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. Will
