On Wed, Apr 14, 2021 at 05:12:11PM +0200, David Hildenbrand wrote:

On 07.04.21 19:26, Mike Rapoport wrote:

quoted

From: Mike Rapoport <redacted>

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 ;-)

quoted

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 <redacted>
---
  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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