DAX error inject/page poison

[Mike Kravetz]

We were trying to simulate pmem errors in an environment where a DAX
filesystem is used (ext4 although I suspect it does not matter).  The
sequence attempted on a DAX filesystem is:
- Populate a file in the DAX filesystem
- mmap the file
- madvise(MADV_HWPOISON)

The madvise operation fails with EFAULT.  This appears to come from
get_user_pages() as there are no struct pages for such mappings?

The idea is to make sure an application can recover from such errors
by hole punching and repopulating with another page.

A couple questions:
It seems like madvise(MADV_HWPOISON) is not going to work (ever?) in
such situations.  If so, should we perhaps add a IS_DAX like check and
return something like EINVAL?  Or, at least document expected behavior?

If madvise(MADV_HWPOISON) will not work, how can one inject errors to
test error handling code?

-- 
Mike Kravetz