linux-kernel - Re: [PATCH v2 2/5] mm: Teach pfn_to_online_page() to consider subsection validity

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Date:   Tue, 12 Jan 2021 14:20:40 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Linux MM <linux-mm@...ck.org>, Qian Cai <cai@....pw>,
        Michal Hocko <mhocko@...e.com>,
        Vishal L Verma <vishal.l.verma@...el.com>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Anshuman Khandual <anshuman.khandual@....com>
Subject: Re: [PATCH v2 2/5] mm: Teach pfn_to_online_page() to consider
 subsection validity

On Tue, Jan 12, 2021 at 1:53 AM David Hildenbrand <david@...hat.com> 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@....pw>
> > Cc: Michal Hocko <mhocko@...e.com>
> > Reported-by: David Hildenbrand <david@...hat.com>
> > ---
> >  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.