Re: [PATCH 1/2] libnvdimm/altmap: Track namespace boundaries in altmap

[Dan Williams <dan.j.williams@intel.com>]

On Tue, Sep 10, 2019 at 1:31 AM Aneesh Kumar K.V
<aneesh.kumar@linux.ibm.com> wrote:
>
> On 9/10/19 1:40 PM, Dan Williams wrote:
> > On Mon, Sep 9, 2019 at 11:29 PM Aneesh Kumar K.V
> > <aneesh.kumar@linux.ibm.com> wrote:
> >>
> >> With PFN_MODE_PMEM namespace, the memmap area is allocated from the device
> >> area. Some architectures map the memmap area with large page size. On
> >> architectures like ppc64, 16MB page for memap mapping can map 262144 pfns.
> >> This maps a namespace size of 16G.
> >>
> >> When populating memmap region with 16MB page from the device area,
> >> make sure the allocated space is not used to map resources outside this
> >> namespace. Such usage of device area will prevent a namespace destroy.
> >>
> >> Add resource end pnf in altmap and use that to check if the memmap area
> >> allocation can map pfn outside the namespace. On ppc64 in such case we fallback
> >> to allocation from memory.
> >
> > Shouldn't this instead be comprehended by nd_pfn_init() to increase
> > the reservation size so that it fits in the alignment? It may not
> > always be possible to fall back to allocation from memory for
> > extremely large pmem devices. I.e. at 64GB of memmap per 1TB of pmem
> > there may not be enough DRAM to store the memmap.
> >
>
> We do switch between DRAM and device for memmap allocation. ppc64
> vmemmap_populate  does
>
> if (altmap && !altmap_cross_boundary(altmap, start, page_size)) {
>         p = altmap_alloc_block_buf(page_size, altmap);
>         if (!p)
>                 pr_debug("altmap block allocation failed, falling back to system memory");
>         }
>         if (!p)
>                 p = vmemmap_alloc_block_buf(page_size, node);
>
>
> With that we should be using DRAM for the first and the last mapping,
> rest of the memmap should be backed by device.

Ah, ok, makes sense.