linux-ext4 - Re: dax pmd fault handler never returns to userspace

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Date:	Wed, 18 Nov 2015 11:53:26 -0700
From:	Ross Zwisler <ross.zwisler@...ux.intel.com>
To:	Jeff Moyer <jmoyer@...hat.com>
Cc:	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	Dan Williams <dan.j.williams@...el.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-nvdimm <linux-nvdimm@...1.01.org>,
	linux-ext4 <linux-ext4@...r.kernel.org>,
	Ross Zwisler <ross.zwisler@...el.com>
Subject: Re: dax pmd fault handler never returns to userspace

On Wed, Nov 18, 2015 at 01:32:46PM -0500, Jeff Moyer wrote:
> Ross Zwisler <ross.zwisler@...ux.intel.com> writes:
> 
> > Yea, my first round of testing was broken, sorry about that.
> >
> > It looks like this test causes the PMD fault handler to be called repeatedly
> > over and over until you kill the userspace process.  This doesn't happen for
> > XFS because when using XFS this test doesn't hit PMD faults, only PTE faults.
> 
> Hmm, I wonder why not?  

Well, whether or not you get PMDs is dependent on the block allocator for the
filesystem.  We ask the FS how much space is contiguous via get_blocks(), and
if it's less than PMD_SIZE (2 MiB) we fall back to the regular 4k page fault
path.   This code all lives in __dax_pmd_fault().  There are also a bunch of
other reasons why we'd fall back to 4k faults - the virtual address isn't 2
MiB aligned, etc.   It's actually pretty hard to get everything right so you
actually get PMD faults.

Anyway, my guess is that we're failing to meet one of our criteria in XFS, so
we just always fall back to PTEs for this test.

> Sounds like that will need investigating as well, right?

Yep, on it.
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