Re: [RFC/RFT PATCH 1/3] memblock: update initialization of reserved pages

[David Hildenbrand <david@redhat.com>]

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Mike Rapoport <rppt@kernel.org> schrieb am Mi. 14. Apr. 2021 um 22:06:

> On Wed, Apr 14, 2021 at 05:12:11PM +0200, David Hildenbrand wrote:
> > On 07.04.21 19:26, Mike Rapoport wrote:
> > > From: Mike Rapoport <rppt@linux.ibm.com>
> > >
> > > The struct pages representing a reserved memory region are initialized
> > > using reserve_bootmem_range() function. This function is called for
> each
> > > reserved region just before the memory is freed from memblock to the
> buddy
> > > page allocator.
> > >
> > > The struct pages for MEMBLOCK_NOMAP regions are kept with the default
> > > values set by the memory map initialization which makes it necessary to
> > > have a special treatment for such pages in pfn_valid() and
> > > pfn_valid_within().
> >
> > I assume these pages are never given to the buddy, because we don't have
> a
> > direct mapping. So to the kernel, it's essentially just like a memory
> hole
> > with benefits.
>
> The pages should not be accessed as normal memory so they do not have a
> direct (or in ARMish linear) mapping and are never given to buddy.
> After looking at ACPI standard I don't see a fundamental reason for this
> but they've already made this mess and we need to cope with it.
>
> > I can spot that we want to export such memory like any special memory
> > thingy/hole in /proc/iomem -- "reserved", which makes sense.
>
> It does, but let's wait with /proc/iomem changes. We don't really have a
> 100% consistent view of it on different architectures, so adding yet
> another type there does not seem, well, urgent.
>

To clarify: this is already done on arm64.


> > I would assume that MEMBLOCK_NOMAP is a special type of *reserved*
> memory.
> > IOW, that for_each_reserved_mem_range() should already succeed on these
> as
> > well -- we should mark anything that is MEMBLOCK_NOMAP implicitly as
> > reserved. Or are there valid reasons not to do so? What can anyone do
> with
> > that memory?
> >
> > I assume they are pretty much useless for the kernel, right? Like other
> > reserved memory ranges.
>
> I agree that there is a lot of commonality between NOMAP and reserved. The
> problem is that even semantics for reserved is different between
> architectures. Moreover, on the same architecture there could be
> E820_TYPE_RESERVED and memblock.reserved with different properties.
>
> I'd really prefer moving in baby steps here because any change in the boot
> mm can bear several month of early hangs debugging ;-)


Yeah I know. We just should have the desired target state figured out :)



>
> > > Split out initialization of the reserved pages to a function with a
> > > meaningful name and treat the MEMBLOCK_NOMAP regions the same way as
> the
> > > reserved regions and mark struct pages for the NOMAP regions as
> > > PageReserved.
> > >
> > > Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
> > > ---
> > >   mm/memblock.c | 23 +++++++++++++++++++++--
> > >   1 file changed, 21 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/mm/memblock.c b/mm/memblock.c
> > > index afaefa8fc6ab..6b7ea9d86310 100644
> > > --- a/mm/memblock.c
> > > +++ b/mm/memblock.c
> > > @@ -2002,6 +2002,26 @@ static unsigned long __init
> __free_memory_core(phys_addr_t start,
> > >     return end_pfn - start_pfn;
> > >   }
> > > +static void __init memmap_init_reserved_pages(void)
> > > +{
> > > +   struct memblock_region *region;
> > > +   phys_addr_t start, end;
> > > +   u64 i;
> > > +
> > > +   /* initialize struct pages for the reserved regions */
> > > +   for_each_reserved_mem_range(i, &start, &end)
> > > +           reserve_bootmem_region(start, end);
> > > +
> > > +   /* and also treat struct pages for the NOMAP regions as
> PageReserved */
> > > +   for_each_mem_region(region) {
> > > +           if (memblock_is_nomap(region)) {
> > > +                   start = region->base;
> > > +                   end = start + region->size;
> > > +                   reserve_bootmem_region(start, end);
> > > +           }
> > > +   }
> > > +}
> > > +
> > >   static unsigned long __init free_low_memory_core_early(void)
> > >   {
> > >     unsigned long count = 0;
> > > @@ -2010,8 +2030,7 @@ static unsigned long __init
> free_low_memory_core_early(void)
> > >     memblock_clear_hotplug(0, -1);
> > > -   for_each_reserved_mem_range(i, &start, &end)
> > > -           reserve_bootmem_region(start, end);
> > > +   memmap_init_reserved_pages();
> > >     /*
> > >      * We need to use NUMA_NO_NODE instead of NODE_DATA(0)->node_id
>
> --
> Sincerely yours,
> Mike.
>
> --
Thanks,

David / dhildenb

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