Re: DAX can not work on virtual nvdimm device

Ross Zwisler
2016-09-08T20:47:08Z

On Tue, Sep 06, 2016 at 05:06:20PM +0200, Jan Kara wrote:
> On Thu 01-09-16 20:57:38, Ross Zwisler wrote:
> > On Wed, Aug 31, 2016 at 04:44:47PM +0800, Xiao Guangrong wrote:
> > > On 08/31/2016 01:09 AM, Dan Williams wrote:
> > > >
> > > > Can you post your exact reproduction steps? This test is not failing for me.
> > > >
> > >
> > > Sure.
> > >
> > > 1. make the guest kernel based on your tree, the top commit is
> > > 10d7902fa0e82b (dax: unmap/truncate on device shutdown) and
> > > the config file can be found in this thread.
> > >
> > > 2. add guest kernel command line: memmap=6G!10G
> > >
> > > 3: start the guest:
> > > x86_64-softmmu/qemu-system-x86_64 -machine pc,nvdimm --enable-kvm \
> > > -smp 16 -m 32G,maxmem=100G,slots=100 /other/VMs/centos6.img -monitor stdio
> > >
> > > 4: in guest:
> > > mkfs.ext4 /dev/pmem0
> > > mount -o dax /dev/pmem0 /mnt/pmem/
> > > echo > /mnt/pmem/xxx
> > > ./mmap /mnt/pmem/xxx
> > > ./read /mnt/pmem/xxx
> > >
> > > The source code of mmap and read has been attached in this mail.
> > >
> > > Hopefully, you can detect the error triggered by read test.
> > >
> > > Thanks!
> >
> > Okay, I think I've isolated this issue. Xiao's VM was an old CentOS 6 system,
> > and for some reason ext4+DAX with the old tools found in that VM fails. I was
> > able to reproduce this failure with a freshly installed CentOS 6.8 VM.
> >
> > You can see the failure with his tests, or perhaps more easily with this
> > series of commands:
> >
> > # mkfs.ext4 /dev/pmem0
> > # mount -o dax /dev/pmem0 /mnt/pmem/
> > # touch /mnt/pmem/x
> > # md5sum /mnt/pmem/x
> > md5sum: /mnt/pmem/x: Bad address
> >
> > This sequence of commands works fine in the old CentOS 6 system if you use XFS
> > instead of ext4, and it works fine with both ext4 and XFS in CentOS 7 and
> > with recent versions of Fedora.
> >
> > I've added the ext4 folks to this mail in case they care, but my guess is that
> > the tools in CentOS 6 are so old that it's not worth worrying about. For
> > reference, the kernel in CentOS 6 is based on 2.6.32. :) DAX was introduced
> > in v4.0.
>
> Hum, can you post 'dumpe2fs -h /dev/pmem0' output from that system when the
> md5sum fails? Because the only idea I have is that mkfs.ext4 in CentOS 6
> creates the filesystem with a different set of features than more recent
> e2fsprogs and so we hit some untested path...

Sure, here's the output:

# dumpe2fs -h /dev/pmem0
dumpe2fs 1.41.12 (17-May-2010)
Filesystem volume name: <none>
Last mounted on: /mnt/pmem
Filesystem UUID: 4cd8a836-cc54-4c59-ae0a-4a26bab0f8bc
Filesystem magic number: 0xEF53
Filesystem revision #: 1 (dynamic)
Filesystem features: has_journal ext_attr resize_inode dir_index filetype
needs_recovery extent flex_bg sparse_super large_file huge_file uninit_bg
dir_nlink extra_isize
Filesystem flags: signed_directory_hash
Default mount options: (none)
Filesystem state: clean
Errors behavior: Continue
Filesystem OS type: Linux
Inode count: 1048576
Block count: 4194304
Reserved block count: 209715
Free blocks: 4084463
Free inodes: 1048565
First block: 0
Block size: 4096
Fragment size: 4096
Reserved GDT blocks: 1023
Blocks per group: 32768
Fragments per group: 32768
Inodes per group: 8192
Inode blocks per group: 512
RAID stride: 1
Flex block group size: 16
Filesystem created: Thu Sep 8 14:45:31 2016
Last mount time: Thu Sep 8 14:45:39 2016
Last write time: Thu Sep 8 14:45:39 2016
Mount count: 1
Maximum mount count: 21
Last checked: Thu Sep 8 14:45:31 2016
Check interval: 15552000 (6 months)
Next check after: Tue Mar 7 13:45:31 2017
Lifetime writes: 388 MB
Reserved blocks uid: 0 (user root)
Reserved blocks gid: 0 (group root)
First inode: 11
Inode size: 256
Required extra isize: 28
Desired extra isize: 28
Journal inode: 8
Default directory hash: half_md4
Directory Hash Seed: 19cad581-c46a-4212-bfa0-d527ff55db49
Journal backup: inode blocks
Journal features: (none)
Journal size: 128M
Journal length: 32768
Journal sequence: 0x00000002
Journal start: 1