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 13:39:29 +0100 |
| |
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()
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.
-- Thanks,
David / dhildenb
| |