[PATCH v3 00/12] Enable per-file/directory DAX operations V3

[ira . weiny]

From: Ira Weiny <ira.weiny@xxxxxxxxx>

Changes from V2:

	* Move i_dax_sem to be a global percpu_rw_sem rather than per inode
		Internal discussions with Dan determined this would be easier,
		just as performant, and slightly less overhead that having it
		in the SB as suggested by Jan
	* Fix locking order in comments and throughout code
	* Change "mode" to "state" throughout commits
	* Add CONFIG_FS_DAX wrapper to disable inode_[un]lock_state() when not
		configured
	* Add static branch for which is activated by a device which supports
		DAX in XFS
	* Change "lock/unlock" to up/down read/write as appropriate
		Previous names were over simplified
	* Update comments/documentation

	* Remove the xfs specific lock to the vfs (global) layer.
	* Fix i_dax_sem locking order and comments

	* Move 'i_mapped' count from struct inode to struct address_space and
		rename it to mmap_count
	* Add inode_has_mappings() call

	* Fix build issues
	* Clean up syntax spacing and minor issues
	* Update man page text for STATX_ATTR_DAX
	* Add reviewed-by's
	* Rebase to latest linux-next

	Rename patch:
		from: fs/xfs: Add lock/unlock state to xfs
		to: fs/xfs: Add write DAX lock to xfs layer
	Add patch:
		fs/xfs: Clarify lockdep dependency for xfs_isilocked()
	Drop patch:
		fs/xfs: Fix truncate up

	https://github.com/weiny2/linux-kernel/tree/dax-file-state-change-v3

At LSF/MM'19 [1] [2] we discussed applications that overestimate memory
consumption due to their inability to detect whether the kernel will
instantiate page cache for a file, and cases where a global dax enable via a
mount option is too coarse.

The following patch series enables selecting the use of DAX on individual files
and/or directories on xfs, and lays some groundwork to do so in ext4.  In this
scheme the dax mount option can be omitted to allow the per-file property to
take effect.

The insight at LSF/MM was to separate the per-mount or per-file "physical"
capability switch from an "effective" attribute for the file.

At LSF/MM we discussed the difficulties of switching the DAX state of a file
with active mappings / page cache.  It was thought the races could be avoided
by limiting DAX state flips to 0-length files.

However, this turns out to not be true.[3] This is because address space
operations (a_ops) may be in use at any time the inode is referenced and users
have expressed a desire to be able to change the DAX state on a file with data
in it.  For those reasons this patch set allows changing the DAX state flag on
a file as long as it is not current mapped.

Furthermore, DAX is a property of the inode and as such, many operations other
than address space operations need to be protected during a DAX state change.

Therefore callbacks are placed within the inode operations and used to lock the
inode as appropriate.

As in V1, Users are able to query the effective and physical flags separately
at any time.  Specifically the addition of the statx attribute bit allows them
to ensure the file is operating in the DAX state they intend.  This 'effective
flag' and physical flags could differ when the filesystem is mounted with the
dax flag for example.

It should be noted that the physical DAX flag inheritance is not shown in this
patch set as it was maintained from previous work on XFS.  The physical DAX
flag and it's inheritance will need to be added to other file systems for user
control. 


[1] https://lwn.net/Articles/787973/
[2] https://lwn.net/Articles/787233/
[3] https://lkml.org/lkml/2019/10/20/96
[4] https://patchwork.kernel.org/patch/11310511/


To: linux-kernel@xxxxxxxxxxxxxxx
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: "Darrick J. Wong" <darrick.wong@xxxxxxxxxx>
Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
Cc: Dave Chinner <david@xxxxxxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxx>
Cc: "Theodore Y. Ts'o" <tytso@xxxxxxx>
Cc: Jan Kara <jack@xxxxxxx>
Cc: linux-ext4@xxxxxxxxxxxxxxx
Cc: linux-xfs@xxxxxxxxxxxxxxx
Cc: linux-fsdevel@xxxxxxxxxxxxxxx


Ira Weiny (12):
  fs/stat: Define DAX statx attribute
  fs/xfs: Isolate the physical DAX flag from effective
  fs/xfs: Separate functionality of xfs_inode_supports_dax()
  fs/xfs: Clean up DAX support check
  fs: remove unneeded IS_DAX() check
  fs/xfs: Check if the inode supports DAX under lock
  fs: Add locking for a dynamic DAX state
  fs/xfs: Clarify lockdep dependency for xfs_isilocked()
  fs/xfs: Add write DAX lock to xfs layer
  fs: Prevent DAX state change if file is mmap'ed
  fs/xfs: Clean up locking in dax invalidate
  fs/xfs: Allow toggle of effective DAX flag

 Documentation/filesystems/vfs.rst | 17 ++++++
 fs/attr.c                         |  1 +
 fs/dax.c                          |  3 ++
 fs/inode.c                        | 15 ++++--
 fs/iomap/buffered-io.c            |  1 +
 fs/open.c                         |  4 ++
 fs/stat.c                         |  5 ++
 fs/super.c                        |  3 ++
 fs/xfs/xfs_inode.c                | 24 +++++++--
 fs/xfs/xfs_inode.h                |  8 ++-
 fs/xfs/xfs_ioctl.c                | 56 ++++++++++++--------
 fs/xfs/xfs_iops.c                 | 51 ++++++++++++++----
 fs/xfs/xfs_iops.h                 |  2 +
 fs/xfs/xfs_super.c                | 16 +++---
 include/linux/fs.h                | 86 +++++++++++++++++++++++++++++--
 include/uapi/linux/stat.h         |  1 +
 mm/fadvise.c                      | 10 +++-
 mm/filemap.c                      |  4 ++
 mm/huge_memory.c                  |  1 +
 mm/khugepaged.c                   |  2 +
 mm/madvise.c                      |  3 ++
 mm/mmap.c                         | 19 ++++++-
 mm/util.c                         |  9 +++-
 23 files changed, 287 insertions(+), 54 deletions(-)

-- 
2.21.0