Re: [Qemu-devel] DAX can not work on virtual nvdimm device

[Ross Zwisler]

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