64-bit writes are /usually/ atomic but alignment or compiler emiting 32-bit opcodes could also do it. I agree there are a few other pieces to this we will chat about separately and come back to this thread. Time for some zzzz...long weekend! -- Computer Architect | Sent from my #ARM Powered Mobile Device On Mar 2, 2015 5:50 AM, Catalin Marinas <catalin.marinas@xxxxxxx> wrote: > > On Mon, Mar 02, 2015 at 12:58:36AM -0500, Jon Masters wrote: > > I've pulled aOn Mon, Mar 02, 2015 at 12:58:36AM -0500, Jon Masters wrote: > I've pulled a couple of all nighters reproducing this hard to trigger > issue and got some data. It looks like the high half of the (note always > userspace) PMD is all zeros or all ones, which makes me wonder if the > logic in update_mmu_cache might be missing something on AArch64. That's worrying but I can tell you offline why ;). Anyway, 64-bit writes are atomic on ARMv8, so you shouldn't see half updates. To make sure the compiler does not generate something weird, change the set_(pte|pmd|pud) to use an inline assembly with a 64-bit STR. One question - is the PMD a table or a block? You mentioned set_pte_at at some point, which leads me to think it's a (transparent) huge page, hence block mapping. > When a kernel is built with 64K pages and 2 levels the PMD is > effectively updated using set_pte_at, which explicitly won't perform a > DSB if the address is userspace (it expects this to happen later, in > update_mmu_cache as an example. > > Can anyone think of an obvious reason why we might not be properly > flushing the changes prior to them being consumed by a hardware walker? Even if you don't have that barrier, the worst that can happen is that you get another trap back in the kernel (from user; translation fault) but the page table read by the kernel is valid and normally the instruction restarted. > Test kernels running with an explicit DSB in all PTE update cases now > running overnight. Just in case. It could be hiding some other problems. -- Catalin -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>