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Date:   Fri, 10 May 2019 21:23:17 -0400 (EDT)
From:   Pankaj Gupta <pagupta@redhat.com>
To:     Dan Williams <dan.j.williams@intel.com>
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Subject: Re: [Qemu-devel] [PATCH v8 3/6] libnvdimm: add dax_dev sync flag
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> > > >
> > > > This patch adds 'DAXDEV_SYNC' flag which is set
> > > > for nd_region doing synchronous flush. This later
> > > > is used to disable MAP_SYNC functionality for
> > > > ext4 & xfs filesystem for devices don't support
> > > > synchronous flush.
> > > >
> > > > Signed-off-by: Pankaj Gupta <pagupta@redhat.com>
> > > > ---
> > > >  drivers/dax/bus.c            |  2 +-
> > > >  drivers/dax/super.c          | 13 ++++++++++++-
> > > >  drivers/md/dm.c              |  3 ++-
> > > >  drivers/nvdimm/pmem.c        |  5 ++++-
> > > >  drivers/nvdimm/region_devs.c |  7 +++++++
> > > >  include/linux/dax.h          |  8 ++++++--
> > > >  include/linux/libnvdimm.h    |  1 +
> > > >  7 files changed, 33 insertions(+), 6 deletions(-)
> > > [..]
> > > > diff --git a/drivers/md/dm.c b/drivers/md/dm.c
> > > > index 043f0761e4a0..ee007b75d9fd 100644
> > > > --- a/drivers/md/dm.c
> > > > +++ b/drivers/md/dm.c
> > > > @@ -1969,7 +1969,8 @@ static struct mapped_device *alloc_dev(int minor)
> > > >         sprintf(md->disk->disk_name, "dm-%d", minor);
> > > >
> > > >         if (IS_ENABLED(CONFIG_DAX_DRIVER)) {
> > > > -               dax_dev = alloc_dax(md, md->disk->disk_name,
> > > > &dm_dax_ops);
> > > > +               dax_dev = alloc_dax(md, md->disk->disk_name,
> > > > &dm_dax_ops,
> > > > +
> > > > DAXDEV_F_SYNC);
> > >
> > > Apologies for not realizing this until now, but this is broken.
> > > Imaging a device-mapper configuration composed of both 'async'
> > > virtio-pmem and 'sync' pmem. The 'sync' flag needs to be unified
> > > across all members. I would change this argument to '0' and then
> > > arrange for it to be set at dm_table_supports_dax() time after
> > > validating that all components support synchronous dax.
> >
> > o.k. Need to set 'DAXDEV_F_SYNC' flag after verifying all the target
> > components support synchronous DAX.
> >
> > Just a question, If device mapper configuration have composed of both
> > virtio-pmem or pmem devices, we want to configure device mapper for async
> > flush?
> 
> If it's composed of both then, yes, it needs to be async flush at the
> device-mapper level. Otherwise MAP_SYNC will succeed and fail to
> trigger fsync on the host file when necessary for the virtio-pmem
> backed portion of the device-mapper device.

o.k. Agree.

Thanks you,
Pankaj
> 
>