doc: update documentation for swap_activate and swap_rw

This documentation for ->swap_activate() has been out-of-date for a long
time.  This patch updates it to match recent changes, and adds
documentation for the associated ->swap_rw()

Link: https://lkml.kernel.org/r/164859778126.29473.6778751233552859461.stgit@noble.brown
Signed-off-by: NeilBrown <neilb@suse.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Tested-by: David Howells <dhowells@redhat.com>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: Hugh Dickins <hughd@google.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Trond Myklebust <trond.myklebust@hammerspace.com>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: neilbrown

Documentation/filesystems/locking.rst

@@ -258,8 +258,9 @@ prototypes::
 	int (*launder_folio)(struct folio *);
 	bool (*is_partially_uptodate)(struct folio *, size_t from, size_t count);
 	int (*error_remove_page)(struct address_space *, struct page *);
-	int (*swap_activate)(struct file *);
+	int (*swap_activate)(struct swap_info_struct *sis, struct file *f, sector_t *span)
 	int (*swap_deactivate)(struct file *);
+	int (*swap_rw)(struct kiocb *iocb, struct iov_iter *iter);
 
 locking rules:
 	All except dirty_folio and freepage may block
@@ -287,6 +288,7 @@ is_partially_uptodate:	yes
 error_remove_page:	yes
 swap_activate:		no
 swap_deactivate:	no
+swap_rw:		yes, unlocks
 ======================	======================== =========	===============
 
 ->write_begin(), ->write_end() and ->readpage() may be called from
@@ -386,15 +388,19 @@ cleaned, or an error value if not. Note that in order to prevent the folio
 getting mapped back in and redirtied, it needs to be kept locked
 across the entire operation.
 
-->swap_activate will be called with a non-zero argument on
-files backing (non block device backed) swapfiles. A return value
-of zero indicates success, in which case this file can be used for
-backing swapspace. The swapspace operations will be proxied to the
-address space operations.
+->swap_activate() will be called to prepare the given file for swap.  It
+should perform any validation and preparation necessary to ensure that
+writes can be performed with minimal memory allocation.  It should call
+add_swap_extent(), or the helper iomap_swapfile_activate(), and return
+the number of extents added.  If IO should be submitted through
+->swap_rw(), it should set SWP_FS_OPS, otherwise IO will be submitted
+directly to the block device ``sis->bdev``.
 
 ->swap_deactivate() will be called in the sys_swapoff()
 path after ->swap_activate() returned success.
 
+->swap_rw will be called for swap IO if SWP_FS_OPS was set by ->swap_activate().
+
 file_lock_operations
 ====================
 

Documentation/filesystems/vfs.rst

@@ -749,8 +749,9 @@ cache in your filesystem.  The following members are defined:
 					       size_t count);
 		void (*is_dirty_writeback) (struct page *, bool *, bool *);
 		int (*error_remove_page) (struct mapping *mapping, struct page *page);
-		int (*swap_activate)(struct file *);
+		int (*swap_activate)(struct swap_info_struct *sis, struct file *f, sector_t *span)
 		int (*swap_deactivate)(struct file *);
+		int (*swap_rw)(struct kiocb *iocb, struct iov_iter *iter);
 	};
 
 ``writepage``
@@ -952,15 +953,21 @@ cache in your filesystem.  The following members are defined:
 	unless you have them locked or reference counts increased.
 
 ``swap_activate``
-	Called when swapon is used on a file to allocate space if
-	necessary and pin the block lookup information in memory.  A
-	return value of zero indicates success, in which case this file
-	can be used to back swapspace.
+
+	Called to prepare the given file for swap.  It should perform
+	any validation and preparation necessary to ensure that writes
+	can be performed with minimal memory allocation.  It should call
+	add_swap_extent(), or the helper iomap_swapfile_activate(), and
+	return the number of extents added.  If IO should be submitted
+	through ->swap_rw(), it should set SWP_FS_OPS, otherwise IO will
+	be submitted directly to the block device ``sis->bdev``.
 
 ``swap_deactivate``
 	Called during swapoff on files where swap_activate was
 	successful.
 
+``swap_rw``
+	Called to read or write swap pages when SWP_FS_OPS is set.
 
 The File Object
 ===============