[PATCH v4 00/12] Change readahead API

[PATCH v4 00/12] Change readahead API

[Matthew Wilcox]

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

I would particularly value feedback on this from the gfs2 and ocfs2
maintainers.  They have non-trivial changes, and a review on patch 5
would be greatly appreciated.

This series adds a readahead address_space operation to eventually
replace the readpages operation.  The key difference is that
pages are added to the page cache as they are allocated (and
then looked up by the filesystem) instead of passing them on a
list to the readpages operation and having the filesystem add
them to the page cache.  It's a net reduction in code for each
implementation, more efficient than walking a list, and solves
the direct-write vs buffered-read problem reported by yu kuai at
https://lore.kernel.org/linux-fsdevel/20200116063601.39201-1-yukuai3@huawei.com/

v4:
 - Rebase on current Linus (a62aa6f7f50a ("Merge tag 'gfs2-for-5.6'"))
 - Add comment to __do_page_cache_readahead() acknowledging we don't
   care _that_ much about setting PageReadahead.
 - Fix the return value check of add_to_page_cache_lru().
 - Add a missing call to put_page() in __do_page_cache_readahead() if
   we fail to insert the page.
 - Improve the documentation of ->readahead (including indentation
   problem identified by Randy).
 - Fix off by one error in read_pages() (Dave Chinner).
 - Fix nr_pages manipulation in btrfs (Dave Chinner).
 - Remove bogus refcount fix in erofs (Gao Xiang, Dave Chinner).
 - Update ext4 patch for Merkle tree readahead.
 - Update f2fs patch for Merkle tree readahead.
 - Reinstate next_page label in f2fs_readpages() now it's used by the
   compression code.
 - Reinstate call to fuse_wait_on_page_writeback (Miklos Szeredi).
 - Remove a double-unlock in the error path in fuse.
 - Remove an odd fly-speck in fuse_readpages().
 - Make nr_pages loop in fuse_readpages less convoluted (Dave Chinner).

Matthew Wilcox (Oracle) (12):
  mm: Fix the return type of __do_page_cache_readahead
  readahead: Ignore return value of ->readpages
  readahead: Put pages in cache earlier
  mm: Add readahead address space operation
  fs: Convert mpage_readpages to mpage_readahead
  btrfs: Convert from readpages to readahead
  erofs: Convert uncompressed files from readpages to readahead
  erofs: Convert compressed files from readpages to readahead
  ext4: Convert from readpages to readahead
  f2fs: Convert from readpages to readahead
  fuse: Convert from readpages to readahead
  iomap: Convert from readpages to readahead

 Documentation/filesystems/locking.rst |  7 ++-
 Documentation/filesystems/vfs.rst     | 14 +++++
 drivers/staging/exfat/exfat_super.c   |  9 +--
 fs/block_dev.c                        |  9 +--
 fs/btrfs/extent_io.c                  | 19 +++---
 fs/btrfs/extent_io.h                  |  2 +-
 fs/btrfs/inode.c                      | 18 +++---
 fs/erofs/data.c                       | 33 ++++------
 fs/erofs/zdata.c                      | 21 +++----
 fs/ext2/inode.c                       | 12 ++--
 fs/ext4/ext4.h                        |  5 +-
 fs/ext4/inode.c                       | 24 ++++----
 fs/ext4/readpage.c                    | 20 +++---
 fs/ext4/verity.c                      | 16 +++--
 fs/f2fs/data.c                        | 35 +++++------
 fs/f2fs/f2fs.h                        |  5 +-
 fs/f2fs/verity.c                      | 16 +++--
 fs/fat/inode.c                        |  8 +--
 fs/fuse/file.c                        | 37 +++++------
 fs/gfs2/aops.c                        | 20 +++---
 fs/hpfs/file.c                        |  8 +--
 fs/iomap/buffered-io.c                | 74 +++++-----------------
 fs/iomap/trace.h                      |  2 +-
 fs/isofs/inode.c                      |  9 +--
 fs/jfs/inode.c                        |  8 +--
 fs/mpage.c                            | 38 ++++--------
 fs/nilfs2/inode.c                     | 13 ++--
 fs/ocfs2/aops.c                       | 32 +++++-----
 fs/omfs/file.c                        |  8 +--
 fs/qnx6/inode.c                       |  8 +--
 fs/reiserfs/inode.c                   | 10 +--
 fs/udf/inode.c                        |  8 +--
 fs/xfs/xfs_aops.c                     | 10 +--
 include/linux/fs.h                    |  2 +
 include/linux/iomap.h                 |  2 +-
 include/linux/mpage.h                 |  2 +-
 include/linux/pagemap.h               | 12 ++++
 include/trace/events/erofs.h          |  6 +-
 include/trace/events/f2fs.h           |  6 +-
 mm/internal.h                         |  2 +-
 mm/migrate.c                          |  2 +-
 mm/readahead.c                        | 89 ++++++++++++++++++---------
 42 files changed, 332 insertions(+), 349 deletions(-)

