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

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Date:   Wed, 17 Jun 2020 09:04:06 +0100
From:   Will Deacon <will@...nel.org>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Xu <peterx@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Gerald Schaefer <gerald.schaefer@...ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        openrisc@...ts.librecores.org,
        linux-arch <linux-arch@...r.kernel.org>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        linux-s390 <linux-s390@...r.kernel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 00/25] mm: Page fault accounting cleanups

On Wed, Jun 17, 2020 at 10:55:14AM +1000, Michael Ellerman wrote:
> Linus Torvalds <torvalds@...ux-foundation.org> writes:
> > On Mon, Jun 15, 2020 at 3:16 PM Peter Xu <peterx@...hat.com> wrote:
> >> 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>
> 
> > 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