linux-kernel - DAX error inject/page poison

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Date:   Tue, 19 Sep 2017 16:15:13 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     linux-nvdimm@...ts.01.org,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Vishal L Verma <vishal.l.verma@...el.com>
Subject: DAX error inject/page poison

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