On Tue, Jan 12, 2021 at 1:53 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 12.01.21 10:34, Dan Williams wrote:
> > pfn_section_valid() determines pfn validity on subsection granularity.
> >
> > pfn_valid_within() internally uses pfn_section_valid(), but gates it
> > with early_section() to preserve the traditional behavior of pfn_valid()
> > before subsection support was added.
> >
> > pfn_to_online_page() wants the explicit precision that pfn_valid() does
> > not offer, so use pfn_section_valid() directly. Since
> > pfn_to_online_page() already open codes the validity of the section
> > number vs NR_MEM_SECTIONS, there's not much value to using
> > pfn_valid_within(), just use pfn_section_valid(). This loses the
> > valid_section() check that pfn_valid_within() was performing, but that
> > was already redundant with the online check.
> >
> > Fixes: b13bc35193d9 ("mm/hotplug: invalid PFNs from pfn_to_online_page()")
> > Cc: Qian Cai <cai@xxxxxx>
> > Cc: Michal Hocko <mhocko@xxxxxxxx>
> > Reported-by: David Hildenbrand <david@xxxxxxxxxx>
> > ---
> > mm/memory_hotplug.c | 16 ++++++++++++----
> > 1 file changed, 12 insertions(+), 4 deletions(-)
> >
> > diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> > index 55a69d4396e7..a845b3979bc0 100644
> > --- a/mm/memory_hotplug.c
> > +++ b/mm/memory_hotplug.c
> > @@ -308,11 +308,19 @@ static int check_hotplug_memory_addressable(unsigned long pfn,
> > struct page *pfn_to_online_page(unsigned long pfn)
> > {
> > unsigned long nr = pfn_to_section_nr(pfn);
> > + struct mem_section *ms;
> > +
> > + if (nr >= NR_MEM_SECTIONS)
> > + return NULL;
> > +
> > + ms = __nr_to_section(nr);
> > + if (!online_section(ms))
> > + return NULL;
> > +
> > + if (!pfn_section_valid(ms, pfn))
> > + return NULL;
>
> That's not sufficient for alternative implementations of pfn_valid().
>
> You still need some kind of pfn_valid(pfn) for alternative versions of
> pfn_valid(). Consider arm64 memory holes in the memmap. See their
> current (yet to be fixed/reworked) pfn_valid() implementation.
> (pfn_valid_within() is implicitly active on arm64)
>
> Actually, I think we should add something like the following, to make
> this clearer (pfn_valid_within() is confusing)
>
> #ifdef CONFIG_HAVE_ARCH_PFN_VALID
> /* We might have to check for holes inside the memmap. */
> if (!pfn_valid())
> return NULL;
> #endif
Looks good to me, I'll take Oscar's version that uses IS_ENABLED().
Skipping the call to pfn_valid() saves 16-bytes of code text on x86_64.
