Messages in this thread | | | Subject | Re: [PATCH V2 1/2] arm64/mm: Fix pfn_valid() for ZONE_DEVICE based memory | From | David Hildenbrand <> | Date | Tue, 2 Feb 2021 13:56:10 +0100 |
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On 02.02.21 13:51, Will Deacon wrote: > On Tue, Feb 02, 2021 at 01:39:29PM +0100, David Hildenbrand wrote: >> On 02.02.21 13:35, Will Deacon wrote: >>> On Tue, Feb 02, 2021 at 12:32:15PM +0000, Will Deacon wrote: >>>> On Tue, Feb 02, 2021 at 09:41:53AM +0530, Anshuman Khandual wrote: >>>>> pfn_valid() validates a pfn but basically it checks for a valid struct page >>>>> backing for that pfn. It should always return positive for memory ranges >>>>> backed with struct page mapping. But currently pfn_valid() fails for all >>>>> ZONE_DEVICE based memory types even though they have struct page mapping. >>>>> >>>>> pfn_valid() asserts that there is a memblock entry for a given pfn without >>>>> MEMBLOCK_NOMAP flag being set. The problem with ZONE_DEVICE based memory is >>>>> that they do not have memblock entries. Hence memblock_is_map_memory() will >>>>> invariably fail via memblock_search() for a ZONE_DEVICE based address. This >>>>> eventually fails pfn_valid() which is wrong. memblock_is_map_memory() needs >>>>> to be skipped for such memory ranges. As ZONE_DEVICE memory gets hotplugged >>>>> into the system via memremap_pages() called from a driver, their respective >>>>> memory sections will not have SECTION_IS_EARLY set. >>>>> >>>>> Normal hotplug memory will never have MEMBLOCK_NOMAP set in their memblock >>>>> regions. Because the flag MEMBLOCK_NOMAP was specifically designed and set >>>>> for firmware reserved memory regions. memblock_is_map_memory() can just be >>>>> skipped as its always going to be positive and that will be an optimization >>>>> for the normal hotplug memory. Like ZONE_DEVICE based memory, all normal >>>>> hotplugged memory too will not have SECTION_IS_EARLY set for their sections >>>>> >>>>> Skipping memblock_is_map_memory() for all non early memory sections would >>>>> fix pfn_valid() problem for ZONE_DEVICE based memory and also improve its >>>>> performance for normal hotplug memory as well. >>>> >>>> Hmm. Although I follow your logic, this does seem to rely on an awful lot of >>>> assumptions to continue to hold true as the kernel evolves. In particular, >>>> how do we ensure that early sections are always fully backed with >>> >>> Sorry, typo here: ^^^ should be *non-early* sections. >> >> It might be a good idea to have a look at generic >> include/linux/mmzone.h:pfn_valid() > > The generic implementation already makes assumptions that aren't true on > arm64, so that's why we've ended up with our own implementation. But the > patches here put us in a position where I worry that pfn_valid() may return > 'true' in future for cases where the underlying struct page is either > non-existent or bogus, and debugging those failures really sucks. We had a > raft of those back when NOMAP was introduced and I don't want to re-live > that experience.
Yeah, and I agree when it comes to boot mem. However, the way generic memory hotplug/memremap infrastructure (->!early sections) works does not allow for such special cases you mention and would break quite some other code if messed up. So I wouldn't worry about that part too much for now.
> >> As I expressed already, long term we should really get rid of the arm64 >> variant and rather special-case the generic one. Then we won't go out of >> sync - just as it happened with ZONE_DEVICE handling here. > > Why does this have to be long term? This ZONE_DEVICE stuff could be the > carrot on the stick :)
Yes, I suggested to do it now, but Anshuman convinced me that doing a simple fix upfront might be cleaner --- for example when it comes to backporting :)
-- Thanks,
David / dhildenb
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