io.c => swap_io.c} | 2 --
 4 files changed, 3 insertions(+), 5 deletions(-)
 rename mm/{page_io.c => swap_io.c} (99%)

diff --git a/MAINTAINERS b/MAINTAINERS
index 16874c32e288..bb7c1031886d 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -16915,7 +16915,7 @@ F:	Documentation/mm/swap-table.rst
 F:	include/linux/swap.h
 F:	include/linux/swapfile.h
 F:	include/linux/swapops.h
-F:	mm/page_io.c
+F:	mm/swap_io.c
 F:	mm/swap.c
 F:	mm/swap.h
 F:	mm/swap_table.h
diff --git a/mm/Makefile b/mm/Makefile
index 8ad2ab08244e..a65ac900096a 100644
--- a/mm/Makefile
+++ b/mm/Makefile
@@ -75,7 +75,7 @@ ifdef CONFIG_MMU
 	obj-$(CONFIG_ADVISE_SYSCALLS)	+= madvise.o
 endif
 
-obj-$(CONFIG_SWAP)	+= page_io.o swap_state.o swapfile.o
+obj-$(CONFIG_SWAP)	+= swap_io.o swap_state.o swapfile.o
 obj-$(CONFIG_ZSWAP)	+= zswap.o
 obj-$(CONFIG_HAS_DMA)	+= dmapool.o
 obj-$(CONFIG_HUGETLBFS)	+= hugetlb.o hugetlb_sysfs.o hugetlb_sysctl.o
diff --git a/mm/swap.h b/mm/swap.h
index a77016f2423b..161185057993 100644
--- a/mm/swap.h
+++ b/mm/swap.h
@@ -214,7 +214,7 @@ extern void __swap_cluster_free_entries(struct swap_info_struct *si,
 					struct swap_cluster_info *ci,
 					unsigned int ci_off, unsigned int nr_pages);
 
-/* linux/mm/page_io.c */
+/* linux/mm/swap_io.c */
 int sio_pool_init(void);
 struct swap_iocb;
 void swap_read_folio(struct folio *folio, struct swap_iocb **plug);
diff --git a/mm/page_io.c b/mm/swap_io.c
similarity index 99%
rename from mm/page_io.c
rename to mm/swap_io.c
index 70cea9e24d2f..91b33d955e63 100644
--- a/mm/page_io.c
+++ b/mm/swap_io.c
@@ -1,7 +1,5 @@
 // SPDX-License-Identifier: GPL-2.0
 /*
- *  linux/mm/page_io.c
- *
  *  Copyright (C) 1991, 1992, 1993, 1994  Linus Torvalds
  *
  *  Swap reorganised 29.12.95, 
-- 
2.39.3 (Apple Git-146)

Re: [PATCH v2 1/3] mm/swap: rename mm/page_io.c to mm/swap_io.c

[Chris Li]

Hi Barry,

Thanks for the patch.

Acked-by: Chris Li <chrisl@kernel.org>

Chris

On Sat, Mar 28, 2026 at 12:58 AM Barry Song <21cnbao@gmail.com> wrote:
>
> From: Baoquan He <bhe@redhat.com>
>
> Codes in mm/page_io.c are only related to swap io, it has
> nothing to do with other page io.
>
> Rename it to avoid confusion.
>
> Reviewed-by: Nhat Pham <nphamcs@gmail.com>
> Reviewed-by: Barry Song <baohua@kernel.org>
> Acked-by: Kairui Song <kasong@tencent.com>
> Signed-off-by: Baoquan He <bhe@redhat.com>
> Signed-off-by: Barry Song <baohua@kernel.org>
> ---
>  MAINTAINERS                 | 2 +-
>  mm/Makefile                 | 2 +-
>  mm/swap.h                   | 2 +-
>  mm/{page_io.c => swap_io.c} | 2 --
>  4 files changed, 3 insertions(+), 5 deletions(-)
>  rename mm/{page_io.c => swap_io.c} (99%)
>
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 16874c32e288..bb7c1031886d 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -16915,7 +16915,7 @@ F:      Documentation/mm/swap-table.rst
>  F:     include/linux/swap.h
>  F:     include/linux/swapfile.h
>  F:     include/linux/swapops.h
> -F:     mm/page_io.c
> +F:     mm/swap_io.c
>  F:     mm/swap.c
>  F:     mm/swap.h
>  F:     mm/swap_table.h
> diff --git a/mm/Makefile b/mm/Makefile
> index 8ad2ab08244e..a65ac900096a 100644
> --- a/mm/Makefile
> +++ b/mm/Makefile
> @@ -75,7 +75,7 @@ ifdef CONFIG_MMU
>         obj-$(CONFIG_ADVISE_SYSCALLS)   += madvise.o
>  endif
>
> -obj-$(CONFIG_SWAP)     += page_io.o swap_state.o swapfile.o
> +obj-$(CONFIG_SWAP)     += swap_io.o swap_state.o swapfile.o
>  obj-$(CONFIG_ZSWAP)    += zswap.o
>  obj-$(CONFIG_HAS_DMA)  += dmapool.o
>  obj-$(CONFIG_HUGETLBFS)        += hugetlb.o hugetlb_sysfs.o hugetlb_sysctl.o
> diff --git a/mm/swap.h b/mm/swap.h
> index a77016f2423b..161185057993 100644
> --- a/mm/swap.h
> +++ b/mm/swap.h
> @@ -214,7 +214,7 @@ extern void __swap_cluster_free_entries(struct swap_info_struct *si,
>                                         struct swap_cluster_info *ci,
>                                         unsigned int ci_off, unsigned int nr_pages);
>
> -/* linux/mm/page_io.c */
> +/* linux/mm/swap_io.c */
>  int sio_pool_init(void);
>  struct swap_iocb;
>  void swap_read_folio(struct folio *folio, struct swap_iocb **plug);
> diff --git a/mm/page_io.c b/mm/swap_io.c
> similarity index 99%
> rename from mm/page_io.c
> rename to mm/swap_io.c
> index 70cea9e24d2f..91b33d955e63 100644
> --- a/mm/page_io.c
> +++ b/mm/swap_io.c
> @@ -1,7 +1,5 @@
>  // SPDX-License-Identifier: GPL-2.0
>  /*
> - *  linux/mm/page_io.c
> - *
>   *  Copyright (C) 1991, 1992, 1993, 1994  Linus Torvalds
>   *
>   *  Swap reorganised 29.12.95,
> --
> 2.39.3 (Apple Git-146)
>