-- 
2.24.1

[PATCH v4 03/12] readahead: Put pages in cache earlier

[Matthew Wilcox]

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

At allocation time, put the pages in the cache unless we're using
->readpages.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: linux-btrfs@vger.kernel.org
Cc: linux-erofs@lists.ozlabs.org
Cc: linux-ext4@vger.kernel.org
Cc: linux-f2fs-devel@lists.sourceforge.net
Cc: linux-xfs@vger.kernel.org
Cc: cluster-devel@redhat.com
Cc: ocfs2-devel@oss.oracle.com
---
 mm/readahead.c | 64 ++++++++++++++++++++++++++++++++++----------------
 1 file changed, 44 insertions(+), 20 deletions(-)

diff --git a/mm/readahead.c b/mm/readahead.c
index fc77d13af556..7daef0038b14 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -114,10 +114,10 @@ int read_cache_pages(struct address_space *mapping, struct list_head *pages,
 EXPORT_SYMBOL(read_cache_pages);
 
 static void read_pages(struct address_space *mapping, struct file *filp,
-		struct list_head *pages, unsigned int nr_pages, gfp_t gfp)
+		struct list_head *pages, pgoff_t start,
+		unsigned int nr_pages)
 {
 	struct blk_plug plug;
-	unsigned page_idx;
 
 	blk_start_plug(&plug);
 
@@ -125,18 +125,17 @@ static void read_pages(struct address_space *mapping, struct file *filp,
 		mapping->a_ops->readpages(filp, mapping, pages, nr_pages);
 		/* Clean up the remaining pages */
 		put_pages_list(pages);
-		goto out;
-	}
+	} else {
+		struct page *page;
+		unsigned long index;
 
-	for (page_idx = 0; page_idx < nr_pages; page_idx++) {
-		struct page *page = lru_to_page(pages);
-		list_del(&page->lru);
-		if (!add_to_page_cache_lru(page, mapping, page->index, gfp))
+		xa_for_each_range(&mapping->i_pages, index, page, start,
+				start + nr_pages - 1) {
 			mapping->a_ops->readpage(filp, page);
-		put_page(page);
+			put_page(page);
+		}
 	}
 
-out:
 	blk_finish_plug(&plug);
 }
 
@@ -153,13 +152,14 @@ unsigned long __do_page_cache_readahead(struct address_space *mapping,
 		unsigned long lookahead_size)
 {
 	struct inode *inode = mapping->host;
-	struct page *page;
 	unsigned long end_index;	/* The last page we want to read */
 	LIST_HEAD(page_pool);
 	int page_idx;
+	pgoff_t page_offset;
 	unsigned long nr_pages = 0;
 	loff_t isize = i_size_read(inode);
 	gfp_t gfp_mask = readahead_gfp_mask(mapping);
+	bool use_list = mapping->a_ops->readpages;
 
 	if (isize == 0)
 		goto out;
@@ -170,21 +170,32 @@ unsigned long __do_page_cache_readahead(struct address_space *mapping,
 	 * Preallocate as many pages as we will need.
 	 */
 	for (page_idx = 0; page_idx < nr_to_read; page_idx++) {
-		pgoff_t page_offset = offset + page_idx;
+		struct page *page;
 
