Re: [RFC PATCH v2 8/8] dax: Fix incorrect list of dcache aliasing architectures

[Dave Chinner <david@fromorbit.com>]

On Tue, Jan 30, 2024 at 11:52:55AM -0500, Mathieu Desnoyers wrote:
> commit d92576f1167c ("dax: does not work correctly with virtual aliasing caches")
> prevents DAX from building on architectures with virtually aliased
> dcache with:
> 
>   depends on !(ARM || MIPS || SPARC)
> 
> This check is too broad (e.g. recent ARMv7 don't have virtually aliased
> dcaches), and also misses many other architectures with virtually
> aliased dcache.
> 
> This is a regression introduced in the v5.13 Linux kernel where the
> dax mount option is removed for 32-bit ARMv7 boards which have no dcache
> aliasing, and therefore should work fine with FS_DAX.
> 
> This was turned into the following implementation of dax_is_supported()
> by a preparatory change:
> 
>         return !IS_ENABLED(CONFIG_ARM) &&
>                !IS_ENABLED(CONFIG_MIPS) &&
>                !IS_ENABLED(CONFIG_SPARC);
> 
> Use dcache_is_aliasing() instead to figure out whether the environment
> has aliasing dcaches.
> 
> Fixes: d92576f1167c ("dax: does not work correctly with virtual aliasing caches")
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: linux-mm@kvack.org
> Cc: linux-arch@vger.kernel.org
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: Vishal Verma <vishal.l.verma@intel.com>
> Cc: Dave Jiang <dave.jiang@intel.com>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Russell King <linux@armlinux.org.uk>
> Cc: nvdimm@lists.linux.dev
> Cc: linux-cxl@vger.kernel.org
> Cc: linux-fsdevel@vger.kernel.org
> ---
>  include/linux/dax.h | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/dax.h b/include/linux/dax.h
> index cfc8cd4a3eae..f59e604662e4 100644
> --- a/include/linux/dax.h
> +++ b/include/linux/dax.h
> @@ -5,6 +5,7 @@
>  #include <linux/fs.h>
>  #include <linux/mm.h>
>  #include <linux/radix-tree.h>
> +#include <linux/cacheinfo.h>
>  
>  typedef unsigned long dax_entry_t;
>  
> @@ -80,9 +81,7 @@ static inline bool daxdev_mapping_supported(struct vm_area_struct *vma,
>  }
>  static inline bool dax_is_supported(void)
>  {
> -	return !IS_ENABLED(CONFIG_ARM) &&
> -	       !IS_ENABLED(CONFIG_MIPS) &&
> -	       !IS_ENABLED(CONFIG_SPARC);
> +	return !dcache_is_aliasing();

Yeah, if this is just a one liner should go into
fs_dax_get_by_bdev(), similar to the blk_queue_dax() check at the
start of the function.

I also noticed that device mapper uses fs_dax_get_by_bdev() to
determine if it can support DAX, but this patch set does not address
that case. Hence it really seems to me like fs_dax_get_by_bdev() is
the right place to put this check.

-Dave.
-- 
Dave Chinner
david@fromorbit.com