[PATCH v2 2/3] mm/swap: use swap_ops to register swap device's methods

[Barry Song]

From: Baoquan He <bhe@redhat.com>

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 opeations adapted into swap_ops.

Suggested-by: Chris Li <chrisl@kernel.org>
Signed-off-by: Baoquan He <bhe@redhat.com>
Co-developed-by: Barry Song <baohua@kernel.org>
Signed-off-by: Barry Song <baohua@kernel.org>
---
 include/linux/swap.h |  2 +
 mm/swap.h            | 10 ++++-
 mm/swap_io.c         | 99 +++++++++++++++++++++++++-------------------
 mm/swapfile.c        |  1 +
 mm/zswap.c           |  3 +-
 5 files changed, 70 insertions(+), 45 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 1930f81e6be4..b52f30dad72b 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -243,6 +243,7 @@ struct swap_sequential_cluster {
 	unsigned int next[SWAP_NR_ORDERS]; /* Likely next allocation offset */
 };
 
+struct swap_ops;
 /*
  * The in-memory structure used to track swap areas.
  */
@@ -283,6 +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/swap.h b/mm/swap.h
index 161185057993..219dbcb3ffb1 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/swap_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);
+};
+void setup_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/swap_io.c b/mm/swap_io.c
index 91b33d955e63..ad0895af6ed8 100644
--- a/mm/swap_io.c
+++ b/mm/swap_io.c
@@ -238,6 +238,7 @@ 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);
 
 	if (folio_free_swap(folio))
 		goto out_unlock;
@@ -283,7 +284,8 @@ int swap_writeout(struct folio *folio, struct swap_iocb **swap_plug)
 	}
 	rcu_read_unlock();
 
-	__swap_writepage(folio, swap_plug);
+	VM_WARN_ON_ONCE(!sis->ops || !sis->ops->write_folio);
+	sis->ops->write_folio(sis, folio, swap_plug);
 	return 0;
 out_unlock:
 	folio_unlock(folio);
@@ -373,10 +375,11 @@ 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);
 
@@ -409,8 +412,9 @@ 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;
@@ -429,8 +433,9 @@ 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;
 
@@ -446,29 +451,6 @@ 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;
@@ -537,9 +519,10 @@ 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);
 
@@ -571,8 +554,9 @@ 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;
@@ -593,8 +577,9 @@ 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;
 
@@ -608,6 +593,39 @@ 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,
+};
+
+void setup_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;
+}
+
 void swap_read_folio(struct folio *folio, struct swap_iocb **plug)
 {
 	struct swap_info_struct *sis = __swap_entry_to_info(folio->swap);
@@ -642,13 +660,8 @@ 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);
-	}
+	VM_WARN_ON_ONCE(!sis->ops || !sis->ops->read_folio);
+	sis->ops->read_folio(sis, folio, plug);
 
 finish:
 	if (workingset) {
diff --git a/mm/swapfile.c b/mm/swapfile.c
index ff315b752afd..c9eb95141c8f 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -3732,6 +3732,7 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 	si->list.prio = -si->prio;
 	si->avail_list.prio = -si->prio;
 	si->swap_file = swap_file;
+	setup_swap_ops(si);
 
 	/* Sets SWP_WRITEOK, resurrect the percpu ref, expose the swap device */
 	enable_swap_info(si);
diff --git a/mm/zswap.c b/mm/zswap.c
index 4b5149173b0e..9bacb1733e1c 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -1054,7 +1054,8 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
 	folio_set_reclaim(folio);
 
 	/* start writeback */
-	__swap_writepage(folio, NULL);
+	VM_WARN_ON_ONCE(!sis->ops || !sis->ops->write_folio);
+	si->ops->write_folio(si, folio, NULL);
 
 out:
 	if (ret && ret != -EEXIST) {
-- 
2.39.3 (Apple Git-146)

Re: [PATCH v2 2/3] mm/swap: use swap_ops to register swap device's methods

[kernel test robot]

Hi Barry,

kernel test robot noticed the following build errors:

