Re: [PATCH 00/25] mm: Page fault accounting cleanups

On Wed, Jun 17, 2020 at 10:55:14AM +1000, Michael Ellerman wrote:

Linus Torvalds [off-list ref] writes:

quoted

On Mon, Jun 15, 2020 at 3:16 PM Peter Xu [off-list ref] wrote:

quoted

This series tries to address all of them by introducing mm_fault_accounting()
first, so that we move all the page fault accounting into the common code base,
then call it properly from arch pf handlers just like handle_mm_fault().

Hmm.

So having looked at this a bit more, I'd actually like to go even
further, and just get rid of the per-architecture code _entirely_.

<snip>

quoted

One detail worth noting: I do wonder if we should put the

    perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, addr);

just in the arch code at the top of the fault handling, and consider
it entirely unrelated to the major/minor fault handling. The
major/minor faults fundamnetally are about successes. But the plain
PERF_COUNT_SW_PAGE_FAULTS could be about things that fail, including
things that never even get to this point at all.

Yeah I think we should keep it in the arch code at roughly the top.

I agree. It's a nice idea to consolidate the code, but I don't see that
it's really possible for PERF_COUNT_SW_PAGE_FAULTS without significantly
changing the semantics (and a potentially less useful way. Of course,
moving more of do_page_fault() out of the arch code would be great, but
that's a much bigger effort.

If it's moved to the end you could have a process spinning taking bad
page faults (and fixing them up), and see no sign of it from the perf
page fault counters.

The current arm64 behaviour is that we record PERF_COUNT_SW_PAGE_FAULTS
if _all_ of the following are true:

  1. The fault isn't handled by kprobes
  2. The pagefault handler is enabled
  3. We have an mm (current->mm)
  4. The fault isn't an unexpected kernel fault on a user address (we oops
     and kill the task in this case)

Which loosely corresponds to "we took a fault on a user address that it
looks like we can handle".

That said, I'm happy to tweak this if it brings us into line with other
architectures.

Will