Re: [PATCH v13] mm: report per-page metadata information

[Pasha Tatashin]

On Mon, Aug 5, 2024 at 7:06 PM Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
>
> Pasha Tatashin wrote:
> [..]
> > Thank you for the heads up. Can you please attach a full config file,
> > also was anyone able to reproduce this problem in qemu with emulated
> > nvdimm?
>
> Yes, I can reproduce the crash just by trying to reconfigure the mode of
> a pmem namespace:
>
> # ndctl create-namespace -m raw -f -e namespace0.0
>
> ...where namespace0.0 results from:
>
>     memmap=4G!4G
>
> ...passed on the kernel command line.
>
> Kernel config here:
>
> https://gist.github.com/djbw/143705077103d43a735c179395d4f69a

Excellent, I was able to reproduce this problem.

The problem appear to be caused by this code:

Calling page_pgdat() in depopulate_section_memmap():

static void depopulate_section_memmap(unsigned long pfn, unsigned long nr_pages,
                struct vmem_altmap *altmap)
{
        unsigned long start = (unsigned long) pfn_to_page(pfn);
        unsigned long end = start + nr_pages * sizeof(struct page);

        mod_node_page_state(page_pgdat(pfn_to_page(pfn)), NR_MEMMAP,
<<<< We cannot do it.
                            -1L * (DIV_ROUND_UP(end - start, PAGE_SIZE)));
        vmemmap_free(start, end, altmap);
}

The page_pgdat() returns NULL starting from:
pageunmap_range()
    remove_pfn_range_from_zone() <- page is removed from the zone.

Pasha