[auto build test ERROR on akpm-mm/mm-everything]

url:    https://github.com/intel-lab-lkp/linux/commits/Barry-Song/mm-swap-rename-mm-page_io-c-to-mm-swap_io-c/20260328-170852
base:   https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-everything
patch link:    https://lore.kernel.org/r/20260328075812.11060-3-21cnbao%40gmail.com
patch subject: [PATCH v2 2/3] mm/swap: use swap_ops to register swap device's methods
config: arm-randconfig-001-20260329 (https://download.01.org/0day-ci/archive/20260329/202603291844.TWh2Ellp-lkp@intel.com/config)
compiler: clang version 23.0.0git (https://github.com/llvm/llvm-project 054e11d1a17e5ba88bb1a8ef32fad3346e80b186)
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20260329/202603291844.TWh2Ellp-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202603291844.TWh2Ellp-lkp@intel.com/

All errors (new ones prefixed by >>):

>> mm/zswap.c:1057:19: error: use of undeclared identifier 'sis'; did you mean 'si'?
    1057 |         VM_WARN_ON_ONCE(!sis->ops || !sis->ops->write_folio);
         |                          ^~~
         |                          si
   include/linux/mmdebug.h:133:52: note: expanded from macro 'VM_WARN_ON_ONCE'
     133 | #define VM_WARN_ON_ONCE(cond) BUILD_BUG_ON_INVALID(cond)
         |                                                    ^~~~
   include/linux/build_bug.h:30:63: note: expanded from macro 'BUILD_BUG_ON_INVALID'
      30 | #define BUILD_BUG_ON_INVALID(e) ((void)(sizeof((__force long)(e))))
         |                                                               ^
   mm/zswap.c:995:27: note: 'si' declared here
     995 |         struct swap_info_struct *si;
         |                                  ^
   mm/zswap.c:1057:32: error: use of undeclared identifier 'sis'; did you mean 'si'?
    1057 |         VM_WARN_ON_ONCE(!sis->ops || !sis->ops->write_folio);
         |                                       ^~~
         |                                       si
   include/linux/mmdebug.h:133:52: note: expanded from macro 'VM_WARN_ON_ONCE'
     133 | #define VM_WARN_ON_ONCE(cond) BUILD_BUG_ON_INVALID(cond)
         |                                                    ^~~~
   include/linux/build_bug.h:30:63: note: expanded from macro 'BUILD_BUG_ON_INVALID'
      30 | #define BUILD_BUG_ON_INVALID(e) ((void)(sizeof((__force long)(e))))
         |                                                               ^
   mm/zswap.c:995:27: note: 'si' declared here
     995 |         struct swap_info_struct *si;
         |                                  ^
   2 errors generated.


vim +1057 mm/zswap.c

   971	
   972	/*********************************
   973	* writeback code
   974	**********************************/
   975	/*
   976	 * Attempts to free an entry by adding a folio to the swap cache,
   977	 * decompressing the entry data into the folio, and issuing a
   978	 * bio write to write the folio back to the swap device.
   979	 *
   980	 * This can be thought of as a "resumed writeback" of the folio
   981	 * to the swap device.  We are basically resuming the same swap
   982	 * writeback path that was intercepted with the zswap_store()
   983	 * in the first place.  After the folio has been decompressed into
   984	 * the swap cache, the compressed version stored by zswap can be
   985	 * freed.
   986	 */
   987	static int zswap_writeback_entry(struct zswap_entry *entry,
   988					 swp_entry_t swpentry)
   989	{
   990		struct xarray *tree;
   991		pgoff_t offset = swp_offset(swpentry);
   992		struct folio *folio;
   993		struct mempolicy *mpol;
   994		bool folio_was_allocated;
   995		struct swap_info_struct *si;
   996		int ret = 0;
   997	
   998		/* try to allocate swap cache folio */
   999		si = get_swap_device(swpentry);
  1000		if (!si)
  1001			return -EEXIST;
  1002	
  1003		mpol = get_task_policy(current);
  1004		folio = swap_cache_alloc_folio(swpentry, GFP_KERNEL, mpol,
  1005					       NO_INTERLEAVE_INDEX, &folio_was_allocated);
  1006		put_swap_device(si);
  1007		if (!folio)
  1008			return -ENOMEM;
  1009	
  1010		/*
  1011		 * Found an existing folio, we raced with swapin or concurrent
  1012		 * shrinker. We generally writeback cold folios from zswap, and
  1013		 * swapin means the folio just became hot, so skip this folio.
  1014		 * For unlikely concurrent shrinker case, it will be unlinked
  1015		 * and freed when invalidated by the concurrent shrinker anyway.
  1016		 */
  1017		if (!folio_was_allocated) {
  1018			ret = -EEXIST;
  1019			goto out;
  1020		}
  1021	
  1022		/*
  1023		 * folio is locked, and the swapcache is now secured against
  1024		 * concurrent swapping to and from the slot, and concurrent
  1025		 * swapoff so we can safely dereference the zswap tree here.
  1026		 * Verify that the swap entry hasn't been invalidated and recycled
  1027		 * behind our backs, to avoid overwriting a new swap folio with
  1028		 * old compressed data. Only when this is successful can the entry
  1029		 * be dereferenced.
  1030		 */
  1031		tree = swap_zswap_tree(swpentry);
  1032		if (entry != xa_load(tree, offset)) {
  1033			ret = -ENOMEM;
  1034			goto out;
  1035		}
  1036	
  1037		if (!zswap_decompress(entry, folio)) {
  1038			ret = -EIO;
  1039			goto out;
  1040		}
  1041	
  1042		xa_erase(tree, offset);
  1043	
  1044		count_vm_event(ZSWPWB);
  1045		if (entry->objcg)
  1046			count_objcg_events(entry->objcg, ZSWPWB, 1);
  1047	
  1048		zswap_entry_free(entry);
  1049	
  1050		/* folio is up to date */
  1051		folio_mark_uptodate(folio);
  1052	
  1053		/* move it to the tail of the inactive list after end_writeback */
  1054		folio_set_reclaim(folio);
  1055	
  1056		/* start writeback */
> 1057		VM_WARN_ON_ONCE(!sis->ops || !sis->ops->write_folio);
  1058		si->ops->write_folio(si, folio, NULL);
  1059	
  1060	out:
  1061		if (ret && ret != -EEXIST) {
  1062			swap_cache_del_folio(folio);
  1063			folio_unlock(folio);
  1064		}
  1065		folio_put(folio);
  1066		return ret;
  1067	}
  1068	