+		page_offset = offset + page_idx;
 		if (page_offset > end_index)
 			break;
 
 		page = xa_load(&mapping->i_pages, page_offset);
 		if (page && !xa_is_value(page)) {
 			/*
-			 * Page already present?  Kick off the current batch of
-			 * contiguous pages before continuing with the next
-			 * batch.
+			 * Page already present?  Kick off the current batch
+			 * of contiguous pages before continuing with the
+			 * next batch.
 			 */
 			if (nr_pages)
-				read_pages(mapping, filp, &page_pool, nr_pages,
-						gfp_mask);
+				read_pages(mapping, filp, &page_pool,
+						page_offset - nr_pages,
+						nr_pages);
+			/*
+			 * It's possible this page is the page we should
+			 * be marking with PageReadahead.  However, we
+			 * don't have a stable ref to this page so it might
+			 * be reallocated to another user before we can set
+			 * the bit.  There's probably another page in the
+			 * cache marked with PageReadahead from the other
+			 * process which accessed this file.
+			 */
 			nr_pages = 0;
 			continue;
 		}
@@ -192,8 +203,20 @@ unsigned long __do_page_cache_readahead(struct address_space *mapping,
 		page = __page_cache_alloc(gfp_mask);
 		if (!page)
 			break;
-		page->index = page_offset;
-		list_add(&page->lru, &page_pool);
+		if (use_list) {
+			page->index = page_offset;
+			list_add(&page->lru, &page_pool);
+		} else if (add_to_page_cache_lru(page, mapping, page_offset,
+					gfp_mask) < 0) {
+			if (nr_pages)
+				read_pages(mapping, filp, &page_pool,
+						page_offset - nr_pages,
+						nr_pages);
+			put_page(page);
+			nr_pages = 0;
+			continue;
+		}
+
 		if (page_idx == nr_to_read - lookahead_size)
 			SetPageReadahead(page);
 		nr_pages++;
@@ -205,7 +228,8 @@ unsigned long __do_page_cache_readahead(struct address_space *mapping,
 	 * will then handle the error.
 	 */
 	if (nr_pages)
-		read_pages(mapping, filp, &page_pool, nr_pages, gfp_mask);
+		read_pages(mapping, filp, &page_pool, page_offset - nr_pages,
+				nr_pages);
 	BUG_ON(!list_empty(&page_pool));
 out:
 	return nr_pages;
-- 
2.24.1

[PATCH v4 04/12] mm: Add readahead address space operation

[Matthew Wilcox]

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

This replaces ->readpages with a saner interface:
 - Return the number of pages not read 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>
Cc: linux-btrfs@vger.kernel.org
Cc: linux-erofs@lists.ozlabs.org
Cc: linux-ext4@vger.kernel.org
Cc: linux-f2fs-devel@lists.sourceforge.net
Cc: linux-xfs@vger.kernel.org
Cc: cluster-devel@redhat.com
Cc: ocfs2-devel@oss.oracle.com
---
 Documentation/filesystems/locking.rst |  7 ++++++-
 Documentation/filesystems/vfs.rst     | 14 ++++++++++++++
 include/linux/fs.h                    |  2 ++
 include/linux/pagemap.h               | 12 ++++++++++++
 mm/readahead.c                        | 13 ++++++++++++-
 5 files changed, 46 insertions(+), 2 deletions(-)

