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

[Mathieu Desnoyers]

On 2024-01-30 22:13, Dan Williams wrote:
> 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@xxxxxxxxxxxx>
> > > 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: nvdimm@xxxxxxxxxxxxxxx
> > > Cc: linux-cxl@xxxxxxxxxxxxxxx
> > > Cc: linux-fsdevel@xxxxxxxxxxxxxxx
> > > ---
> > >   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.

I've had a quick look at other users of alloc_dax() and
alloc_dax_region(), and so far I figure that all of those
really want to bail out on alloc_dax failure. Is dm.c the
only special-case we need to fix to make it non-fatal ?

Thanks,

Mathieu

--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com