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki

Re: [PATCH v2 2/3] mm/swap: use swap_ops to register swap device's methods

[Barry Song]

On Sun, Mar 29, 2026 at 6:49 PM kernel test robot <lkp@intel.com> wrote:
>
> Hi Barry,
>
> kernel test robot noticed the following build errors:
>
> [auto build test ERROR on akpm-mm/mm-everything]
>
> url:    https://github.com/intel-lab-lkp/linux/commits/Barry-Song/mm-swap-rename-mm-page_io-c-to-mm-swap_io-c/20260328-170852
> base:   https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-everything
> patch link:    https://lore.kernel.org/r/20260328075812.11060-3-21cnbao%40gmail.com
> patch subject: [PATCH v2 2/3] mm/swap: use swap_ops to register swap device's methods
> config: arm-randconfig-001-20260329 (https://download.01.org/0day-ci/archive/20260329/202603291844.TWh2Ellp-lkp@intel.com/config)
> compiler: clang version 23.0.0git (https://github.com/llvm/llvm-project 054e11d1a17e5ba88bb1a8ef32fad3346e80b186)
> reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20260329/202603291844.TWh2Ellp-lkp@intel.com/reproduce)
>
> If you fix the issue in a separate patch/commit (i.e. not just a new version of
> the same patch/commit), kindly add following tags
> | Reported-by: kernel test robot <lkp@intel.com>
> | Closes: https://lore.kernel.org/oe-kbuild-all/202603291844.TWh2Ellp-lkp@intel.com/
>
> All errors (new ones prefixed by >>):
>
> >> mm/zswap.c:1057:19: error: use of undeclared identifier 'sis'; did you mean 'si'?
>     1057 |         VM_WARN_ON_ONCE(!sis->ops || !sis->ops->write_folio);
>          |                          ^~~
>          |                          si
>    include/linux/mmdebug.h:133:52: note: expanded from macro 'VM_WARN_ON_ONCE'
>      133 | #define VM_WARN_ON_ONCE(cond) BUILD_BUG_ON_INVALID(cond)
>          |                                                    ^~~~
>    include/linux/build_bug.h:30:63: note: expanded from macro 'BUILD_BUG_ON_INVALID'
>       30 | #define BUILD_BUG_ON_INVALID(e) ((void)(sizeof((__force long)(e))))
>          |                                                               ^
>    mm/zswap.c:995:27: note: 'si' declared here
>      995 |         struct swap_info_struct *si;
>          |                                  ^
>    mm/zswap.c:1057:32: error: use of undeclared identifier 'sis'; did you mean 'si'?
>     1057 |         VM_WARN_ON_ONCE(!sis->ops || !sis->ops->write_folio);
>          |                                       ^~~
>          |                                       si

Deeply sorry for the last-minute edit—I fat-fingered it.
It should be si, not sis. My sincere apologies.

diff --git a/mm/zswap.c b/mm/zswap.c
index 9bacb1733e1c..b60821758169 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 */
-	VM_WARN_ON_ONCE(!sis->ops || !sis->ops->write_folio);
+	VM_WARN_ON_ONCE(!si->ops || !si->ops->write_folio);
 	si->ops->write_folio(si, folio, NULL);
 
 out:
-- 
Best Regards
Barry

Re: [PATCH v2 2/3] mm/swap: use swap_ops to register swap device's methods

[kernel test robot]

Hi Barry,

kernel test robot noticed the following build errors:

[auto build test ERROR on akpm-mm/mm-everything]

url:    https://github.com/intel-lab-lkp/linux/commits/Barry-Song/mm-swap-rename-mm-page_io-c-to-mm-swap_io-c/20260328-170852
base:   https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-everything
patch link:    https://lore.kernel.org/r/20260328075812.11060-3-21cnbao%40gmail.com
patch subject: [PATCH v2 2/3] mm/swap: use swap_ops to register swap device's methods
config: um-randconfig-r072-20260329 (https://download.01.org/0day-ci/archive/20260329/202603291919.Y8MwFsok-lkp@intel.com/config)
compiler: gcc-14 (Debian 14.2.0-19) 14.2.0
smatch: v0.5.0-9004-gb810ac53
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20260329/202603291919.Y8MwFsok-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202603291919.Y8MwFsok-lkp@intel.com/

