[00/13] Allow readpage to return a locked page

Message

Matthew Wilcox (Oracle) Sept. 17, 2020, 3:10 p.m. UTC

Linus recently made the page lock more fair.  That means that the old
pattern where we returned from ->readpage with the page unlocked and
then attempted to re-lock it will send us to the back of the queue for
this page's lock.

Ideally all filesystems would return from ->readpage with the
page Uptodate and Locked, but it's a bit painful to convert all the
asynchronous readpage implementations to synchronous.  These ones are
already synchronous, so convert them while I work on iomap.

A further benefit is that a synchronous readpage implementation allows
us to return an error to someone who might actually care about it.
There's no need to SetPageError, but I don't want to learn about how
a dozen filesystems handle I/O errors (hint: they're all different),
so I have not attempted to change that.

Please review your filesystem carefully.  I've tried to catch all the
places where a filesystem calls its own internal readpage implementation
without going through ->readpage, but I may have missed some.

Matthew Wilcox (Oracle) (13):
  mm: Add AOP_UPDATED_PAGE return value
  9p: Tell the VFS that readpage was synchronous
  afs: Tell the VFS that readpage was synchronous
  ceph: Tell the VFS that readpage was synchronous
  cifs: Tell the VFS that readpage was synchronous
  cramfs: Tell the VFS that readpage was synchronous
  ecryptfs: Tell the VFS that readpage was synchronous
  fuse: Tell the VFS that readpage was synchronous
  hostfs: Tell the VFS that readpage was synchronous
  jffs2: Tell the VFS that readpage was synchronous
  ubifs: Tell the VFS that readpage was synchronous
  udf: Tell the VFS that readpage was synchronous
  vboxsf: Tell the VFS that readpage was synchronous

 Documentation/filesystems/locking.rst |  7 ++++---
 Documentation/filesystems/vfs.rst     | 21 ++++++++++++++-------
 fs/9p/vfs_addr.c                      |  6 +++++-
 fs/afs/file.c                         |  3 ++-
 fs/ceph/addr.c                        |  9 +++++----
 fs/cifs/file.c                        |  8 ++++++--
 fs/cramfs/inode.c                     |  5 ++---
 fs/ecryptfs/mmap.c                    | 11 ++++++-----
 fs/fuse/file.c                        |  2 ++
 fs/hostfs/hostfs_kern.c               |  2 ++
 fs/jffs2/file.c                       |  6 ++++--
 fs/ubifs/file.c                       | 16 ++++++++++------
 fs/udf/file.c                         |  3 +--
 fs/vboxsf/file.c                      |  2 ++
 include/linux/fs.h                    |  5 +++++
 mm/filemap.c                          | 12 ++++++++++--
 16 files changed, 80 insertions(+), 38 deletions(-)

Comments

Jeffrey Layton Sept. 17, 2020, 4:49 p.m. UTC | #1

On Thu, 2020-09-17 at 16:10 +0100, Matthew Wilcox (Oracle) wrote:
> The ceph readpage implementation was already synchronous, so use
> AOP_UPDATED_PAGE to avoid cycling the page lock.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <[e-mail]>
> ---
>  fs/ceph/addr.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/ceph/addr.c b/fs/ceph/addr.c
> index 6ea761c84494..b2bf8bf7a312 100644
> --- a/fs/ceph/addr.c
> +++ b/fs/ceph/addr.c
> @@ -291,10 +291,11 @@ static int ceph_do_readpage(struct file *filp, struct page *page)
>  static int ceph_readpage(struct file *filp, struct page *page)
>  {
>  	int r = ceph_do_readpage(filp, page);
> -	if (r != -EINPROGRESS)
> -		unlock_page(page);
> -	else
> -		r = 0;
> +	if (r == -EINPROGRESS)
> +		return 0;
> +	if (r == 0)
> +		return AOP_UPDATED_PAGE;
> +	unlock_page(page);
>  	return r;
>  }
>  

Looks good to me. I assume you'll merge all of these as a set since the
early ones are a prerequisite?

Reviewed-by: Jeff Layton <[e-mail]>