Messages in this thread | | | From | David Hildenbrand <> | Subject | Re: [RFC/RFT PATCH 1/3] memblock: update initialization of reserved pages | Date | Wed, 14 Apr 2021 17:52:57 +0200 |
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On 14.04.21 17:27, Ard Biesheuvel wrote: > On Wed, 14 Apr 2021 at 17:14, David Hildenbrand <david@redhat.com> 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. >> >> I can spot that we want to export such memory like any special memory >> thingy/hole in /proc/iomem -- "reserved", which makes sense. >> >> 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. >> > > On ARM, we need to know whether any physical regions that do not > contain system memory contain something with device semantics or not. > One of the examples is ACPI tables: these are in reserved memory, and > so they are not covered by the linear region. However, when the ACPI > core ioremap()s an arbitrary memory region, we don't know whether it > is mapping a memory region or a device region unless we keep track of > this in some way. (Device mappings require device attributes, but > firmware tables require memory attributes, as they might be accessed > using misaligned reads)
Using generically sounding NOMAP ("don't create direct mapping") to identify device regions feels like a hack. I know, it was introduced just for that purpose.
Looking at memblock_mark_nomap(), we consider "device regions"
1) ACPI tables
2) VIDEO_TYPE_EFI memory
3) some device-tree regions in of/fdt.c
IIUC, right now we end up creating a memmap for this NOMAP memory, but hide it away in pfn_valid(). This patch set at least fixes that.
Assuming these pages are never mapped to user space via the struct page (which better be the case), we could further use a new pagetype to mark these pages in a special way, such that we can identify them directly via pfn_to_page().
Then, we could mostly avoid having to query memblock at runtime to figure out that this is special memory. This would obviously be an extension to this series. Just a thought.
-- Thanks,
David / dhildenb
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