All errors (new ones prefixed by >>):

   In file included from include/asm-generic/bug.h:7,
                    from ./arch/um/include/generated/asm/bug.h:1,
                    from arch/x86/include/asm/alternative.h:9,
                    from arch/x86/um/asm/barrier.h:6,
                    from include/linux/list.h:11,
                    from include/linux/module.h:12,
                    from mm/zswap.c:16:
   mm/zswap.c: In function 'zswap_writeback_entry':
>> mm/zswap.c:1057:26: error: 'sis' undeclared (first use in this function); did you mean 'si'?
    1057 |         VM_WARN_ON_ONCE(!sis->ops || !sis->ops->write_folio);
         |                          ^~~
   include/linux/once_lite.h:28:41: note: in definition of macro 'DO_ONCE_LITE_IF'
      28 |                 bool __ret_do_once = !!(condition);                     \
         |                                         ^~~~~~~~~
   include/linux/mmdebug.h:123:37: note: in expansion of macro 'WARN_ON_ONCE'
     123 | #define VM_WARN_ON_ONCE(cond) (void)WARN_ON_ONCE(cond)
         |                                     ^~~~~~~~~~~~
   mm/zswap.c:1057:9: note: in expansion of macro 'VM_WARN_ON_ONCE'
    1057 |         VM_WARN_ON_ONCE(!sis->ops || !sis->ops->write_folio);
         |         ^~~~~~~~~~~~~~~
   mm/zswap.c:1057:26: note: each undeclared identifier is reported only once for each function it appears in
    1057 |         VM_WARN_ON_ONCE(!sis->ops || !sis->ops->write_folio);
         |                          ^~~
   include/linux/once_lite.h:28:41: note: in definition of macro 'DO_ONCE_LITE_IF'
      28 |                 bool __ret_do_once = !!(condition);                     \
         |                                         ^~~~~~~~~
   include/linux/mmdebug.h:123:37: note: in expansion of macro 'WARN_ON_ONCE'
     123 | #define VM_WARN_ON_ONCE(cond) (void)WARN_ON_ONCE(cond)
         |                                     ^~~~~~~~~~~~
   mm/zswap.c:1057:9: note: in expansion of macro 'VM_WARN_ON_ONCE'
    1057 |         VM_WARN_ON_ONCE(!sis->ops || !sis->ops->write_folio);
         |         ^~~~~~~~~~~~~~~


vim +1057 mm/zswap.c

   971	
   972	/*********************************
   973	* writeback code
   974	**********************************/
   975	/*
   976	 * Attempts to free an entry by adding a folio to the swap cache,
   977	 * decompressing the entry data into the folio, and issuing a
   978	 * bio write to write the folio back to the swap device.
   979	 *
   980	 * This can be thought of as a "resumed writeback" of the folio
   981	 * to the swap device.  We are basically resuming the same swap
   982	 * writeback path that was intercepted with the zswap_store()
   983	 * in the first place.  After the folio has been decompressed into
   984	 * the swap cache, the compressed version stored by zswap can be
   985	 * freed.
   986	 */
   987	static int zswap_writeback_entry(struct zswap_entry *entry,
   988					 swp_entry_t swpentry)
   989	{
   990		struct xarray *tree;
   991		pgoff_t offset = swp_offset(swpentry);
   992		struct folio *folio;
   993		struct mempolicy *mpol;
   994		bool folio_was_allocated;
   995		struct swap_info_struct *si;
   996		int ret = 0;
   997	
   998		/* try to allocate swap cache folio */
   999		si = get_swap_device(swpentry);
  1000		if (!si)
  1001			return -EEXIST;
  1002	
  1003		mpol = get_task_policy(current);
  1004		folio = swap_cache_alloc_folio(swpentry, GFP_KERNEL, mpol,
  1005					       NO_INTERLEAVE_INDEX, &folio_was_allocated);
  1006		put_swap_device(si);
  1007		if (!folio)
  1008			return -ENOMEM;
  1009	
  1010		/*
  1011		 * Found an existing folio, we raced with swapin or concurrent
  1012		 * shrinker. We generally writeback cold folios from zswap, and
  1013		 * swapin means the folio just became hot, so skip this folio.
  1014		 * For unlikely concurrent shrinker case, it will be unlinked
  1015		 * and freed when invalidated by the concurrent shrinker anyway.
  1016		 */
  1017		if (!folio_was_allocated) {
  1018			ret = -EEXIST;
  1019			goto out;
  1020		}
  1021	
  1022		/*
  1023		 * folio is locked, and the swapcache is now secured against
  1024		 * concurrent swapping to and from the slot, and concurrent
  1025		 * swapoff so we can safely dereference the zswap tree here.
  1026		 * Verify that the swap entry hasn't been invalidated and recycled
  1027		 * behind our backs, to avoid overwriting a new swap folio with
  1028		 * old compressed data. Only when this is successful can the entry
  1029		 * be dereferenced.
  1030		 */
  1031		tree = swap_zswap_tree(swpentry);
  1032		if (entry != xa_load(tree, offset)) {
  1033			ret = -ENOMEM;
  1034			goto out;
  1035		}
  1036	
  1037		if (!zswap_decompress(entry, folio)) {
  1038			ret = -EIO;
  1039			goto out;
  1040		}
  1041	
  1042		xa_erase(tree, offset);
  1043	
  1044		count_vm_event(ZSWPWB);
  1045		if (entry->objcg)
  1046			count_objcg_events(entry->objcg, ZSWPWB, 1);
  1047	
  1048		zswap_entry_free(entry);
  1049	
  1050		/* folio is up to date */
  1051		folio_mark_uptodate(folio);
  1052	
  1053		/* move it to the tail of the inactive list after end_writeback */
  1054		folio_set_reclaim(folio);
  1055	
  1056		/* start writeback */
> 1057		VM_WARN_ON_ONCE(!sis->ops || !sis->ops->write_folio);
  1058		si->ops->write_folio(si, folio, NULL);
  1059	
  1060	out:
  1061		if (ret && ret != -EEXIST) {
  1062			swap_cache_del_folio(folio);
  1063			folio_unlock(folio);
  1064		}
  1065		folio_put(folio);
  1066		return ret;
  1067	}
  1068	

