[PATCH v12 00/20] DAX: Page cache bypass for filesystems on memory storage

[Matthew Wilcox]

DAX is a replacement for the variation of XIP currently supported by
the ext2 filesystem.  We have three different things in the tree called
'XIP', and the new focus is on access to data rather than executables,
so a name change was in order.  DAX stands for Direct Access.  The X is
for eXciting.

The new focus on data access has resulted in more careful attention to
races that exist in the current XIP code, but are not hit by the use-case
that it was designed for.  XIP's architecture worked fine for ext2, but
DAX is architected to work with modern filsystems such as ext4 and XFS.
DAX is not intended for use with btrfs; the value that btrfs adds relies
on manipulating data and writing data to different locations, while DAX's
value is for write-in-place and keeping the kernel from touching the data.

DAX was developed in order to support NV-DIMMs, but it's become clear that
its usefuless extends beyond NV-DIMMs and there are several potential
customers including the tracing machinery.  Other people want to place
the kernel log in an area of memory, as long as they have a BIOS that
does not clear DRAM on reboot.

Patch 1 is a bug fix.  It is obviously correct, and should be included
into 3.18.

Patch 2 starts the transformation by changing how ->direct_access works.
Much code is moved from the drivers and filesystems into the block layer,
and we add the flexibility of being able to map more than one page at
a time.  It would be good to get this patch into 3.18 as it is also
useful for people who are pursuing non-DAX approaches to working with
persistent memory.

Patch 3 is also a bug fix, probably worth including in 3.18.

Patches 4 & 5 are infrastructure for DAX.

Patches 6-10 replace the XIP code with its DAX equivalents, transforming
ext2 to use the DAX code as we go.  Note that patch 10 is the
Documentation patch.

Patches 11-17 clean up after the XIP code, removing the infrastructure
that is no longer needed and renaming various XIP things to DAX.
Most of these patches were added after Jan found things he didn't
like in an earlier version of the ext4 patch ... that had been copied
from ext2.  So ext2 i being transformed to do things the same way that
ext4 will later.  The ability to mount ext2 filesystems with the 'xip'
option is retained, although the 'dax' option is now preferred.

Patch 18 adds some DAX infrastructure to support ext4.

Patch 19 adds DAX support to ext4.  It is broadly similar to ext2's DAX
support, but it is more efficient than ext4's due to its support for
unwritten extents.

Patch 20 is another cleanup patch renaming XIP to DAX.


My thanks to Mathieu Desnoyers for his reviews of the v11 patchset.  Most
of the changes below were based on his feedback.

Changes since v11:
 - Rebased to 3.18-rc1, dropping patch "vfs: Add copy_to_iter(),
   copy_from_iter() and iov_iter_zero()" as it was merged through Al's tree.
 - Added cc to stable@xxxxxxxxxxxxxxx on patch 1
 - Fixed comment style in brd.c (Mathieu)
 - Make more functions in fs.h common with and without CONFIG_FS_DAX set
 - Improve type-checking with !CONFIG_FS_DAX
 - Simplify check for holes in dax_io()
 - Harden the loop in dax_clear_blocks()
 - Add missing check against truncate of a page covering a hole
 - Fix the page-fault handler to work for block devices too
 - Change a few more places that mentioned 'XIP' into 'DAX'
 - Update DAX documentation in a couple of places

Matthew Wilcox (19):
  axonram: Fix bug in direct_access
  block: Change direct_access calling convention
  mm: Fix XIP fault vs truncate race
  mm: Allow page fault handlers to perform the COW
  vfs,ext2: Introduce IS_DAX(inode)
  dax,ext2: Replace XIP read and write with DAX I/O
  dax,ext2: Replace ext2_clear_xip_target with dax_clear_blocks
  dax,ext2: Replace the XIP page fault handler with the DAX page fault
    handler
  dax,ext2: Replace xip_truncate_page with dax_truncate_page
  dax: Replace XIP documentation with DAX documentation
  vfs: Remove get_xip_mem
  ext2: Remove ext2_xip_verify_sb()
  ext2: Remove ext2_use_xip
  ext2: Remove xip.c and xip.h
  vfs,ext2: Remove CONFIG_EXT2_FS_XIP and rename CONFIG_FS_XIP to
    CONFIG_FS_DAX
  ext2: Remove ext2_aops_xip
  ext2: Get rid of most mentions of XIP in ext2
  dax: Add dax_zero_page_range
  brd: Rename XIP to DAX

Ross Zwisler (1):
  ext4: Add DAX functionality

 Documentation/filesystems/00-INDEX |   5 +-
 Documentation/filesystems/Locking  |   3 -
 Documentation/filesystems/dax.txt  |  91 +++++++
 Documentation/filesystems/ext2.txt |   5 +-
 Documentation/filesystems/ext4.txt |   4 +
 Documentation/filesystems/vfs.txt  |   7 -
 Documentation/filesystems/xip.txt  |  68 -----
 MAINTAINERS                        |   6 +
 arch/powerpc/sysdev/axonram.c      |  19 +-
 drivers/block/Kconfig              |  13 +-
 drivers/block/brd.c                |  28 +-
 drivers/s390/block/dcssblk.c       |  21 +-
 fs/Kconfig                         |  21 +-
 fs/Makefile                        |   1 +
 fs/block_dev.c                     |  40 +++
 fs/dax.c                           | 530 +++++++++++++++++++++++++++++++++++++
 fs/exofs/inode.c                   |   1 -
 fs/ext2/Kconfig                    |  11 -
 fs/ext2/Makefile                   |   1 -
 fs/ext2/ext2.h                     |  10 +-
 fs/ext2/file.c                     |  45 +++-
 fs/ext2/inode.c                    |  38 +--
 fs/ext2/namei.c                    |  13 +-
 fs/ext2/super.c                    |  53 ++--
 fs/ext2/xip.c                      |  91 -------
 fs/ext2/xip.h                      |  26 --
 fs/ext4/ext4.h                     |   6 +
 fs/ext4/file.c                     |  50 +++-
 fs/ext4/indirect.c                 |  18 +-
 fs/ext4/inode.c                    |  89 +++++--
 fs/ext4/namei.c                    |  10 +-
 fs/ext4/super.c                    |  39 ++-
 fs/open.c                          |   5 +-
 include/linux/blkdev.h             |   6 +-
 include/linux/fs.h                 |  34 +--
 include/linux/mm.h                 |   1 +
 include/linux/rmap.h               |   2 +-
 mm/Makefile                        |   1 -
 mm/fadvise.c                       |   6 +-
 mm/filemap.c                       |  25 +-
 mm/filemap_xip.c                   | 483 ---------------------------------
 mm/madvise.c                       |   2 +-
 mm/memory.c                        |  33 ++-
 scripts/diffconfig                 |   1 -
 44 files changed, 1069 insertions(+), 893 deletions(-)
 create mode 100644 Documentation/filesystems/dax.txt
 delete mode 100644 Documentation/filesystems/xip.txt
 create mode 100644 fs/dax.c
 delete mode 100644 fs/ext2/xip.c
 delete mode 100644 fs/ext2/xip.h
 delete mode 100644 mm/filemap_xip.c

-- 
2.1.1

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