On Wed, 29 Aug 2018, James Morse wrote: > Hi Michal, > > (CC: +Ard) > > On 24/08/18 12:41, Michal Hocko wrote: > > On Thu 23-08-18 15:06:08, James Morse wrote: > > [...] > >> My best-guess is that pfn_valid_within() shouldn't be optimised out if > > ARCH_HAS_HOLES_MEMORYMODEL, even if HOLES_IN_ZONE isn't set. > >> > >> Does something like this solve the problem?: > >> ============================%<============================ > >> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h > >> index 32699b2dc52a..5e27095a15f4 100644 > >> --- a/include/linux/mmzone.h > >> +++ b/include/linux/mmzone.h > >> @@ -1295,7 +1295,7 @@ void memory_present(int nid, unsigned long start, unsigned > >> long end); > >> * pfn_valid_within() should be used in this case; we optimise this away > >> * when we have no holes within a MAX_ORDER_NR_PAGES block. > >> */ > >> -#ifdef CONFIG_HOLES_IN_ZONE > >> +#if defined(CONFIG_HOLES_IN_ZONE) || defined(CONFIG_ARCH_HAS_HOLES_MEMORYMODEL) > >> #define pfn_valid_within(pfn) pfn_valid(pfn) > >> #else > >> #define pfn_valid_within(pfn) (1) > >> ============================%<============================ > > After plenty of greping, git-archaeology and help from others, I think I've a > clearer picture of what these options do. > > > Please correct me if I've explained something wrong here: > > > This is the first time I hear about CONFIG_ARCH_HAS_HOLES_MEMORYMODEL. > > The comment in include/linux/mmzone.h describes this as relevant when parts the > memmap have been free()d. This would happen on systems where memory is smaller > than a sparsemem-section, and the extra struct pages are expensive. > pfn_valid() on these systems returns true for the whole sparsemem-section, so an > extra memmap_valid_within() check is needed. > > This is independent of nomap, and isn't relevant on arm64 as our pfn_valid() > always tests the page in memblock due to nomap pages, which can occur anywhere. > (I will propose a patch removing ARCH_HAS_HOLES_MEMORYMODEL for arm64.) > > > HOLES_IN_ZONE is similar, if some memory is smaller than MAX_ORDER_NR_PAGES, > possibly due to nomap holes. > > 6d526ee26ccd only enabled it for NUMA systems on arm64, because the NUMA code > was first to fall foul of this, but there is nothing NUMA specific about nomap > holes within a MAX_ORDER_NR_PAGES region. > > I'm convinced arm64 should always enable HOLES_IN_ZONE because nomap pages can > occur anywhere. I'll post a fix. But x86 had the same bug - https://bugzilla.redhat.com/show_bug.cgi?id=1598462 And x86 fixed it without enabling HOLES_IN_ZONE. On x86, the BIOS can also reserve any memory range - so you can have arbitrary holes there that are not predictable when the kernel is compiled. Currently HOLES_IN_ZONE is selected only for ia64, mips/octeon - so does it mean that all the other architectures don't have holes in the memory map? What should be architecture-independent way how to handle the holes? Mikulas