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki

Re: [PATCH v2 2/3] mm/swap: use swap_ops to register swap device's methods

[Chris Li]

On Sat, Mar 28, 2026 at 12:58 AM Barry Song <21cnbao@gmail.com> wrote:
>
> From: Baoquan He <bhe@redhat.com>
>
> 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 opeations adapted into swap_ops.

Should add that there is no functional change expected from this patch.


> Suggested-by: Chris Li <chrisl@kernel.org>
> Signed-off-by: Baoquan He <bhe@redhat.com>
> Co-developed-by: Barry Song <baohua@kernel.org>
> Signed-off-by: Barry Song <baohua@kernel.org>

Have some nitpicks follows.

> ---
>  include/linux/swap.h |  2 +
>  mm/swap.h            | 10 ++++-
>  mm/swap_io.c         | 99 +++++++++++++++++++++++++-------------------
>  mm/swapfile.c        |  1 +
>  mm/zswap.c           |  3 +-
>  5 files changed, 70 insertions(+), 45 deletions(-)
>
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 1930f81e6be4..b52f30dad72b 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -243,6 +243,7 @@ struct swap_sequential_cluster {
>         unsigned int next[SWAP_NR_ORDERS]; /* Likely next allocation offset */
>  };
>
> +struct swap_ops;
>  /*
>   * The in-memory structure used to track swap areas.
>   */
> @@ -283,6 +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/swap.h b/mm/swap.h
> index 161185057993..219dbcb3ffb1 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/swap_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);
> +};
> +void setup_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/swap_io.c b/mm/swap_io.c
> index 91b33d955e63..ad0895af6ed8 100644
> --- a/mm/swap_io.c
> +++ b/mm/swap_io.c
> @@ -238,6 +238,7 @@ 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);
>
>         if (folio_free_swap(folio))
>                 goto out_unlock;
> @@ -283,7 +284,8 @@ int swap_writeout(struct folio *folio, struct swap_iocb **swap_plug)
>         }
>         rcu_read_unlock();
>
> -       __swap_writepage(folio, swap_plug);
> +       VM_WARN_ON_ONCE(!sis->ops || !sis->ops->write_folio);
> +       sis->ops->write_folio(sis, folio, swap_plug);
>         return 0;
>  out_unlock:
>         folio_unlock(folio);
> @@ -373,10 +375,11 @@ 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);
>
> @@ -409,8 +412,9 @@ 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;
> @@ -429,8 +433,9 @@ 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;
>
> @@ -446,29 +451,6 @@ 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;
> @@ -537,9 +519,10 @@ 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);
>
> @@ -571,8 +554,9 @@ 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;
> @@ -593,8 +577,9 @@ 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;
>
> @@ -608,6 +593,39 @@ 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,
> +};
> +
> +void setup_swap_ops(struct swap_info_struct *sis)

setup_swap_ops()` needs to return an indication of an error.
The error is that sis->ops is NULL or op->read_folio == NULL or
op->write_folio == NULL.
If there is an error, we should fail the swapon.

See my other comments regardin g the error checking at the dereference side.

> +{
> +       /*
> +        * ->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;
> +}
> +
>  void swap_read_folio(struct folio *folio, struct swap_iocb **plug)
>  {
>         struct swap_info_struct *sis = __swap_entry_to_info(folio->swap);
> @@ -642,13 +660,8 @@ 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);
> -       }
> +       VM_WARN_ON_ONCE(!sis->ops || !sis->ops->read_folio);

WARN_ON_ONCE is not enough. If we hit a WARN here, the kernel will
crash on the next dereference of ops->read_folios().
In other words, we will get kernel OOPS immediately after the warning.
This makes no make sense.

We should refuse to swapon at setup_swap_ops() above.  Then no
checking here is needed.

