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From: Mike Kravetz <mike.kravetz@oracle.com>
Subject: DAX error inject/page poison
Message-ID: <11e8c954-7d54-ae4f-f4fe-459da79c2990@oracle.com>
Date: Tue, 19 Sep 2017 16:15:13 -0700
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To: linux-nvdimm@lists.01.org, "linux-mm@kvack.org" <linux-mm@kvack.org>, linux-kernel <linux-kernel@vger.kernel.org>
Cc: Dan Williams <dan.j.williams@intel.com>, Ross Zwisler <ross.zwisler@linux.intel.com>, Vishal L Verma <vishal.l.verma@intel.com>


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

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