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

[Peter Xu]

On Wed, Jun 17, 2020 at 09:04:06AM +0100, Will Deacon wrote:
> On Wed, Jun 17, 2020 at 10:55:14AM +1000, Michael Ellerman wrote:
> > Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> writes:
> > > On Mon, Jun 15, 2020 at 3:16 PM Peter Xu <peterx@xxxxxxxxxx> 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.

I see.  I'll keep the semantics for PERF_COUNT_SW_PAGE_FAULTS in the next
version.  Thanks for all the comments!

-- 
Peter Xu