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Re: [BUGFIX][PATCH -rc/-mm] prevent kprobes from catching spurious page faults
- From: Masami Hiramatsu <mhiramat at redhat dot com>
- To: Mathieu Desnoyers <mathieu dot desnoyers at polymtl dot ca>
- Cc: Ingo Molnar <mingo at elte dot hu>, 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: Fri, 06 Feb 2009 11:30:51 -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> <20090206011320.GA7161@Krystal>
Mathieu Desnoyers wrote:
> * 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 ?
As you can see, these functions are a kind of fixup code.
If it succeed fixup a fault, do_page_fault() has to return because
the fault is fixed.
Since tracepoint itself is just a watchpoint, it should not
change code path. So, I think just moving kmmio_fault() to
notify_page_fault() is enough.
> I'd need to clean up the patchset a little bit to fold a few patches,
> but that would be straightforward enough.
Anyway, I agree with the idea to push tracepoint in the pagefault.
It is very useful for watching system behavior.
Thanks!
>
> Mathieu
>
--
Masami Hiramatsu
Software Engineer
Hitachi Computer Products (America) Inc.
Software Solutions Division
e-mail: mhiramat@redhat.com