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From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
To: linux-fsdevel@vger.kernel.org
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>,
	linux-mm@kvack.org, v9fs-developer@lists.sourceforge.net,
	linux-kernel@vger.kernel.org, linux-afs@lists.infradead.org,
	ceph-devel@vger.kernel.org, linux-cifs@vger.kernel.org,
	ecryptfs@vger.kernel.org, linux-um@lists.infradead.org,
	linux-mtd@lists.infradead.org,
	Richard Weinberger <richard@nod.at>,
	linux-xfs@vger.kernel.org
Subject: [PATCH v2 01/16] mm: Add AOP_UPDATED_PAGE return value
Date: Fri,  9 Oct 2020 15:30:49 +0100
Message-ID: <20201009143104.22673-2-willy@infradead.org> (raw)
In-Reply-To: <20201009143104.22673-1-willy@infradead.org>

Allow synchronous ->readpage implementations to execute more
efficiently by skipping the re-locking of the page.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 Documentation/filesystems/locking.rst |  7 ++++---
 Documentation/filesystems/vfs.rst     | 21 ++++++++++++++-------
 include/linux/fs.h                    |  5 +++++
 mm/filemap.c                          | 15 +++++++++++++--
 4 files changed, 36 insertions(+), 12 deletions(-)

diff --git a/Documentation/filesystems/locking.rst b/Documentation/filesystems/locking.rst
index 64f94a18d97e..06a7a8bf2362 100644
--- a/Documentation/filesystems/locking.rst
+++ b/Documentation/filesystems/locking.rst
@@ -269,7 +269,7 @@ locking rules:
 ops			PageLocked(page)	 i_rwsem
 ======================	======================== =========
 writepage:		yes, unlocks (see below)
-readpage:		yes, unlocks
+readpage:		yes, may unlock
 writepages:
 set_page_dirty		no
 readahead:		yes, unlocks
@@ -294,8 +294,9 @@ swap_deactivate:	no
 ->write_begin(), ->write_end() and ->readpage() may be called from
 the request handler (/dev/loop).
 
-->readpage() unlocks the page, either synchronously or via I/O
-completion.
+->readpage() may return AOP_UPDATED_PAGE if the page is now Uptodate
+or 0 if the page will be unlocked asynchronously by I/O completion.
+If it returns -errno, it should unlock the page.
 
 ->readahead() unlocks the pages that I/O is attempted on like ->readpage().
 
diff --git a/Documentation/filesystems/vfs.rst b/Documentation/filesystems/vfs.rst
index ca52c82e5bb5..16248c299aaa 100644
--- a/Documentation/filesystems/vfs.rst
+++ b/Documentation/filesystems/vfs.rst
@@ -643,7 +643,7 @@ set_page_dirty to write data into the address_space, and writepage and
 writepages to writeback data to storage.
 
 Adding and removing pages to/from an address_space is protected by the
-inode's i_mutex.
+inode's i_rwsem held exclusively.
 
 When data is written to a page, the PG_Dirty flag should be set.  It
 typically remains set until writepage asks for it to be written.  This
@@ -757,12 +757,19 @@ cache in your filesystem.  The following members are defined:
 
 ``readpage``
 	called by the VM to read a page from backing store.  The page
-	will be Locked when readpage is called, and should be unlocked
-	and marked uptodate once the read completes.  If ->readpage
-	discovers that it needs to unlock the page for some reason, it
-	can do so, and then return AOP_TRUNCATED_PAGE.  In this case,
-	the page will be relocated, relocked and if that all succeeds,
-	->readpage will be called again.
+	will be Locked and !Uptodate when readpage is called.  Ideally,
+	the filesystem will bring the page Uptodate and return
+	AOP_UPDATED_PAGE.  If the filesystem encounters an error, it
+	should unlock the page and return a negative errno without marking
+	the page Uptodate.  It does not need to mark the page as Error.
+	If the filesystem returns 0, this means the page will be unlocked
+	asynchronously by I/O completion.  The VFS will wait for the
+	page to be unlocked, so there is no advantage to executing this
+	operation asynchronously.
+
+	The filesystem can also return AOP_TRUNCATED_PAGE to indicate
+	that it had to unlock the page to avoid a deadlock.  The caller
+	will re-check the page cache and call ->readpage again.
 
