fall back from direct to buffered I/O when stable writes are required

[Christoph Hellwig <hch@lst.de>]

Hi all,

we've had a long standing issue that direct I/O to and from devices that
require stable writes can corrupt data because the user memory can be
modified while in flight.  This series tries to address this by falling
back to uncached buffered I/O.  Given that this requires an extra copy it
is usually going to be a slow down, especially for very high bandwith
use cases, so I'm not exactly happy about.

I suspect we need a way to opt out of this for applications that know
what they are doing, and I can think of a few ways to do that:

1a) Allow a mount option to override the behavior

	This allows the sysadmin to get back to the previous state.
	This is fairly easy to implement, but the scope might be to wide.

1b) Sysfs attribute

	Same as above.  Slightly easier to modify, but a more unusual
	interface.

2) Have a per-inode attribute

	Allows to set it on a specific file.  Would require an on-disk
	format change for the usual attr options.

3) Have a fcntl or similar to allow an application to override it

	Fine granularity.  Requires application change.  We might not
	allow any application to force this as it could be used to inject
	corruption.

In other words, they are all kinda horrible.

Diffstat:
 fs/ext4/file.c      |    2 -
 fs/xfs/xfs_file.c   |   59 +++++++++++++++++++++++++++++++++++++++++++---------
 fs/xfs/xfs_iops.c   |    6 +++++
 include/linux/fs.h  |   11 +++++----
 io_uring/io_uring.c |    2 -
 5 files changed, 63 insertions(+), 17 deletions(-)