diff --git a/Documentation/filesystems/locking.rst b/Documentation/filesystems/locking.rst
index 5057e4d9dcd1..3d10729caf44 100644
--- a/Documentation/filesystems/locking.rst
+++ b/Documentation/filesystems/locking.rst
@@ -239,6 +239,8 @@ prototypes::
 	int (*readpage)(struct file *, struct page *);
 	int (*writepages)(struct address_space *, struct writeback_control *);
 	int (*set_page_dirty)(struct page *page);
+	unsigned (*readahead)(struct file *, struct address_space *,
+				 pgoff_t start, unsigned nr_pages);
 	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 +273,8 @@ 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 +298,8 @@ 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..c2bc345f2169 100644
--- a/Documentation/filesystems/vfs.rst
+++ b/Documentation/filesystems/vfs.rst
@@ -706,6 +706,8 @@ 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);
+		unsigned (*readahead)(struct file *filp, struct address_space *mapping,
+				 pgoff_t start, unsigned nr_pages);
 		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,
@@ -781,6 +783,18 @@ 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 attempting 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, return the number
+	of pages which were not read so the caller can 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.
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 41584f50af0d..3bfc142e7d10 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -375,6 +375,8 @@ struct address_space_operations {
 	 */
 	int (*readpages)(struct file *filp, struct address_space *mapping,
 			struct list_head *pages, unsigned nr_pages);
+	unsigned (*readahead)(struct file *, struct address_space *,
+			pgoff_t start, unsigned nr_pages);
 
 	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 ccb14b6a16b5..a2cf007826f2 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -630,6 +630,18 @@ static inline int add_to_page_cache(struct page *page,
 	return error;
 }
 
+/*
+ * Only call this from a ->readahead implementation.
+ */
+static inline
+struct page *readahead_page(struct address_space *mapping, pgoff_t index)
+{
+	struct page *page = xa_load(&mapping->i_pages, index);
+	VM_BUG_ON_PAGE(!PageLocked(page), page);
+
+	return page;
+}
+
 static inline unsigned long dir_pages(struct inode *inode)
 {
 	return (unsigned long)(inode->i_size + PAGE_SIZE - 1) >>
diff --git a/mm/readahead.c b/mm/readahead.c
index 7daef0038b14..b2ed0baf3a5d 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -121,7 +121,18 @@ static void read_pages(struct address_space *mapping, struct file *filp,
 
 	blk_start_plug(&plug);
 
-	if (mapping->a_ops->readpages) {
+	if (mapping->a_ops->readahead) {
+		unsigned left = mapping->a_ops->readahead(filp, mapping,
+				start, nr_pages);
+
+		while (left) {
+			struct page *page = readahead_page(mapping,
+					start + nr_pages - left);
+			unlock_page(page);
+			put_page(page);
+			left--;
+		}
+	} else if (mapping->a_ops->readpages) {
 		mapping->a_ops->readpages(filp, mapping, pages, nr_pages);
 		/* Clean up the remaining pages */
 		put_pages_list(pages);
-- 
2.24.1

[PATCH v4 09/12] ext4: Convert from readpages to readahead

[Matthew Wilcox]

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

Use the new readahead operation in ext4

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: linux-ext4@vger.kernel.org
---
 fs/ext4/ext4.h     |  5 ++---
 fs/ext4/inode.c    | 24 ++++++++++++------------
 fs/ext4/readpage.c | 20 +++++++-------------
 fs/ext4/verity.c   | 16 +++++++++++-----
 4 files changed, 32 insertions(+), 33 deletions(-)

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 9a2ee2428ecc..346ad9e1403b 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -3275,9 +3275,8 @@ static inline void ext4_set_de_type(struct super_block *sb,
 }
 
 /* readpages.c */
-extern int ext4_mpage_readpages(struct address_space *mapping,
-				struct list_head *pages, struct page *page,
-				unsigned nr_pages, bool is_readahead);
+extern int ext4_mpage_readpages(struct address_space *mapping, pgoff_t start,
+		struct page *page, unsigned nr_pages, bool is_readahead);
 extern int __init ext4_init_post_read_processing(void);
 extern void ext4_exit_post_read_processing(void);
 