> +       sis->ops->read_folio(sis, folio, plug);
>
>  finish:
>         if (workingset) {
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index ff315b752afd..c9eb95141c8f 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -3732,6 +3732,7 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
>         si->list.prio = -si->prio;
>         si->avail_list.prio = -si->prio;
>         si->swap_file = swap_file;
> +       setup_swap_ops(si);
>
>         /* Sets SWP_WRITEOK, resurrect the percpu ref, expose the swap device */
>         enable_swap_info(si);
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 4b5149173b0e..9bacb1733e1c 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -1054,7 +1054,8 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
>         folio_set_reclaim(folio);
>
>         /* start writeback */
> -       __swap_writepage(folio, NULL);
> +       VM_WARN_ON_ONCE(!sis->ops || !sis->ops->write_folio);

Again, WARN_ON is not enough, the kernel will panic here if a warning
is triggered.
Check at setup_swap_ops() and refuse to swapon if ops->write_folio is NULL.

With the other "sis" fix, the patch looks good otherwise.

Chris


> +       si->ops->write_folio(si, folio, NULL);
>
>  out:
>         if (ret && ret != -EEXIST) {
> --
> 2.39.3 (Apple Git-146)
>

Re: [PATCH v2 2/3] mm/swap: use swap_ops to register swap device's methods

[Barry Song]

On Tue, Mar 31, 2026 at 4:31 PM Chris Li <chrisl@kernel.org> wrote:
>
> On Sat, Mar 28, 2026 at 12:58 AM Barry Song <21cnbao@gmail.com> wrote:
> >
> > From: Baoquan He <bhe@redhat.com>
> >
> > 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 opeations adapted into swap_ops.
>
> Should add that there is no functional change expected from this patch.

Sounds good to me.

[...]

> > +static const struct swap_ops bdev_async_swap_ops = {
> > +       .read_folio = swap_read_folio_bdev_async,
> > +       .write_folio = swap_writepage_bdev_async,
> > +};
> > +
> > +void setup_swap_ops(struct swap_info_struct *sis)
>
> setup_swap_ops()` needs to return an indication of an error.
> The error is that sis->ops is NULL or op->read_folio == NULL or
> op->write_folio == NULL.
> If there is an error, we should fail the swapon.

Right now there is no possibility for any of the ops (read_folio or
write_folio) to be NULL, so I was using VM_WARN_ON as an assertion
to catch bugs in kernel code.

That said, I agree we can return an error earlier instead of letting
swapon fail later.

I think it may still make sense to keep a VM_WARN_ON in
setup_swap_ops() before returning -EINVAL or a similar error?

Thanks
Barry

Re: [PATCH v2 2/3] mm/swap: use swap_ops to register swap device's methods

[Chris Li]

On Tue, Mar 31, 2026 at 2:22 AM Barry Song <baohua@kernel.org> wrote:
>
> On Tue, Mar 31, 2026 at 4:31 PM Chris Li <chrisl@kernel.org> wrote:
> >
> > On Sat, Mar 28, 2026 at 12:58 AM Barry Song <21cnbao@gmail.com> wrote:
> > >
> > > From: Baoquan He <bhe@redhat.com>
> > >
> > > 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 opeations adapted into swap_ops.
> >
> > Should add that there is no functional change expected from this patch.
>
> Sounds good to me.

Ack

>
> > > +static const struct swap_ops bdev_async_swap_ops = {
> > > +       .read_folio = swap_read_folio_bdev_async,
> > > +       .write_folio = swap_writepage_bdev_async,
> > > +};
> > > +
> > > +void setup_swap_ops(struct swap_info_struct *sis)
> >
> > setup_swap_ops()` needs to return an indication of an error.
> > The error is that sis->ops is NULL or op->read_folio == NULL or
> > op->write_folio == NULL.
> > If there is an error, we should fail the swapon.
>
> Right now there is no possibility for any of the ops (read_folio or
> write_folio) to be NULL, so I was using VM_WARN_ON as an assertion
> to catch bugs in kernel code.

Ack. Someone might register a new swap_ops later that introduces a
NULL swap_write_folio. The check is for catching that.

>
> That said, I agree we can return an error earlier instead of letting
> swapon fail later.
>
> I think it may still make sense to keep a VM_WARN_ON in
> setup_swap_ops() before returning -EINVAL or a similar error?

Sure. We can just print  pr_err() rather than VM_WARN_ON.  VM_WARN_ON
is nop when VM debug is disabled. It is a kernel swap code bug on the
swapon path, so the back trace portion of the warning is not very
valuable. A normal error print will suffice. Because the swapon
failed, the user will notice this error.

Chris

[PATCH v2 3/3] mm/swap_io.c: rename swap_writepage_* to swap_write_folio_*

[Barry Song]

From: Baoquan He <bhe@redhat.com>

All these swap_writepage_* functions are hanlding passed in folio, but
not page. And this renaming make them consistent with the their
counterpart swap_read_folio_* functions.

