[PATCH v6 06/16] mm: Add readahead address space operation

[Matthew Wilcox <willy@infradead.org>]

From: "Matthew Wilcox (Oracle)" <willy@infradead.org>

This replaces ->readpages with a saner interface:
 - Return void instead of an ignored error code.
 - Pages are already in the page cache when ->readahead is called.
 - Implementation looks up the pages in the page cache instead of
   having them passed in a linked list.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 Documentation/filesystems/locking.rst |  6 +++++-
 Documentation/filesystems/vfs.rst     | 13 +++++++++++++
 include/linux/fs.h                    |  2 ++
 include/linux/pagemap.h               | 18 ++++++++++++++++++
 mm/readahead.c                        |  8 +++++++-
 5 files changed, 45 insertions(+), 2 deletions(-)

diff --git a/Documentation/filesystems/locking.rst b/Documentation/filesystems/locking.rst
index 5057e4d9dcd1..0ebc4491025a 100644
--- a/Documentation/filesystems/locking.rst
+++ b/Documentation/filesystems/locking.rst
@@ -239,6 +239,7 @@ prototypes::
 	int (*readpage)(struct file *, struct page *);
 	int (*writepages)(struct address_space *, struct writeback_control *);
 	int (*set_page_dirty)(struct page *page);
+	void (*readahead)(struct readahead_control *);
 	int (*readpages)(struct file *filp, struct address_space *mapping,
 			struct list_head *pages, unsigned nr_pages);
 	int (*write_begin)(struct file *, struct address_space *mapping,
@@ -271,7 +272,8 @@ writepage:		yes, unlocks (see below)
 readpage:		yes, unlocks
 writepages:
 set_page_dirty		no
-readpages:
+readahead:		yes, unlocks
+readpages:		no
 write_begin:		locks the page		 exclusive
 write_end:		yes, unlocks		 exclusive
 bmap:
@@ -295,6 +297,8 @@ the request handler (/dev/loop).
 ->readpage() unlocks the page, either synchronously or via I/O
 completion.
 
+->readahead() unlocks the pages like ->readpage().
+
 ->readpages() populates the pagecache with the passed pages and starts
 I/O against them.  They come unlocked upon I/O completion.
 
diff --git a/Documentation/filesystems/vfs.rst b/Documentation/filesystems/vfs.rst
index 7d4d09dd5e6d..81ab30fbe45c 100644
--- a/Documentation/filesystems/vfs.rst
+++ b/Documentation/filesystems/vfs.rst
@@ -706,6 +706,7 @@ cache in your filesystem.  The following members are defined:
 		int (*readpage)(struct file *, struct page *);
 		int (*writepages)(struct address_space *, struct writeback_control *);
 		int (*set_page_dirty)(struct page *page);
+		void (*readahead)(struct readahead_control *);
 		int (*readpages)(struct file *filp, struct address_space *mapping,
 				 struct list_head *pages, unsigned nr_pages);
 		int (*write_begin)(struct file *, struct address_space *mapping,
@@ -781,12 +782,24 @@ cache in your filesystem.  The following members are defined:
 	If defined, it should set the PageDirty flag, and the
 	PAGECACHE_TAG_DIRTY tag in the radix tree.
 
+``readahead``
+	Called by the VM to read pages associated with the address_space
+	object.  The pages are consecutive in the page cache and are
+	locked.  The implementation should decrement the page refcount
+	after starting I/O on each page.  Usually the page will be
+	unlocked by the I/O completion handler.  If the function does
+	not attempt I/O on some pages, the caller will decrement the page
+	refcount and unlock the pages for you.	Set PageUptodate if the
+	I/O completes successfully.  Setting PageError on any page will
+	be ignored; simply unlock the page if an I/O error occurs.
+
 ``readpages``
 	called by the VM to read pages associated with the address_space
 	object.  This is essentially just a vector version of readpage.
 	Instead of just one page, several pages are requested.
 	readpages is only used for read-ahead, so read errors are
 	ignored.  If anything goes wrong, feel free to give up.
+	This interface is deprecated; implement readahead instead.
 
 ``write_begin``
 	Called by the generic buffered write code to ask the filesystem
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 3cd4fe6b845e..d4e2d2964346 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -292,6 +292,7 @@ enum positive_aop_returns {
 struct page;
 struct address_space;
 struct writeback_control;
+struct readahead_control;
 
 /*
  * Write life time hint values.
@@ -375,6 +376,7 @@ struct address_space_operations {
 	 */
 	int (*readpages)(struct file *filp, struct address_space *mapping,
 			struct list_head *pages, unsigned nr_pages);
+	void (*readahead)(struct readahead_control *);
 
 	int (*write_begin)(struct file *, struct address_space *mapping,
 				loff_t pos, unsigned len, unsigned flags,
diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 3613154e79e4..bd4291f78f41 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -665,6 +665,24 @@ static inline void readahead_next(struct readahead_control *rac)
 #define readahead_for_each(rac, page)					\
 	for (; (page = readahead_page(rac)); readahead_next(rac))
 
+/* The byte offset into the file of this readahead block */
+static inline loff_t readahead_offset(struct readahead_control *rac)
+{
+	return (loff_t)rac->_start * PAGE_SIZE;
+}
+
+/* The number of bytes in this readahead block */
+static inline loff_t readahead_length(struct readahead_control *rac)
+{
+	return (loff_t)rac->_nr_pages * PAGE_SIZE;
+}
+
+/* The index of the first page in this readahead block */
+static inline unsigned int readahead_index(struct readahead_control *rac)
+{
+	return rac->_start;
+}
+
 /* The number of pages in this readahead block */
 static inline unsigned int readahead_count(struct readahead_control *rac)
 {
diff --git a/mm/readahead.c b/mm/readahead.c
index 7663de534734..5be7e1cb8666 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -121,7 +121,13 @@ static void read_pages(struct readahead_control *rac, struct list_head *pages)
 
 	blk_start_plug(&plug);
 
-	if (aops->readpages) {
+	if (aops->readahead) {
+		aops->readahead(rac);
+		readahead_for_each(rac, page) {
+			unlock_page(page);
+			put_page(page);
+		}
+	} else if (aops->readpages) {
 		aops->readpages(rac->file, rac->mapping, pages,
 				readahead_count(rac));
 		/* Clean up the remaining pages */
-- 
2.25.0