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

    > 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 ;-)

    > > 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.

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