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

[Dan Williams]

On Mon, May 13, 2019 at 10:32 AM Pankaj Gupta <pagupta@xxxxxxxxxx> wrote:
>
>
> Hi Dan,
>
> While testing device mapper with DAX, I faced a bug with the commit:
>
> commit ad428cdb525a97d15c0349fdc80f3d58befb50df
> Author: Dan Williams <dan.j.williams@xxxxxxxxx>
> 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.

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