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    SubjectRe: [PATCH 0/2 x86] fix some page faults in nmi if kmemcheck is enabled
    On Mon, 2012-02-27 at 11:58 +0100, Peter Zijlstra wrote:
    > On Thu, 2012-02-23 at 17:53 +0800, Li Zhong wrote:
    > > I will think further about it, and would appreciate it if you could give
    > > some good ideas.
    > *sigh*.. or you could do your own damn work..

    I'm still doing the work ...

    > > > > 2. If CONFIG_KMEMCHECK is enabled, the pages allocated through slab will
    > > > > be marked as non-present, to capture uninitialized memory access. More
    > > > > information in Documentation/kmemcheck.txt .
    > > >
    > > > So then kmemcheck is buggy, since the nmiaction structure is initialized
    > > > in register_nmi_handler(), so it should most definitely not be marked
    > > > non-present.
    > > >
    > >
    > > I'm not sure whether I understand it correctly. Do you mean that
    > > nmiaction is initialized in register_nmi_handler(), which indicates it
    > > will be used in nmi, so it shouldn't be marked non-present?
    > No, you said that it marks memory non-present to detect uninitialized
    > stuff, but since it is initialized, it shouldn't then be non-present,
    > right?

    From my understanding of kmemcheck, the checking is based on the
    non-present page. So while handling page fault, if the memory hasn't
    been written before read, kmemcheck knows that it is uninitialized.

    I think it is used to find code errors, so it need mark all non-present,
    to check if there are any access to uninitialized memory.

    > > But for kmemcheck, why need it know the information that page fault is
    > > not allowed in nmi?
    > Uh, what?

    Please ignore it, as I misunderstood your point previously.

    > > > > 3. From the log, there are some memories accessed in nmi, which are in
    > > > > pages marked as non-present by kmemcheck, as they are allocated by
    > > > > something like kmalloc().
    > > >
    > > > So figure out why and fix that instead of writing ugly ass patches that
    > > > seemingly work around the problem without actually thinking about it.
    > > >
    > >
    > > I think the reason is that kmalloc ( or kzalloc ... ) uses malloc_sizes
    > > slab caches to allocate memory. The malloc_sizes slab caches is set up
    > > without SLAB_NOTRACK flag, then kmemcheck marks the pages non-present to
    > > do its check in page fault handling code. I think we shouldn't disable
    > > kmemechek for the general malloc_sizes caches.
    > Nobody said you should.. there's plenty of solutions that aren't ass
    > backward stupid nor as ugly.
    > First you need to figure out why the page is marked non-present since
    > the data structure is initialized (I've got a fair idea why), then look
    > if you can tell kmemcheck not to be silly like that.
    > Alternatively you can change the nmi stuff to use static storage like
    > other notifiers (see notifier_block).

    OK, I will try to update the nmi one using this way.

    But I think it couldn't be used to the perf stuff.
    For perf, maybe it's good for kmemcheck to have some flag like
    __GFP_NO_PAGE_FAULT? Currently it seems only has flags like
    __GFP_NOTRACK, which will still mark page non-present.

    > What you don't ever do is write alternative code paths that are never
    > ever used except for debugging, that is just asking for problems.

    Got it, thanks for the reminder! Previously, I thought the biggest
    problem was wasting memory ...


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