 ``writepages``
 	called by the VM to write out pages associated with the
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 7519ae003a08..badf80e133fd 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -273,6 +273,10 @@ struct iattr {
  *  			reference, it should drop it before retrying.  Returned
  *  			by readpage().
  *
+ * @AOP_UPDATED_PAGE: The readpage method has brought the page Uptodate
+ * without releasing the page lock.  This is suitable for synchronous
+ * implementations of readpage.
+ *
  * address_space_operation functions return these large constants to indicate
  * special semantics to the caller.  These are much larger than the bytes in a
  * page to allow for functions that return the number of bytes operated on in a
@@ -282,6 +286,7 @@ struct iattr {
 enum positive_aop_returns {
 	AOP_WRITEPAGE_ACTIVATE	= 0x80000,
 	AOP_TRUNCATED_PAGE	= 0x80001,
+	AOP_UPDATED_PAGE	= 0x80002,
 };
 
 #define AOP_FLAG_CONT_EXPAND		0x0001 /* called from cont_expand */
diff --git a/mm/filemap.c b/mm/filemap.c
index 1aaea26556cc..95b68ec1f22c 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2254,8 +2254,13 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb,
 		 * PG_error will be set again if readpage fails.
 		 */
 		ClearPageError(page);
-		/* Start the actual read. The read will unlock the page. */
+		/* Start the actual read. The read may unlock the page. */
 		error = mapping->a_ops->readpage(filp, page);
+		if (error == AOP_UPDATED_PAGE) {
+			unlock_page(page);
+			error = 0;
+			goto page_ok;
+		}
 
 		if (unlikely(error)) {
 			if (error == AOP_TRUNCATED_PAGE) {
@@ -2619,7 +2624,7 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
 	 */
 	if (unlikely(!PageUptodate(page)))
 		goto page_not_uptodate;
-
+page_ok:
 	/*
 	 * We've made it this far and we had to drop our mmap_lock, now is the
 	 * time to return to the upper layer and have it re-find the vma and
@@ -2654,6 +2659,8 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
 	ClearPageError(page);
 	fpin = maybe_unlock_mmap_for_io(vmf, fpin);
 	error = mapping->a_ops->readpage(file, page);
+	if (error == AOP_UPDATED_PAGE)
+		goto page_ok;
 	if (!error) {
 		wait_on_page_locked(page);
 		if (!PageUptodate(page))
@@ -2867,6 +2874,10 @@ static struct page *do_read_cache_page(struct address_space *mapping,
 			err = filler(data, page);
 		else
 			err = mapping->a_ops->readpage(data, page);
+		if (err == AOP_UPDATED_PAGE) {
+			unlock_page(page);
+			goto out;
+		}
 
 		if (err < 0) {
 			put_page(page);
-- 
2.28.0


  reply index

Thread overview: 28+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-10-09 14:30 [PATCH v2 00/16] Allow readpage to return a locked page Matthew Wilcox (Oracle)
2020-10-09 14:30 ` Matthew Wilcox (Oracle) [this message]
2020-10-15  9:06   ` [PATCH v2 01/16] mm: Add AOP_UPDATED_PAGE return value Christoph Hellwig
2020-10-15 14:06     ` Matthew Wilcox
2020-10-09 14:30 ` [PATCH v2 02/16] mm: Inline wait_on_page_read into its one caller Matthew Wilcox (Oracle)
2020-10-15  9:08   ` Christoph Hellwig
2020-10-09 14:30 ` [PATCH v2 03/16] 9p: Tell the VFS that readpage was synchronous Matthew Wilcox (Oracle)
2020-10-09 14:30 ` [PATCH v2 04/16] afs: " Matthew Wilcox (Oracle)
2020-10-09 14:30 ` [PATCH v2 05/16] ceph: " Matthew Wilcox (Oracle)
2020-10-09 14:30 ` [PATCH v2 06/16] cifs: " Matthew Wilcox (Oracle)
2020-10-09 14:30 ` [PATCH v2 07/16] cramfs: " Matthew Wilcox (Oracle)
2020-10-09 14:30 ` [PATCH v2 08/16] ecryptfs: " Matthew Wilcox (Oracle)
2020-10-09 14:30 ` [PATCH v2 09/16] fuse: " Matthew Wilcox (Oracle)
2020-10-09 14:30 ` [PATCH v2 10/16] hostfs: " Matthew Wilcox (Oracle)
2020-10-09 14:30 ` [PATCH v2 11/16] jffs2: " Matthew Wilcox (Oracle)
2020-10-09 14:31 ` [PATCH v2 12/16] ubifs: " Matthew Wilcox (Oracle)
2020-10-09 14:31 ` [PATCH v2 13/16] udf: " Matthew Wilcox (Oracle)
2020-10-09 14:31 ` [PATCH v2 14/16] vboxsf: " Matthew Wilcox (Oracle)
2020-10-09 14:31 ` [PATCH v2 15/16] iomap: Inline iomap_iop_set_range_uptodate into its one caller Matthew Wilcox (Oracle)
2020-10-15  9:08   ` Christoph Hellwig
2020-10-09 14:31 ` [PATCH v2 16/16] iomap: Make readpage synchronous Matthew Wilcox (Oracle)
2020-10-15  9:42   ` Christoph Hellwig
2020-10-15 16:43     ` Matthew Wilcox
2020-10-15 17:58       ` Christoph Hellwig
2020-10-15 19:03         ` Matthew Wilcox
2020-10-16  6:35           ` Christoph Hellwig
2020-10-15  9:02 ` [PATCH v2 00/16] Allow readpage to return a locked page Christoph Hellwig
2020-10-15 11:49   ` Matthew Wilcox

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