Hi Kirill, On Thu, Feb 21, 2019 at 05:57:06PM +0300, Kirill A. Shutemov wrote: > On Thu, Feb 21, 2019 at 02:46:18PM +0000, Steven Price wrote: > > On 21/02/2019 14:28, Kirill A. Shutemov wrote: > > > On Thu, Feb 21, 2019 at 11:34:52AM +0000, Steven Price wrote: > > >> From: James Morse <james.morse@xxxxxxx> > > >> > > >> Exposing the pud/pgd levels of the page tables to walk_page_range() means > > >> we may come across the exotic large mappings that come with large areas > > >> of contiguous memory (such as the kernel's linear map). > > >> > > >> For architectures that don't provide p?d_large() macros, provided a > > >> does nothing default. > > > > > > Nak, sorry. > > > > > > Power will get broken by the patch. It has pmd_large() inline function, > > > that will be overwritten by the define from this patch. > > > > > > I believe it requires more ground work on arch side in general. > > > All architectures that has huge page support has to provide these helpers > > > (and matching defines) before you can use it in a generic code. > > > > Sorry about that, I had compile tested on power, but obviously not the > > right config to actually see the breakage. > > I don't think you'll catch it at compile-time. It would silently override > the helper with always-false. Can you explain why the compiler would override the helper define in, e.g. arch/powerpc/include/asm/pgtable.h with the generic (0)? Actually, I've tried to compile this on power with deliberately adding errors to both power-specific and the generic definition of pmd_large and the compilation failed the way I expected in the power-specific helper. > > I'll do some grepping - hopefully this is just a case of exposing the > > functions/defines that already exist for those architectures. > > I see the same type of breakage on s390 and sparc. > > > Note that in terms of the new page walking code, these new defines are > > only used when walking a page table without a VMA (which isn't currently > > done), so architectures which don't use p?d_large currently will work > > fine with the generic versions. They only need to provide meaningful > > definitions when switching to use the walk-without-a-VMA functionality. > > How other architectures would know that they need to provide the helpers > to get walk-without-a-VMA functionality? This looks very fragile to me. > > -- > Kirill A. Shutemov -- Sincerely yours, Mike.