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 3313168b680f..e1f5864e5ecf 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -3218,7 +3218,7 @@ static sector_t ext4_bmap(struct address_space *mapping, sector_t block)
 static int ext4_readpage(struct file *file, struct page *page)
 {
 	int ret = -EAGAIN;
-	struct inode *inode = page->mapping->host;
+	struct inode *inode = file_inode(file);
 
 	trace_ext4_readpage(page);
 
@@ -3226,23 +3226,23 @@ static int ext4_readpage(struct file *file, struct page *page)
 		ret = ext4_readpage_inline(inode, page);
 
 	if (ret == -EAGAIN)
-		return ext4_mpage_readpages(page->mapping, NULL, page, 1,
-						false);
+		return ext4_mpage_readpages(page->mapping, 0, page, 1, false);
 
 	return ret;
 }
 
-static int
-ext4_readpages(struct file *file, struct address_space *mapping,
-		struct list_head *pages, unsigned nr_pages)
+static unsigned
+ext4_readahead(struct file *file, struct address_space *mapping,
+		pgoff_t start, unsigned nr_pages)
 {
 	struct inode *inode = mapping->host;
 
-	/* If the file has inline data, no need to do readpages. */
+	/* If the file has inline data, no need to do readahead. */
 	if (ext4_has_inline_data(inode))
-		return 0;
+		return nr_pages;
 
-	return ext4_mpage_readpages(mapping, pages, NULL, nr_pages, true);
+	ext4_mpage_readpages(mapping, start, NULL, nr_pages, true);
+	return 0;
 }
 
 static void ext4_invalidatepage(struct page *page, unsigned int offset,
@@ -3587,7 +3587,7 @@ static int ext4_set_page_dirty(struct page *page)
 
 static const struct address_space_operations ext4_aops = {
 	.readpage		= ext4_readpage,
-	.readpages		= ext4_readpages,
+	.readahead		= ext4_readahead,
 	.writepage		= ext4_writepage,
 	.writepages		= ext4_writepages,
 	.write_begin		= ext4_write_begin,
@@ -3604,7 +3604,7 @@ static const struct address_space_operations ext4_aops = {
 
 static const struct address_space_operations ext4_journalled_aops = {
 	.readpage		= ext4_readpage,
-	.readpages		= ext4_readpages,
+	.readahead		= ext4_readahead,
 	.writepage		= ext4_writepage,
 	.writepages		= ext4_writepages,
 	.write_begin		= ext4_write_begin,
@@ -3620,7 +3620,7 @@ static const struct address_space_operations ext4_journalled_aops = {
 
 static const struct address_space_operations ext4_da_aops = {
 	.readpage		= ext4_readpage,
-	.readpages		= ext4_readpages,
+	.readahead		= ext4_readahead,
 	.writepage		= ext4_writepage,
 	.writepages		= ext4_writepages,
 	.write_begin		= ext4_da_write_begin,
diff --git a/fs/ext4/readpage.c b/fs/ext4/readpage.c
index c1769afbf799..aaeb298c8fdb 100644
--- a/fs/ext4/readpage.c
+++ b/fs/ext4/readpage.c
@@ -7,8 +7,8 @@
  *
  * This was originally taken from fs/mpage.c
  *
- * The intent is the ext4_mpage_readpages() function here is intended
- * to replace mpage_readpages() in the general case, not just for
+ * The ext4_mpage_readahead() function here is intended to
+ * replace mpage_readahead() in the general case, not just for
  * encrypted files.  It has some limitations (see below), where it
  * will fall back to read_block_full_page(), but these limitations
  * should only be hit when page_size != block_size.
@@ -221,9 +221,8 @@ static inline loff_t ext4_readpage_limit(struct inode *inode)
 	return i_size_read(inode);
 }
 
