On 13 Aug 2020, at 22:36, Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > On Thu, Aug 13, 2020 at 11:55:21PM +0300, Mike Rapoport wrote: >> +/* >> + * In the kernel's mapped region we know everything is in region number 5, so >> + * as an optimisation its PGD already points to the area for that region. > > Is it actually an optimisation? Are there any benchmarks where this > makes any difference whatsoever? Or should the comment be closer to > > /* ia64 is gratuitously different. Cope with it. */ > > I might suggest that pgd be changed to be the same as every other pgd > in the system, and every other architecture. IA-64 is just weird. There's an optional hardware page table walker, and two different options for how to use it within that, short-format and long-format. My understanding is that Linux uses the short-format, and that means that the VHPTs (Virtual Hashed Page Table) are per-region. My assumption therefore is that the kernel's PGD is a completely separate allocation from the userspace ones, and it's not just a case of "moving the pointer back to the start" but that "that is all there is", so unifying the two would be a significant undertaking that avoids one special case at the cost of invasive refactoring and a performance hit by duplication and/or unused space. My knowledge of IA-64 translation is pretty limited, that's about all I know (and this likely has mistakes in it), but I know enough to avoid finding out too much more. Never make the mistake of assuming IA-64 can be made to look like any other architecture :) Jess
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