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

[Dan Williams <dan.j.williams@intel.com>]

Dave Chinner wrote:
> 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.

Oh, good catch. Yes, I agree this can definitely be pushed down, but
then I think it should be pushed down all the way to make alloc_dax()
fail. That will need some additional fixups like:

diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index 8dcabf84d866..a35e60e62440 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -2126,12 +2126,12 @@ static struct mapped_device *alloc_dev(int minor)
                md->dax_dev = alloc_dax(md, &dm_dax_ops);
                if (IS_ERR(md->dax_dev)) {
                        md->dax_dev = NULL;
-                       goto bad;
+               } else {
+                       set_dax_nocache(md->dax_dev);
+                       set_dax_nomc(md->dax_dev);
+                       if (dax_add_host(md->dax_dev, md->disk))
+                               goto bad;
                }
-               set_dax_nocache(md->dax_dev);
-               set_dax_nomc(md->dax_dev);
-               if (dax_add_host(md->dax_dev, md->disk))
-                       goto bad;
        }
 
        format_dev_t(md->name, MKDEV(_major, minor));

...to make it not fatal to fail to register the dax_dev.