Re: [Qemu-devel] [PATCH v2 5/8] migration/ram: ensure write persistence on loading zero pages to PMEM

[Pankaj Gupta <pagupta@redhat.com>]

> 
> On 02/07/18 19:52 +0800, Haozhong Zhang wrote:
> > On 02/07/18 11:38 +0000, Dr. David Alan Gilbert wrote:
> > > * Haozhong Zhang (haozhong.zhang@intel.com) wrote:
> > > > When loading a zero page, check whether it will be loaded to
> > > > persistent memory If yes, load it by libpmem function
> > > > pmem_memset_nodrain().  Combined with a call to pmem_drain() at the
> > > > end of RAM loading, we can guarantee all those zero pages are
> > > > persistently loaded.
> > > 
> > > I'm surprised pmem is this invasive to be honest;   I hadn't expected
> > > the need for special memcpy's etc everywhere.  We're bound to miss some.
> > > I assume there's separate kernel work needed to make postcopy work;
> > > certainly the patches here don't seem to touch the postcopy path.
> > 
> > This link at
> > https://wiki.qemu.org/Features/PostCopyLiveMigration#Conflicts shows
> > that postcopy with memory-backend-file requires kernel support. Can
> > you point me the details of the required kernel support, so that I can
> > understand what would be needed to NVDIMM postcopy?
> 
> I saw test_ramblock_postcopiable() requires the ram block not be
> mmap'ed with MAP_SHARED. The only pmem device (i.e., device DAX e.g.,
> /dev/dax0.0) that can be safely used as the backend of vNVDIMM must be
> shared mapped which is required by kernel, so postcopy does not work
> with pmem right now. Even if the private mmap was supported for device
> dax, it would still make little sense to private mmap it in QEMU,
> because vNVDIMM intends to be non-volatile.

o.k that's the reason post-copy pages does not work with THP.

If we want to support device DAX with postcopy we need to handle
this use-case? 

Adding 'Andrea' to the thread for more clear answer.


Thanks,
Pankaj