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Date:   Fri, 14 Jun 2019 09:36:33 -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>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>
Subject: Re: [PATCH -next] mm/hotplug: skip bad PFNs from pfn_to_online_page()

On Fri, Jun 14, 2019 at 9:26 AM Aneesh Kumar K.V
<aneesh.kumar@...ux.ibm.com> wrote:
>
> On 6/14/19 9:52 PM, Dan Williams wrote:
> > 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
>
> That should be SECTION_SIZE_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.
> >
>
>
> persistent memory namespaces are not portable across archs because they
> have PAGE_SIZE dependency.

We can fix that by reserving mem_map capacity assuming the smallest
PAGE_SIZE across archs.

> Then we have dependencies like the page size
> with which we map the vmemmap area.

How does that lead to cross-arch incompatibility? Even on a single
arch the vmemmap area will be mapped with 2MB pages for 128MB aligned
spans of pmem address space and 4K pages for subsections.

> Why not let the arch
> arch decide the SUBSECTION_SHIFT and default to one subsection per
> section if arch is not enabled to work with subsection.

Because that keeps the implementation from ever reaching a point where
a namespace might be able to be moved from one arch to another. If we
can squash these arch differences then we can have a common tool to
initialize namespaces outside of the kernel. The one wrinkle is
device-dax that wants to enforce the mapping size, but I think we can
have a module-option compatibility override for that case for the
admin to say "yes, I know this namespace is defined for 2MB x86 pages,
but I want to force enable it with 64K pages on PowerPC"

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