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

[Dan Williams]

On Thu, Feb 6, 2020 at 7:10 PM Baoquan He <bhe@xxxxxxxxxx> wrote:
>
> Hi Dan,
>
> On 02/06/20 at 06:19pm, Dan Williams wrote:
> > On Thu, Feb 6, 2020 at 3:17 PM Wei Yang <richardw.yang@xxxxxxxxxxxxxxx> wrote:
> > > 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.
>
> The practical effects is that the memmap for the first unaligned section will be lost
> when destroy namespace to hot remove it. Because we encode the ->section_mem_map
> into mem_section, and get memmap from the related mem_section to free it in
> section_deactivate(). In fact in vmemmap, we don't need to encode the ->section_mem_map
> with memmap.

Right, but can you actually trigger that in the SPARSEMEM_VMEMMAP=n case?

> By the way, sub-section support is only valid in vmemmap case, right?

Yes.

> Seems yes from code, but I don't find any document to prove it.

check_pfn_span() enforces this requirement.