Re: [PATCH] Linux Kernel Markers 0.5 for Linux 2.6.17 (with probe management)

Ingo Molnar
2006-09-22T17:02:01Z

* Mathieu Desnoyers <compudj@krystal.dyndns.org> wrote:

> > > Then you lose the ability to trace in-kernel minor page faults.
> >
> > that's wrong, minor pagefaults go through __handle_mm_fault() just as
> > much.
> >
>
> Hi Ingo,
>
> On a 2.6.17 kernel tree :

> It seems like a shortcut path that will never call __handle_mm_fault.
> This path is precisely used to handle vmalloc faults.

yes, but you said "minor fault", not "vmalloc fault".

minor faults are the things that happen when a task does read-after-COW
or read-mmap-ed-pagecache-page, and they very much go through
__handle_mm_fault().

vmalloc faults are extremely rare, x86-specific and they are a pure
kernel-internal matter. (I'd never want to trace them, especially if it
pushes tracepoints into every architecture's page fault handler. I
implemented the initial version of them IIRC, but my memory fails
precisely why. I think it was 4:4 related, but i'm unsure.)

(i now realize that above you said "in-kernel minor faults" - under that
you meant vmalloc faults?)

Ingo