linux-kernel - Re: [BUGFIX][PATCH -rc/-mm] prevent kprobes from catching spurious page faults

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Date:	Fri, 6 Feb 2009 03:04:29 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Masami Hiramatsu <mhiramat@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Greg KH <greg@...ah.com>, Nick Piggin <npiggin@...e.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Jim Keniston <jkenisto@...ibm.com>,
	systemtap-ml <systemtap@...rces.redhat.com>,
	"Frank Ch. Eigler" <fche@...hat.com>
Subject: Re: [BUGFIX][PATCH -rc/-mm] prevent kprobes from catching spurious
	page  faults


* Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca> wrote:

> * Ingo Molnar (mingo@...e.hu) wrote:
> > 
> > * Masami Hiramatsu <mhiramat@...hat.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.

yes, that would be an option too - it depends on the details of how it looks 
like and what kind of complexity it hides.

	Ingo
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