Re: [PATCH v8 15/15] xfs: don't enable DAX on zoned inodes

[Christoph Hellwig <hch@lst.de>]

On Wed, Nov 11, 2026 at 08:02:44PM -0500, Chris Mason wrote:
> A store through a shared mapping can dereference a NULL pointer if the
> file is zoned and has S_DAX.
> 
> DAX writes in place,

DAX does not actually require in-place writes, we had patches to
support DAX with reflink for example.

> and zoned inodes are only written out of place, so
> the two can't work together. XFS doesn't check for this: with dax=always
> or FS_XFLAG_DAX, an inode gets S_DAX if its device does DAX, and an
> internal zoned section on pmem does. The write fault then takes the
> zoned path:
> 
>     xfs_filemap_fault()
>       xfs_write_fault()
>         xfs_write_fault_zoned()
>           page_folio(vmf->page)     /* NULL in a DAX fault */

but this is real issue, and xfs_write_fault_zoned would need to
find a way to deal with pfn faults.

I think thi could be supported, but there's no real reason to bother
as there is no good use yet (at least right now).  So I think your
change is fine, but the commit log and comments are not.  Just say
that the zone code does not support DAX, and maybe mention this
callchain.