-int ext4_mpage_readpages(struct address_space *mapping,
-			 struct list_head *pages, struct page *page,
-			 unsigned nr_pages, bool is_readahead)
+int ext4_mpage_readpages(struct address_space *mapping, pgoff_t start,
+		struct page *page, unsigned nr_pages, bool is_readahead)
 {
 	struct bio *bio = NULL;
 	sector_t last_block_in_bio = 0;
@@ -251,14 +250,10 @@ int ext4_mpage_readpages(struct address_space *mapping,
 		int fully_mapped = 1;
 		unsigned first_hole = blocks_per_page;
 
-		if (pages) {
-			page = lru_to_page(pages);
+		if (is_readahead) {
+			page = readahead_page(mapping, start++);
 
 			prefetchw(&page->flags);
-			list_del(&page->lru);
-			if (add_to_page_cache_lru(page, mapping, page->index,
-				  readahead_gfp_mask(mapping)))
-				goto next_page;
 		}
 
 		if (page_has_buffers(page))
@@ -406,10 +401,9 @@ int ext4_mpage_readpages(struct address_space *mapping,
 		else
 			unlock_page(page);
 	next_page:
-		if (pages)
+		if (is_readahead)
 			put_page(page);
 	}
-	BUG_ON(pages && !list_empty(pages));
 	if (bio)
 		submit_bio(bio);
 	return 0;
diff --git a/fs/ext4/verity.c b/fs/ext4/verity.c
index dc5ec724d889..40a550c0da2b 100644
--- a/fs/ext4/verity.c
+++ b/fs/ext4/verity.c
@@ -351,7 +351,6 @@ static int ext4_get_verity_descriptor(struct inode *inode, void *buf,
 static void ext4_merkle_tree_readahead(struct address_space *mapping,
 				       pgoff_t start_index, unsigned long count)
 {
-	LIST_HEAD(pages);
 	unsigned int nr_pages = 0;
 	struct page *page;
 	pgoff_t index;
@@ -360,16 +359,23 @@ static void ext4_merkle_tree_readahead(struct address_space *mapping,
 	for (index = start_index; index < start_index + count; index++) {
 		page = xa_load(&mapping->i_pages, index);
 		if (!page || xa_is_value(page)) {
-			page = __page_cache_alloc(readahead_gfp_mask(mapping));
+			gfp_t gfp = readahead_gfp_mask(mapping);
+			page = __page_cache_alloc(gfp);
 			if (!page)
 				break;
-			page->index = index;
-			list_add(&page->lru, &pages);
+			if (add_to_page_cache_lru(page, mapping, index, gfp)) {
+				put_page(page);
+				break;
+			}
 			nr_pages++;
 		}
 	}
+
+	if (!nr_pages)
+		return;
+
 	blk_start_plug(&plug);
-	ext4_mpage_readpages(mapping, &pages, NULL, nr_pages, true);
+	ext4_mpage_readpages(mapping, start_index, NULL, nr_pages, true);
 	blk_finish_plug(&plug);
 }
 
-- 
2.24.1

Re: [PATCH v4 00/12] Change readahead API

[David Sterba]

On Sat, Feb 01, 2020 at 07:12:28AM -0800, Matthew Wilcox wrote:
> From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
> 
> I would particularly value feedback on this from the gfs2 and ocfs2
> maintainers.  They have non-trivial changes, and a review on patch 5
> would be greatly appreciated.
> 
> This series adds a readahead address_space operation to eventually
> replace the readpages operation.  The key difference is that
> pages are added to the page cache as they are allocated (and
> then looked up by the filesystem) instead of passing them on a
> list to the readpages operation and having the filesystem add
> them to the page cache.  It's a net reduction in code for each
> implementation, more efficient than walking a list, and solves
> the direct-write vs buffered-read problem reported by yu kuai at
> https://lore.kernel.org/linux-fsdevel/20200116063601.39201-1-yukuai3@huawei.com/
> 
> v4:
>  - Rebase on current Linus (a62aa6f7f50a ("Merge tag 'gfs2-for-5.6'"))

I've tried to test the patchset but haven't got very far, it crashes at boot
ritht after VFS mounts the root. The patches are from mailinglist, applied on
current master, bug I saw the same crash with the git branch in your
repo (probably v1).

