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. > 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 :) Will
