dax: add empty static inline for CONFIG_DAX=n

Patch series "Introduce cpu_dcache_is_aliasing() to fix DAX regression",
v6.

This commit introduced in v4.0 prevents building FS_DAX on 32-bit ARM,
even on ARMv7 which does not have virtually aliased data caches:

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

Even though it used to work fine before.

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

This patch series introduces cpu_dcache_is_aliasing() with the new
Kconfig option ARCH_HAS_CPU_CACHE_ALIASING and implements it for all
architectures. The implementation of cpu_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.


This patch (of 9):

When building a kernel with CONFIG_DAX=n, all uses of set_dax_nocache()
and set_dax_nomc() need to be either within regions of code or compile
units which are explicitly not compiled, or they need to rely on compiler
optimizations to eliminate calls to those undefined symbols.

It appears that at least the openrisc and loongarch architectures don't
end up eliminating those undefined symbols even if they are provably
within code which is eliminated due to conditional branches depending on
constants.

Implement empty static inline functions for set_dax_nocache() and
set_dax_nomc() in CONFIG_DAX=n to ensure those undefined references are
removed.

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Reported-by: kernel test robot <[email protected]>
Closes: https://lore.kernel.org/oe-kbuild-all/[email protected]/
Reported-by: kernel test robot <[email protected]>
Closes: https://lore.kernel.org/oe-kbuild-all/[email protected]/
Fixes: 7ac5360cd4d0 ("dax: remove the copy_from_iter and copy_to_iter methods")
Signed-off-by: Mathieu Desnoyers <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: Vishal Verma <[email protected]>
Cc: Dave Jiang <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Russell King <[email protected]>
Cc: Dave Chinner <[email protected]>
Cc: Michael Sclafani <[email protected]>
Cc: Alasdair Kergon <[email protected]>
Cc: Heiko Carstens <[email protected]>
Cc: Mike Snitzer <[email protected]>
Cc: Mikulas Patocka <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

0 files changed, 0 insertions, 0 deletions