Re: [PATCH 2/3] mm/sparsemem: get physical address to page struct instead of virtual address to pfn

[Baoquan He]

On 02/07/20 at 11:26am, Wei Yang wrote:
> On Thu, Feb 06, 2020 at 06:19:46PM -0800, Dan Williams wrote:
> >On Thu, Feb 6, 2020 at 3:17 PM Wei Yang <richardw.yang@xxxxxxxxxxxxxxx> wrote:
> >>
> >> memmap should be the physical address to page struct instead of virtual
> >> address to pfn.
> >>
> >> Since we call this only for SPARSEMEM_VMEMMAP, pfn_to_page() is valid at
> >> this point.
> >>
> >> Fixes: ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
> >> Signed-off-by: Wei Yang <richardw.yang@xxxxxxxxxxxxxxx>
> >> CC: Dan Williams <dan.j.williams@xxxxxxxxx>
> >> ---
> >>  mm/sparse.c | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/mm/sparse.c b/mm/sparse.c
> >> index b5da121bdd6e..56816f653588 100644
> >> --- a/mm/sparse.c
> >> +++ b/mm/sparse.c
> >> @@ -888,7 +888,7 @@ int __meminit sparse_add_section(int nid, unsigned long start_pfn,
> >>         /* Align memmap to section boundary in the subsection case */
> >>         if (IS_ENABLED(CONFIG_SPARSEMEM_VMEMMAP) &&
> >>                 section_nr_to_pfn(section_nr) != start_pfn)
> >> -               memmap = pfn_to_kaddr(section_nr_to_pfn(section_nr));
> >> +               memmap = pfn_to_page(section_nr_to_pfn(section_nr));
> >
> >Yes, this looks obviously correct. This might be tripping up
> >makedumpfile. Do you see any practical effects of this bug? The kernel
> >mostly avoids ->section_mem_map in the vmemmap case and in the
> >!vmemmap case section_nr_to_pfn(section_nr) should always equal
> >start_pfn.
> 
> I took another look into the code. Looks there is no practical effect after
> this. Because in the vmemmap case, we don't need ->section_mem_map to retrieve
> the real memmap.
> 
> But leave a inconsistent data in section_mem_map is not a good practice.

Yeah, it does has no pratical effect. I tried to create sub-section
alighed namespace, then trigger crash, makedumpfile isn't impacted.
Because pmem memory is only added, but not onlined. We don't report it
to kdump, makedumpfile will ignore it.

I think it's worth fixing it to encode a correct memmap address. We
don't know if in the future this will break anything.

Thanks
Baoquan