[RFC PATCH 0/7] Introduce cache_is_aliasing() to fix DAX regression

[Mathieu Desnoyers]

This commit introduced in v5.13 prevents building FS_DAX on 32-bit ARM,
even on ARMv7 which does not have virtually aliased dcaches:

commit d92576f1167c ("dax: does not work correctly with virtual aliasing caches")

It used to work fine before: I have customers using dax over pmem on
ARMv7, but this regression will likely prevent them from upgrading their
kernel.

The root of the issue here is the fact that DAX was never designed to
handle virtually aliased dcache (VIVT and VIPT with aliased dcache). It
touches the pages through their linear mapping, which is not consistent
with the userspace mappings on virtually aliased dcaches. 

This patch series introduces cache_is_aliasing() with new Kconfig
options:

  * ARCH_HAS_CACHE_ALIASING
  * ARCH_HAS_CACHE_ALIASING_DYNAMIC

and implements it for all architectures. The "DYNAMIC" implementation
implements cache_is_aliasing() as a runtime check, which is what is
needed on architectures like 32-bit ARMV6 and ARMV6K.

With this we can basically narrow down the list of architectures which
are unsupported by DAX to those which are really affected.

Feedback is welcome,

Thanks,

Mathieu

Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: linux-mm@xxxxxxxxx
Cc: linux-arch@xxxxxxxxxxxxxxx
Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
Cc: Vishal Verma <vishal.l.verma@xxxxxxxxx>
Cc: Dave Jiang <dave.jiang@xxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: linux-cxl@xxxxxxxxxxxxxxx
Cc: nvdimm@xxxxxxxxxxxxxxx

Mathieu Desnoyers (7):
  Introduce cache_is_aliasing() across all architectures
  dax: Fix incorrect list of cache aliasing architectures
  erofs: Use dax_is_supported()
  ext2: Use dax_is_supported()
  ext4: Use dax_is_supported()
  fuse: Introduce fuse_dax_is_supported()
  xfs: Use dax_is_supported()

 arch/arc/Kconfig                    |  1 +
 arch/arm/include/asm/cachetype.h    |  3 ++
 arch/arm/mm/Kconfig                 |  2 ++
 arch/csky/Kconfig                   |  1 +
 arch/m68k/Kconfig                   |  1 +
 arch/mips/Kconfig                   |  1 +
 arch/mips/include/asm/cachetype.h   |  9 +++++
 arch/nios2/Kconfig                  |  1 +
 arch/nios2/include/asm/cachetype.h  | 10 ++++++
 arch/parisc/Kconfig                 |  1 +
 arch/sh/Kconfig                     |  1 +
 arch/sparc/Kconfig                  |  1 +
 arch/sparc/include/asm/cachetype.h  | 14 ++++++++
 arch/xtensa/Kconfig                 |  1 +
 arch/xtensa/include/asm/cachetype.h | 10 ++++++
 fs/Kconfig                          |  2 +-
 fs/erofs/super.c                    | 10 +++---
 fs/ext2/super.c                     | 14 ++++----
 fs/ext4/super.c                     | 52 ++++++++++++++---------------
 fs/fuse/file.c                      |  2 +-
 fs/fuse/fuse_i.h                    | 36 +++++++++++++++++++-
 fs/fuse/inode.c                     | 47 +++++++++++++-------------
 fs/fuse/virtio_fs.c                 |  4 +--
 fs/xfs/xfs_super.c                  | 20 +++++++----
 include/linux/cacheinfo.h           |  8 +++++
 include/linux/dax.h                 |  9 +++++
 mm/Kconfig                          | 10 ++++++
 27 files changed, 198 insertions(+), 73 deletions(-)
 create mode 100644 arch/mips/include/asm/cachetype.h
 create mode 100644 arch/nios2/include/asm/cachetype.h
 create mode 100644 arch/sparc/include/asm/cachetype.h
 create mode 100644 arch/xtensa/include/asm/cachetype.h

-- 
2.39.2