Re: PMD update corruption (sync question)

[Catalin Marinas <catalin.marinas@arm.com>]

On 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