[RFC PATCH v2 0/8] Introduce dcache_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 dcache_is_aliasing() with the new Kconfig
option ARCH_HAS_CACHE_ALIASING and implements it for all architectures.
The implementation of dcache_is_aliasing() is either evaluated to a
constant at compile-time or a runtime check, which is what is needed on
ARM.

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

Note that the order of the series was completely changed based on the
feedback received on v1, cache_is_aliasing() is renamed to
dcache_is_aliasing(), ARCH_HAS_CACHE_ALIASING_DYNAMIC is gone,
dcache_is_aliasing() vs ARCH_HAS_CACHE_ALIASING relationship is
simplified, and the dax_is_supported() check was moved to its rightful
place in all filesystems.

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: Arnd Bergmann <arnd@xxxxxxxx>
Cc: Russell King <linux@xxxxxxxxxxxxxxx>
Cc: linux-cxl@xxxxxxxxxxxxxxx
Cc: nvdimm@xxxxxxxxxxxxxxx
Cc: linux-fsdevel@xxxxxxxxxxxxxxx

Mathieu Desnoyers (8):
  dax: Introduce dax_is_supported()
  erofs: Use dax_is_supported()
  ext2: Use dax_is_supported()
  ext4: Use dax_is_supported()
  fuse: Use dax_is_supported()
  xfs: Use dax_is_supported()
  Introduce dcache_is_aliasing() across all architectures
  dax: Fix incorrect list of dcache aliasing architectures

 arch/arc/Kconfig                    |  1 +
 arch/arc/include/asm/cachetype.h    |  9 +++++++++
 arch/arm/Kconfig                    |  1 +
 arch/arm/include/asm/cachetype.h    |  2 ++
 arch/csky/Kconfig                   |  1 +
 arch/csky/include/asm/cachetype.h   |  9 +++++++++
 arch/m68k/Kconfig                   |  1 +
 arch/m68k/include/asm/cachetype.h   |  9 +++++++++
 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/parisc/include/asm/cachetype.h |  9 +++++++++
 arch/sh/Kconfig                     |  1 +
 arch/sh/include/asm/cachetype.h     |  9 +++++++++
 arch/sparc/Kconfig                  |  1 +
 arch/sparc/include/asm/cachetype.h  | 14 ++++++++++++++
 arch/xtensa/Kconfig                 |  1 +
 arch/xtensa/include/asm/cachetype.h | 10 ++++++++++
 fs/Kconfig                          |  1 -
 fs/erofs/super.c                    |  5 ++++-
 fs/ext2/super.c                     |  6 +++++-
 fs/ext4/super.c                     |  5 ++++-
 fs/fuse/dax.c                       |  7 +++++++
 fs/xfs/xfs_iops.c                   |  2 +-
 include/linux/cacheinfo.h           |  6 ++++++
 include/linux/dax.h                 |  9 +++++++++
 mm/Kconfig                          |  6 ++++++
 29 files changed, 142 insertions(+), 5 deletions(-)
 create mode 100644 arch/arc/include/asm/cachetype.h
 create mode 100644 arch/csky/include/asm/cachetype.h
 create mode 100644 arch/m68k/include/asm/cachetype.h
 create mode 100644 arch/mips/include/asm/cachetype.h
 create mode 100644 arch/nios2/include/asm/cachetype.h
 create mode 100644 arch/parisc/include/asm/cachetype.h
 create mode 100644 arch/sh/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