Re: [Qemu-devel] [PATCH v8 3/6] libnvdimm: add dax_dev sync flag

[Pankaj Gupta <pagupta@redhat.com>]

> >
> >
> > Hi Dan,
> >
> > While testing device mapper with DAX, I faced a bug with the commit:
> >
> > commit ad428cdb525a97d15c0349fdc80f3d58befb50df
> > Author: Dan Williams <dan.j.williams@intel.com>
> > Date:   Wed Feb 20 21:12:50 2019 -0800
> >
> > When I reverted the condition to old code[1] it worked for me. I
> > am thinking when we map two different devices (e.g with device mapper),
> > will
> > start & end pfn still point to same pgmap? Or there is something else which
> > I am missing here.
> >
> > Note: I tested only EXT4.
> >
> > [1]
> >
> > -               if (pgmap && pgmap->type == MEMORY_DEVICE_FS_DAX)
> > +               end_pgmap = get_dev_pagemap(pfn_t_to_pfn(end_pfn), NULL);
> > +               if (pgmap && pgmap == end_pgmap && pgmap->type ==
> > MEMORY_DEVICE_FS_DAX
> > +                               && pfn_t_to_page(pfn)->pgmap == pgmap
> > +                               && pfn_t_to_page(end_pfn)->pgmap == pgmap
> > +                               && pfn_t_to_pfn(pfn) ==
> > PHYS_PFN(__pa(kaddr))
> > +                               && pfn_t_to_pfn(end_pfn) ==
> > PHYS_PFN(__pa(end_kaddr)))
> 
> Ugh, yes, device-mapper continues to be an awkward fit for dax (or
> vice versa). We would either need a way to have a multi-level pfn to
> pagemap lookup for composite devices, or a way to discern that even
> though the pagemap is different that the result is still valid / not
> an indication that we have leaked into an unassociated address range.
> Perhaps a per-daxdev callback for ->dax_supported() so that
> device-mapper internals can be used for this validation.

Yes, Will look at it.

> 
> We need to get that fixed up, but I don't see it as a blocker /
> pre-requisite for virtio-pmem.

Agree. Will send virtio-pmem patch series.

Thank you,
Pankaj
> 
>