Messages in this thread | | | Date | Fri, 06 Feb 2009 11:30:51 -0500 | From | Masami Hiramatsu <> | Subject | Re: [BUGFIX][PATCH -rc/-mm] prevent kprobes from catching spurious page faults |
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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
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