linux-ext4 - Re: [PATCH 0/9 v3] ext4: Punch hole and DAX fixes

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Date:	Sat, 7 Nov 2015 09:17:27 +1100
From:	Dave Chinner <david@...morbit.com>
To:	"Boylston, Brian" <brian.boylston@....com>
Cc:	Jan Kara <jack@...e.com>, Ted Tso <tytso@....edu>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	"dan.j.williams@...el.com" <dan.j.williams@...el.com>
Subject: Re: [PATCH 0/9 v3] ext4: Punch hole and DAX fixes

On Fri, Nov 06, 2015 at 05:57:04PM +0000, Boylston, Brian wrote:
> Hi,
> 
> I've written a test tool (included below) that exercises page faults on
> hole-y portions of an mmapped file.  The file is created, sized using
> various methods, mmapped, and then two threads race to write a marker to
> different offsets within each mapped page.  Once the threads have
> finished marking each page, the pages are checked for the presence of
> the markers.
> 
> With vanilla 4.2 and 4.3 kernels, this test easily exposes corruption on
> pmem-backed, DAX-mounted xfs and ext4 file systems.

4.2/4.3 kernels do not have a fix for this race problem in them, so
it's no surprise that they fail the test. The fixes for this problem
in XFS are currently in the for-next branch, and will be merged into
4.4 during the merge window.

And FWIW, with XFS on a ramdisk on a 4.3+for-next branch kernel,
this test passes just fine:

$ ./dax-hole-test -f /mnt/scratch/holetest 1000
holetest r0

INFO: zero-filled test...
INFO: sz = 3e800000, npages = 256000
INFO: vastart = 00007fc5a90bd000
INFO: thread 0 is 7fc5a90bc700
INFO: thread 1 is 7fc5a88bb700
INFO: 0 error(s) detected

INFO: posix_fallocate test...
INFO: sz = 3e800000, npages = 256000
INFO: vastart = 00007fc5a90bd000
INFO: thread 0 is 7fc5a88bb700
INFO: thread 1 is 7fc5a90bc700
INFO: 0 error(s) detected

INFO: fallocate test...
INFO: sz = 3e800000, npages = 256000
INFO: vastart = 00007fc5a90bd000
INFO: thread 0 is 7fc5a90bc700
INFO: thread 1 is 7fc5a88bb700
INFO: 0 error(s) detected

INFO: ftruncate test...
INFO: sz = 3e800000, npages = 256000
INFO: vastart = 00007fc5a90bd000
INFO: thread 0 is 7fc5a88bb700
INFO: thread 1 is 7fc5a90bc700
INFO: 0 error(s) detected

That tends to indicate a bug in the ext4 patchset, not that the
method we've taken to solve the problem is fundamentally wrong.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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