[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 12/12] iomap: Convert from readpages to readahead

[Matthew Wilcox]

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

Use the new readahead operation in XFS and iomap.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: linux-xfs@vger.kernel.org
---
 fs/iomap/buffered-io.c | 72 +++++++++---------------------------------
 fs/iomap/trace.h       |  2 +-
 fs/xfs/xfs_aops.c      | 10 +++---
 include/linux/iomap.h  |  2 +-
 4 files changed, 22 insertions(+), 64 deletions(-)

diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index cb3511eb152a..490b66ea3298 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -216,7 +216,6 @@ struct iomap_readpage_ctx {
 	bool			cur_page_in_bio;
 	bool			is_readahead;
 	struct bio		*bio;
-	struct list_head	*pages;
 };
 
 static void
@@ -367,36 +366,8 @@ iomap_readpage(struct page *page, const struct iomap_ops *ops)
 }
 EXPORT_SYMBOL_GPL(iomap_readpage);
 
-static struct page *
-iomap_next_page(struct inode *inode, struct list_head *pages, loff_t pos,
-		loff_t length, loff_t *done)
-{
-	while (!list_empty(pages)) {
-		struct page *page = lru_to_page(pages);
-
-		if (page_offset(page) >= (u64)pos + length)
-			break;
-
-		list_del(&page->lru);
-		if (!add_to_page_cache_lru(page, inode->i_mapping, page->index,
-				GFP_NOFS))
-			return page;
-
-		/*
-		 * If we already have a page in the page cache at index we are
-		 * done.  Upper layers don't care if it is uptodate after the
-		 * readpages call itself as every page gets checked again once
-		 * actually needed.
-		 */
-		*done += PAGE_SIZE;
-		put_page(page);
-	}
-
-	return NULL;
-}
-
 static loff_t
-iomap_readpages_actor(struct inode *inode, loff_t pos, loff_t length,
+iomap_readahead_actor(struct inode *inode, loff_t pos, loff_t length,
 		void *data, struct iomap *iomap, struct iomap *srcmap)
 {
 	struct iomap_readpage_ctx *ctx = data;
@@ -410,10 +381,8 @@ iomap_readpages_actor(struct inode *inode, loff_t pos, loff_t length,
 			ctx->cur_page = NULL;
 		}
 		if (!ctx->cur_page) {
-			ctx->cur_page = iomap_next_page(inode, ctx->pages,
-					pos, length, &done);
-			if (!ctx->cur_page)
-				break;
+			ctx->cur_page = readahead_page(inode->i_mapping,
+					pos / PAGE_SIZE);
 			ctx->cur_page_in_bio = false;
 		}
 		ret = iomap_readpage_actor(inode, pos + done, length - done,
@@ -423,48 +392,37 @@ iomap_readpages_actor(struct inode *inode, loff_t pos, loff_t length,
 	return done;
 }
 
-int
-iomap_readpages(struct address_space *mapping, struct list_head *pages,
+unsigned
+iomap_readahead(struct address_space *mapping, pgoff_t start,
 		unsigned nr_pages, const struct iomap_ops *ops)
 {
 	struct iomap_readpage_ctx ctx = {
-		.pages		= pages,
 		.is_readahead	= true,
 	};
-	loff_t pos = page_offset(list_entry(pages->prev, struct page, lru));
-	loff_t last = page_offset(list_entry(pages->next, struct page, lru));
-	loff_t length = last - pos + PAGE_SIZE, ret = 0;
+	loff_t pos = start * PAGE_SIZE;
+	loff_t length = nr_pages * PAGE_SIZE;
 