(gdb) l *(ext4_mpage_readpages+0x1da/0xc20)
0xffffffff813753f0 is in ext4_mpage_readpages (fs/ext4/readpage.c:226).
221             return i_size_read(inode);
222     }
223
224     int ext4_mpage_readpages(struct address_space *mapping, pgoff_t start,
225                     struct page *page, unsigned nr_pages, bool is_readahead)
226     {
227             struct bio *bio = NULL;
228             sector_t last_block_in_bio = 0;
229
230             struct inode *inode = mapping->host;

[    8.008531] BUG: kernel NULL pointer dereference, address: 0000000000000000
[    8.011482] #PF: supervisor read access in kernel mode
[    8.014121] #PF: error_code(0x0000) - not-present page
[    8.016767] PGD 0 P4D 0
[    8.018352] Oops: 0000 [#1] SMP
[    8.019716] CPU: 2 PID: 1 Comm: swapper/0 Not tainted 5.5.0-default+ #955
[    8.021746] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.0-59-gc9ba527-rebuilt.opensuse.org 04/01/2014
[    8.025244] RIP: 0010:ext4_mpage_readpages+0x1da/0xc20
[    8.026817] Code: 7c 24 4e 00 0f 85 23 04 00 00 44 29 74 24 3c 83 6c 24 48 01 0f 84 4d 04 00 00 80 7c 24 4e 00 0f 85 fc 05 00 00 48 8b 4c 24 18 <48> 8b 01 f6 c4 20 75 89 4c 8b 69 20 b9 0c 00 00 00 2b 4c 24 38 83
[    8.031957] RSP: 0000:ffffb34f40013988 EFLAGS: 00010292
[    8.033691] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
[    8.035533] RDX: 0000000000000001 RSI: ffffffff960934c0 RDI: ffffffff9681a080
[    8.036900] RBP: 0000000000000001 R08: ffffb34f40013a68 R09: 0000000000000000
[    8.038461] R10: 0000000000000038 R11: 0000000000000000 R12: 0000000000000004
[    8.040698] R13: ffff9668ba4e18e0 R14: 0000000000000001 R15: 0000000000000000
[    8.042805] FS:  0000000000000000(0000) GS:ffff9668bda00000(0000) knlGS:0000000000000000
[    8.045396] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[    8.047233] CR2: 0000000000000000 CR3: 000000002e011001 CR4: 0000000000160ee0
[    8.049337] Call Trace:
[    8.050435]  ? __lock_acquire+0xee0/0x1320
[    8.051833]  ? release_pages+0x310/0x380
[    8.053265]  ? mark_held_locks+0x50/0x80
[    8.054468]  ext4_readahead+0x3b/0x50
[    8.055877]  read_pages+0x65/0x1a0
[    8.057167]  ? put_pages_list+0x90/0x90
[    8.058689]  __do_page_cache_readahead+0x24b/0x2a0
[    8.060394]  generic_file_buffered_read+0x7cf/0x9f0
[    8.062137]  ? sched_clock+0x5/0x10
[    8.063451]  ? up_read+0x18/0x240
[    8.064774]  ? ext4_xattr_get+0x97/0x2c0
[    8.066178]  new_sync_read+0x111/0x1a0
[    8.067423]  vfs_read+0xc5/0x180
[    8.068572]  kernel_read+0x2c/0x40
[    8.069788]  prepare_binprm+0x171/0x1b0
[    8.071311]  load_script+0x1c1/0x250
[    8.072643]  search_binary_handler+0x5f/0x210
[    8.074135]  exec_binprm+0xd7/0x290
[    8.075463]  __do_execve_file.isra.0+0x570/0x800
[    8.077400]  ? rest_init+0x2f1/0x2f5
[    8.078979]  do_execve+0x21/0x30
[    8.080420]  kernel_init+0xa4/0x11b
[    8.081856]  ? rest_init+0x2f5/0x2f5
[    8.083173]  ret_from_fork+0x24/0x30
[    8.084695] Modules linked in:
[    8.086055] CR2: 0000000000000000
[    8.087572] ---[ end trace 0890c371a706b34a ]---
[    8.089417] RIP: 0010:ext4_mpage_readpages+0x1da/0xc20
[    8.116836] BUG: sleeping function called from invalid context at include/linux/percpu-rwsem.h:38
[    8.119626] in_atomic(): 0, irqs_disabled(): 1, non_block: 0, pid: 1, name: swapper/0
[    8.122392] INFO: lockdep is turned off.
[    8.123694] irq event stamp: 18341344
[    8.124735] hardirqs last  enabled at (18341343): [<ffffffff95230c42>] free_unref_page_list+0x232/0x270
[    8.127918] hardirqs last disabled at (18341344): [<ffffffff95002b4b>] trace_hardirqs_off_thunk+0x1a/0x1c
[    8.131145] softirqs last  enabled at (18341250): [<ffffffff95a00358>] __do_softirq+0x358/0x52b
[    8.143060] softirqs last disabled at (18341243): [<ffffffff9508ae3d>] irq_exit+0x9d/0xb0
[    8.145603] CPU: 2 PID: 1 Comm: swapper/0 Tainted: G      D           5.5.0-default+ #955
[    8.148474] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.0-59-gc9ba527-rebuilt.opensuse.org 04/01/2014
[    8.152440] Call Trace:
[    8.153747]  dump_stack+0x71/0xa0
[    8.155238]  ___might_sleep.cold+0xa6/0xf9
[    8.156903]  exit_signals+0x31/0x310
[    8.158431]  ? __do_execve_file.isra.0+0x570/0x800
[    8.160179]  do_exit+0xa8/0xd60
[    8.161632]  ? rest_init+0x2f1/0x2f5
[    8.163204]  rewind_stack_do_exit+0x17/0x20
[    8.164931] Kernel panic - not syncing: Attempted to kill init! exitcode=0x00000009
[    8.167575] Kernel Offset: 0x14000000 from 0xffffffff81000000 (relocation range: 0xffffffff80000000-0xffffffffbfffffff)