Reviewed-by: Barry Song <baohua@kernel.org>
Reviewed-by: Nhat Pham <nphamcs@gmail.com>
Signed-off-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Barry Song <baohua@kernel.org>
---
 mm/swap_io.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/mm/swap_io.c b/mm/swap_io.c
index ad0895af6ed8..063cd005ca0d 100644
--- a/mm/swap_io.c
+++ b/mm/swap_io.c
@@ -375,7 +375,7 @@ 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_write_folio_fs(struct swap_info_struct *sis,
 			      struct folio *folio,
 			      struct swap_iocb **swap_plug)
 {
@@ -412,7 +412,7 @@ 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_write_folio_bdev_sync(struct swap_info_struct *sis,
 				     struct folio *folio,
 				     struct swap_iocb **plug)
 {
@@ -433,7 +433,7 @@ 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_write_folio_bdev_async(struct swap_info_struct *sis,
 				      struct folio *folio,
 				      struct swap_iocb **plug)
 {
@@ -595,17 +595,17 @@ 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,
+	.write_folio = swap_write_folio_fs,
 };
 
 static const struct swap_ops bdev_sync_swap_ops = {
 	.read_folio = swap_read_folio_bdev_sync,
-	.write_folio = swap_writepage_bdev_sync,
+	.write_folio = swap_write_folio_bdev_sync,
 };
 
 static const struct swap_ops bdev_async_swap_ops = {
 	.read_folio = swap_read_folio_bdev_async,
-	.write_folio = swap_writepage_bdev_async,
+	.write_folio = swap_write_folio_bdev_async,
 };
 
 void setup_swap_ops(struct swap_info_struct *sis)
-- 
2.39.3 (Apple Git-146)

Re: [PATCH v2 3/3] mm/swap_io.c: rename swap_writepage_* to swap_write_folio_*

[Chris Li]

Acked-by: Chris Li <chrisl@kernel.org>

Chris

On Sat, Mar 28, 2026 at 12:58 AM Barry Song <21cnbao@gmail.com> wrote:
>
> From: Baoquan He <bhe@redhat.com>
>
> All these swap_writepage_* functions are hanlding passed in folio, but
> not page. And this renaming make them consistent with the their
> counterpart swap_read_folio_* functions.
>
> Reviewed-by: Barry Song <baohua@kernel.org>
> Reviewed-by: Nhat Pham <nphamcs@gmail.com>
> Signed-off-by: Baoquan He <bhe@redhat.com>
> Signed-off-by: Barry Song <baohua@kernel.org>
> ---
>  mm/swap_io.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/mm/swap_io.c b/mm/swap_io.c
> index ad0895af6ed8..063cd005ca0d 100644
> --- a/mm/swap_io.c
> +++ b/mm/swap_io.c
> @@ -375,7 +375,7 @@ 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_write_folio_fs(struct swap_info_struct *sis,
>                               struct folio *folio,
>                               struct swap_iocb **swap_plug)
>  {
> @@ -412,7 +412,7 @@ 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_write_folio_bdev_sync(struct swap_info_struct *sis,
>                                      struct folio *folio,
>                                      struct swap_iocb **plug)
>  {
> @@ -433,7 +433,7 @@ 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_write_folio_bdev_async(struct swap_info_struct *sis,
>                                       struct folio *folio,
>                                       struct swap_iocb **plug)
>  {
> @@ -595,17 +595,17 @@ 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,
> +       .write_folio = swap_write_folio_fs,
>  };
>
>  static const struct swap_ops bdev_sync_swap_ops = {
>         .read_folio = swap_read_folio_bdev_sync,
> -       .write_folio = swap_writepage_bdev_sync,
> +       .write_folio = swap_write_folio_bdev_sync,
>  };
>
>  static const struct swap_ops bdev_async_swap_ops = {
>         .read_folio = swap_read_folio_bdev_async,
> -       .write_folio = swap_writepage_bdev_async,
> +       .write_folio = swap_write_folio_bdev_async,
>  };
>
>  void setup_swap_ops(struct swap_info_struct *sis)
> --
> 2.39.3 (Apple Git-146)
>

Re: [PATCH v2 0/3] mm/swap: use swap_ops to register swap device's methods

[Baoquan He]

On 03/28/26 at 03:58pm, Barry Song wrote:
> From: Barry Song <baohua@kernel.org>
> 
> -v2:
>  * lots of cleanup for patch 2/3: renaming, moving data
>    structures, and using const properly
>  * collected tags from Kairui, Nhat and Barry

Thanks a lot for taking care of this, Barry. I took several days off for
family reason. Sorry for the inconvenience caused by my leave.

> 
> -v1:
>  https://lore.kernel.org/linux-mm/20260302104016.163542-1-bhe@redhat.com/
> 
> This can simplify the code logic and benefit any new type of swap device
> added later.
> 
> And also do renaming in this patchset:
> -------
>    file renaming:
>    ---
>    mm/page_io.c to mm/swap_io.c
> 
>    function renaming:
>    ---
>    swap_writepage_* to swap_write_folio_* in file mm/swap_io.c
> 
> Baoquan He (3):
>   mm/swap: rename mm/page_io.c to mm/swap_io.c
>   mm/swap: use swap_ops to register swap device's methods
>   mm/swap_io.c: rename swap_writepage_* to swap_write_folio_*
> 
>  MAINTAINERS                 |   2 +-
>  include/linux/swap.h        |   2 +
>  mm/Makefile                 |   2 +-
>  mm/swap.h                   |  12 ++++-
>  mm/{page_io.c => swap_io.c} | 101 ++++++++++++++++++++----------------
>  mm/swapfile.c               |   1 +
>  mm/zswap.c                  |   3 +-
>  7 files changed, 73 insertions(+), 50 deletions(-)
>  rename mm/{page_io.c => swap_io.c} (89%)
> 
> -- 
> 2.39.3 (Apple Git-146)
>