On 01/03/2019 12:30, Kirill A. Shutemov wrote: > On Fri, Mar 01, 2019 at 01:53:01PM +0200, Mike Rapoport wrote: >> Him Kirill, >> >> On Fri, Feb 22, 2019 at 12:06:18AM +0300, Kirill A. Shutemov wrote: >>> On Thu, Feb 21, 2019 at 05:16:46PM +0000, Steven Price wrote: >>>>>> 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. >>>> >>>> Yes, you've got a good point there. This would apply to the p?d_large >>>> macros as well - any arch which (inadvertently) uses the generic version >>>> is likely to be fragile/broken. >>>> >>>> I think probably the best option here is to scrap the generic versions >>>> altogether and simply introduce a ARCH_HAS_PXD_LARGE config option which >>>> would enable the new functionality to those arches that opt-in. Do you >>>> think this would be less fragile? >>> >>> These helpers are useful beyond pagewalker. >>> >>> Can we actually do some grinding and make *all* archs to provide correct >>> helpers? Yes, it's tedious, but not that bad. >> >> Many architectures simply cannot support non-leaf entries at the higher >> levels. I think letting the use a generic helper actually does make sense. > > I disagree. > > It's makes sense if the level doesn't exists on the arch. This is what patch 24 [1] of the series does - if the level doesn't exist then appropriate stubs are provided. > But if the level exists, it will be less frugile to ask the arch to > provide the helper. Even if it is dummy always-false. The problem (as I see it), is we need a reliable set of p?d_large() implementations to be able to walk arbitrary page tables. Either the entire functionality of walking page tables without a VMA has to be an opt-in per architecture, or we need to mandate that every architecture provide these implementations. I could provide an asm-generic header to provide a complete set of dummy implementations for architectures that don't support large pages at all, but that seems a bit overkill when most architectures only need to define 2 or 3 implementations (the rest being provided by the folded-levels automatically). Thanks, Steve [1] https://lore.kernel.org/lkml/20190227170608.27963-25-steven.price@xxxxxxx/
