[PATCH 01/13] mm: Add AOP_UPDATED_PAGE return value

Matthew Wilcox (Oracle)
2020-09-17T15:11:41Z

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 | 12 ++++++++++--
4 files changed, 33 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 e019ea2f1347..6fc650050d20 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..131a2aaa1537 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2254,8 +2254,10 @@ 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)
+ goto page_ok;

if (unlikely(error)) {
if (error == AOP_TRUNCATED_PAGE) {
@@ -2619,7 +2621,7 @@ 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 +2656,8 @@ 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 +2871,10 @@ 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);