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Date:   Fri, 14 Jun 2019 09:22:29 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
Cc:     Oscar Salvador <osalvador@...e.de>, Qian Cai <cai@....pw>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        jmoyer <jmoyer@...hat.com>
Subject: Re: [PATCH -next] mm/hotplug: skip bad PFNs from pfn_to_online_page()

On Fri, Jun 14, 2019 at 9:18 AM Aneesh Kumar K.V
<aneesh.kumar@...ux.ibm.com> wrote:
>
> On 6/14/19 9:05 PM, Oscar Salvador wrote:
> > On Fri, Jun 14, 2019 at 02:28:40PM +0530, Aneesh Kumar K.V wrote:
> >> Can you check with this change on ppc64.  I haven't reviewed this series yet.
> >> I did limited testing with change . Before merging this I need to go
> >> through the full series again. The vmemmap poplulate on ppc64 needs to
> >> handle two translation mode (hash and radix). With respect to vmemap
> >> hash doesn't setup a translation in the linux page table. Hence we need
> >> to make sure we don't try to setup a mapping for a range which is
> >> arleady convered by an existing mapping.
> >>
> >> diff --git a/arch/powerpc/mm/init_64.c b/arch/powerpc/mm/init_64.c
> >> index a4e17a979e45..15c342f0a543 100644
> >> --- a/arch/powerpc/mm/init_64.c
> >> +++ b/arch/powerpc/mm/init_64.c
> >> @@ -88,16 +88,23 @@ static unsigned long __meminit vmemmap_section_start(unsigned long page)
> >>    * which overlaps this vmemmap page is initialised then this page is
> >>    * initialised already.
> >>    */
> >> -static int __meminit vmemmap_populated(unsigned long start, int page_size)
> >> +static bool __meminit vmemmap_populated(unsigned long start, int page_size)
> >>   {
> >>      unsigned long end = start + page_size;
> >>      start = (unsigned long)(pfn_to_page(vmemmap_section_start(start)));
> >>
> >> -    for (; start < end; start += (PAGES_PER_SECTION * sizeof(struct page)))
> >> -            if (pfn_valid(page_to_pfn((struct page *)start)))
> >> -                    return 1;
> >> +    for (; start < end; start += (PAGES_PER_SECTION * sizeof(struct page))) {
> >>
> >> -    return 0;
> >> +            struct mem_section *ms;
> >> +            unsigned long pfn = page_to_pfn((struct page *)start);
> >> +
> >> +            if (pfn_to_section_nr(pfn) >= NR_MEM_SECTIONS)
> >> +                    return 0;
> >
> > I might be missing something, but is this right?
> > Having a section_nr above NR_MEM_SECTIONS is invalid, but if we return 0 here,
> > vmemmap_populate will go on and populate it.
>
> I should drop that completely. We should not hit that condition at all.
> I will send a final patch once I go through the full patch series making
> sure we are not breaking any ppc64 details.
>
> Wondering why we did the below
>
> #if defined(ARCH_SUBSECTION_SHIFT)
> #define SUBSECTION_SHIFT (ARCH_SUBSECTION_SHIFT)
> #elif defined(PMD_SHIFT)
> #define SUBSECTION_SHIFT (PMD_SHIFT)
> #else
> /*
>   * Memory hotplug enabled platforms avoid this default because they
>   * either define ARCH_SUBSECTION_SHIFT, or PMD_SHIFT is a constant, but
>   * this is kept as a backstop to allow compilation on
>   * !ARCH_ENABLE_MEMORY_HOTPLUG archs.
>   */
> #define SUBSECTION_SHIFT 21
> #endif
>
> why not
>
> #if defined(ARCH_SUBSECTION_SHIFT)
> #define SUBSECTION_SHIFT (ARCH_SUBSECTION_SHIFT)
> #else
> #define SUBSECTION_SHIFT  SECTION_SHIFT
> #endif
>
> ie, if SUBSECTION is not supported by arch we have one sub-section per
> section?

A couple comments:

The only reason ARCH_SUBSECTION_SHIFT exists is because PMD_SHIFT on
PowerPC was a non-constant value. However, I'm planning to remove the
distinction in the next rev of the patches. Jeff rightly points out
that having a variable subsection size per arch will lead to
situations where persistent memory namespaces are not portable across
archs. So I plan to just make SUBSECTION_SHIFT 21 everywhere.

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