Re: [Qemu-devel] [PATCH V10 4/4] docs: Added MAP_SYNC documentation

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Jan 25, 2019 at 01:26:53AM -0200, Eduardo Habkost wrote:
> On Thu, Jan 24, 2019 at 10:08:37PM -0500, Michael S. Tsirkin wrote:
> > On Thu, Jan 24, 2019 at 05:14:43PM -0200, Eduardo Habkost wrote:
> > > On Thu, Jan 24, 2019 at 02:05:45PM -0500, Michael S. Tsirkin wrote:
> > > > On Thu, Jan 24, 2019 at 04:28:39PM -0200, Eduardo Habkost wrote:
> > > > > On Thu, Jan 24, 2019 at 12:45:54PM -0500, Michael S. Tsirkin wrote:
> > > > > > On Thu, Jan 24, 2019 at 02:59:26PM -0200, Eduardo Habkost wrote:
> > > > > > > On Thu, Jan 24, 2019 at 07:21:03PM +0800, Yi Zhang wrote:
> > > > > > > > On 2019-01-23 at 12:50:50 -0200, Eduardo Habkost wrote:
> > > > > > > > > On Wed, Jan 23, 2019 at 11:00:02AM +0800, Zhang, Yi wrote:
> > > > > > > > > > From: Zhang Yi <yi.z.zhang@linux.intel.com>
> > > > > > > > > > 
> > > > > > > > > > Signed-off-by: Zhang Yi <yi.z.zhang@linux.intel.com>
> > > > > [...]
> > > > > > > > > > + - 'pmem' option of memory-backend-file is 'on':
> > > > > > > > > > +   The backend is a file supporting DAX, e.g., a file on an ext4 or
> > > > > > > > > > +   xfs file system mounted with '-o dax'. if your pmem=on ,but the backend is
> > > > > > > > > > +   not a file supporting DAX, mapping with this flag results in an EOPNOTSUPP
> > > > > > > > > > +   error.
> > > > > > > > > 
> > > > > > > > > Won't this break existing configurations that work today on QEMU
> > > > > > > > > 3.1.0?  Why exactly it is OK to break compatibility here?
> > > > > > > > won't, pmem option default is off, if people who start VM don't know what
> > > > > > > > backend file is, it is suggested and *default to set pmem=off,
> > > > > > > > if people well know the backend file have dax capbility. it is suggest
> > > > > > > > to set pmem=on. 
> > > > > > > > 
> > > > > > > > For a special case that we use /dev/dax as backend, we already have a
> > > > > > > > patch to add MAP_SYNC falg mapiing from device dax mode.
> > > > > > > > see https://lkml.org/lkml/2018/4/22/524 
> > > > > > > > 
> > > > > > > > So, if people force set pmem=on, mapping a regular file, it will results
> > > > > > > > in an EOPNOTSUPP error. 
> > > > > > > 
> > > > > > > This is where compatibility is being broken, isn't it?  People
> > > > > > > currently using pmem=on on a regular file will start getting
> > > > > > > errors after a QEMU upgrade.  Existing VMs with pmem=on may stop
> > > > > > > booting.  Maybe this is OK, but we need to be able to explain why
> > > > > > > it is OK.
> > > > > > 
> > > > > > I think it's OK since pmem explicitly means "persistent":
> > > > > > 
> > > > > > The @option{pmem} option specifies whether the backing file specified
> > > > > > by @option{mem-path} is in host persistent memory that can be accessed
> > > > > > using the SNIA NVM programming model (e.g. Intel NVDIMM).
> > > > > > If @option{pmem} is set to 'on', QEMU will take necessary operations to
> > > > > > guarantee the persistence of its own writes to @option{mem-path}
> > > > > > (e.g. in vNVDIMM label emulation and live migration).
> > > > > 
> > > > > If it's OK, let's at least explicitly document that we are
> > > > > breaking compatibility in those cases.
> > > > > 
> > > > > 
> > > > > > > > 
> > > > > [...]
> > > > > > I think generally MAP_SYNC is required.
> > > > > > But for compatibility reasons we might need to support
> > > > > > !MAP_SYNC on old kernels even though it's risky.
> > > > > 
> > > > > What about making MAP_SYNC optional only on older machine-types?
> > > > 
> > > > I don't think this makes sense. It's not a guest visible change,
> > > > machine types are for that.
> > > 
> > > Losing data written to persistent memory is surely guest-visible
> > > behavior.
> > 
> > I think we need not be purists here. Most people don't lose power and
> > then it's fine and compatible. People who want more robustness need to
> > use more modern kernels, that is all.
> 
> I don't think that's being purist.  I want to avoid hidden bugs
> if we ignore that MAP_SYNC failed for any unexpected reason.  If
> we need to ignore errors in some cases, let's at least limit that
> to cases where we absolutely have to.
> But I would also be happy with just a warning.

Makes sense to me. So if it fails with EOPNOTSUPP,
we try with MAP_SHARED_VALIDATE without MAP_SYNC.
If that succeeds then it's not a dax file, and we warn.
If it fails too then it's an old kernel and we
silently proceed for compatibility reasons.


> 
> -- 
> Eduardo