Re: [PATCH v4 00/12] Change readahead API

[Matthew Wilcox]

On Tue, Feb 04, 2020 at 04:32:27PM +0100, David Sterba wrote:
> On Sat, Feb 01, 2020 at 07:12:28AM -0800, Matthew Wilcox wrote:
> > From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
> > 
> > I would particularly value feedback on this from the gfs2 and ocfs2
> > maintainers.  They have non-trivial changes, and a review on patch 5
> > would be greatly appreciated.
> > 
> > This series adds a readahead address_space operation to eventually
> > replace the readpages operation.  The key difference is that
> > pages are added to the page cache as they are allocated (and
> > then looked up by the filesystem) instead of passing them on a
> > list to the readpages operation and having the filesystem add
> > them to the page cache.  It's a net reduction in code for each
> > implementation, more efficient than walking a list, and solves
> > the direct-write vs buffered-read problem reported by yu kuai at
> > https://lore.kernel.org/linux-fsdevel/20200116063601.39201-1-yukuai3@huawei.com/
> > 
> > v4:
> >  - Rebase on current Linus (a62aa6f7f50a ("Merge tag 'gfs2-for-5.6'"))
> 
> I've tried to test the patchset but haven't got very far, it crashes at boot
> ritht after VFS mounts the root. The patches are from mailinglist, applied on
> current master, bug I saw the same crash with the git branch in your
> repo (probably v1).

Yeah, I wasn't able to test at the time due to what turned out to be
the hpet bug in Linus' tree.  Now that's fixed, I've found & fixed a
couple more bugs.  There'll be a v5 once I fix the remaining problem
(looks like a missing page unlock somewhere).