-	trace_iomap_readpages(mapping->host, nr_pages);
+	trace_iomap_readahead(mapping->host, nr_pages);
 
 	while (length > 0) {
-		ret = iomap_apply(mapping->host, pos, length, 0, ops,
-				&ctx, iomap_readpages_actor);
+		loff_t ret = iomap_apply(mapping->host, pos, length, 0, ops,
+				&ctx, iomap_readahead_actor);
 		if (ret <= 0) {
 			WARN_ON_ONCE(ret == 0);
-			goto done;
+			break;
 		}
 		pos += ret;
 		length -= ret;
 	}
-	ret = 0;
-done:
+
 	if (ctx.bio)
 		submit_bio(ctx.bio);
-	if (ctx.cur_page) {
-		if (!ctx.cur_page_in_bio)
-			unlock_page(ctx.cur_page);
+	if (ctx.cur_page && ctx.cur_page_in_bio)
 		put_page(ctx.cur_page);
-	}
 
-	/*
-	 * Check that we didn't lose a page due to the arcance calling
-	 * conventions..
-	 */
-	WARN_ON_ONCE(!ret && !list_empty(ctx.pages));
-	return ret;
+	return length / PAGE_SIZE;
 }
-EXPORT_SYMBOL_GPL(iomap_readpages);
+EXPORT_SYMBOL_GPL(iomap_readahead);
 
 /*
  * iomap_is_partially_uptodate checks whether blocks within a page are
diff --git a/fs/iomap/trace.h b/fs/iomap/trace.h
index 6dc227b8c47e..d6ba705f938a 100644
--- a/fs/iomap/trace.h
+++ b/fs/iomap/trace.h
@@ -39,7 +39,7 @@ DEFINE_EVENT(iomap_readpage_class, name,	\
 	TP_PROTO(struct inode *inode, int nr_pages), \
 	TP_ARGS(inode, nr_pages))
 DEFINE_READPAGE_EVENT(iomap_readpage);
-DEFINE_READPAGE_EVENT(iomap_readpages);
+DEFINE_READPAGE_EVENT(iomap_readahead);
 
 DECLARE_EVENT_CLASS(iomap_page_class,
 	TP_PROTO(struct inode *inode, struct page *page, unsigned long off,
diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
index 3a688eb5c5ae..4d9da34e759b 100644
--- a/fs/xfs/xfs_aops.c
+++ b/fs/xfs/xfs_aops.c
@@ -621,14 +621,14 @@ xfs_vm_readpage(
 	return iomap_readpage(page, &xfs_read_iomap_ops);
 }
 
-STATIC int
-xfs_vm_readpages(
+STATIC unsigned
+xfs_vm_readahead(
 	struct file		*unused,
 	struct address_space	*mapping,
-	struct list_head	*pages,
+	pgoff_t			start,
 	unsigned		nr_pages)
 {
-	return iomap_readpages(mapping, pages, nr_pages, &xfs_read_iomap_ops);
+	return iomap_readahead(mapping, start, nr_pages, &xfs_read_iomap_ops);
 }
 
 static int
@@ -644,7 +644,7 @@ xfs_iomap_swapfile_activate(
 
 const struct address_space_operations xfs_address_space_operations = {
 	.readpage		= xfs_vm_readpage,
-	.readpages		= xfs_vm_readpages,
+	.readahead		= xfs_vm_readahead,
 	.writepage		= xfs_vm_writepage,
 	.writepages		= xfs_vm_writepages,
 	.set_page_dirty		= iomap_set_page_dirty,
diff --git a/include/linux/iomap.h b/include/linux/iomap.h
index 8b09463dae0d..81c6067e9b61 100644
--- a/include/linux/iomap.h
+++ b/include/linux/iomap.h
@@ -155,7 +155,7 @@ loff_t iomap_apply(struct inode *inode, loff_t pos, loff_t length,
 ssize_t iomap_file_buffered_write(struct kiocb *iocb, struct iov_iter *from,
 		const struct iomap_ops *ops);
 int iomap_readpage(struct page *page, const struct iomap_ops *ops);
-int iomap_readpages(struct address_space *mapping, struct list_head *pages,
+unsigned iomap_readahead(struct address_space *, pgoff_t start,
 		unsigned nr_pages, const struct iomap_ops *ops);
 int iomap_set_page_dirty(struct page *page);
 int iomap_is_partially_uptodate(struct page *page, unsigned long from,
-- 
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).