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Re: [BUGFIX][PATCH -rc/-mm] prevent kprobes from catching spurious page faults
- From: Mathieu Desnoyers <mathieu dot desnoyers at polymtl dot ca>
- To: Ingo Molnar <mingo at elte dot hu>
- Cc: Masami Hiramatsu <mhiramat at redhat dot com>, Andrew Morton <akpm at linux-foundation dot org>, Linus Torvalds <torvalds at linux-foundation dot org>, Greg KH <greg at kroah dot com>, Nick Piggin <npiggin at suse dot de>, LKML <linux-kernel at vger dot kernel dot org>, Ananth N Mavinakayanahalli <ananth at in dot ibm dot com>, Jim Keniston <jkenisto at us dot ibm dot com>, systemtap-ml <systemtap at sources dot redhat dot com>, "Frank Ch. Eigler" <fche at redhat dot com>
- Date: Thu, 5 Feb 2009 20:13:20 -0500
- Subject: Re: [BUGFIX][PATCH -rc/-mm] prevent kprobes from catching spurious page faults
- References: <497FC3B1.7050805@redhat.com> <497FE895.1080708@redhat.com> <20090128154824.GA6025@Krystal> <49808EEF.1020700@redhat.com> <20090128171331.GA9006@Krystal> <49809CCE.40409@redhat.com> <20090128181053.GC9908@Krystal> <498B6457.20302@redhat.com> <20090205235727.GA16040@elte.hu>
* Ingo Molnar (mingo@elte.hu) wrote:
>
> * Masami Hiramatsu <mhiramat@redhat.com> wrote:
>
> > - if (notify_page_fault(regs))
> > - return;
> > if (unlikely(kmmio_fault(regs, address)))
> > return;
> >
> > @@ -634,6 +632,9 @@ void __kprobes do_page_fault(struct pt_r
> > if (spurious_fault(address, error_code))
> > return;
> >
> > + /* kprobes don't want to hook the spurious faults. */
> > + if (notify_page_fault(regs))
> > + return;
> > /*
> > * Don't take the mm semaphore here. If we fixup a prefetch
> > * fault we could otherwise deadlock.
> > @@ -641,6 +642,9 @@ void __kprobes do_page_fault(struct pt_r
> > goto bad_area_nosemaphore;
> > }
> >
> > + /* kprobes don't want to hook the spurious faults. */
> > + if (notify_page_fault(regs))
> > + return;
>
> I dont know - this spreads that callback to two places now. Any
> reason why kprobes cannot call spurious_fault(), if there's a
> probe active?
>
> Also, moving that would remove the planned cleanup of merging these
> two into one call:
>
> if (notify_page_fault(regs))
> return;
> if (unlikely(kmmio_fault(regs, address)))
> return;
>
> We should reduce the probing cross section, not increase it,
> especially in such a critical codepath as the pagefault handler.
>
> Btw., why cannot kprobes install a dynamic probe to the fault
> handler itself? That way the default path would have no such
> callbacks and checks at all.
>
Or we could simply merge my 2 LTTng page fault handler tracepoints per
architecture and be done with it ?
I'd need to clean up the patchset a little bit to fold a few patches,
but that would be straightforward enough.
Mathieu
--
Mathieu Desnoyers
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