lkml.org 
[lkml]   [2012]   [Feb]   [23]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    SubjectRe: [PATCH 0/2 x86] fix some page faults in nmi if kmemcheck is enabled
    From
    Date
    On Tue, 2012-02-21 at 11:17 +0100, Peter Zijlstra wrote:
    > On Tue, 2012-02-21 at 09:42 +0800, Li Zhong wrote:
    >
    > > > Hell no, these are some of the ugliest patches I've seen in a while. Not
    > > > to mention that their changelogs are utter crap since they don't even
    > > > explain why they're doing what they're doing.
    > > >
    > > Hi Peter,
    > >
    > > I agree that the fix is ugly. I'm willing to change if there are some
    > > better ways.
    >
    > There are always better ways..

    Hi Peter,

    I will think further about it, and would appreciate it if you could give
    some good ideas.

    >
    > > The problem here is:
    > > 1. It seems x86 doesn't allow page faults in nmi, and there are checks
    > > in the code, like WARN_ON_ONCE(in_nmi()).
    >
    > I bet that's not x86 only..

    Maybe, I found this problem on x86, didn't check other archs.

    However, from Documentation/kmemcheck.txt, seems kmemcheck only supports
    x86.

    >
    > > 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?

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

    Or maybe I misunderstand your point here?

    > > 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.

    Thanks,
    Zhong






    \
     
     \ /
      Last update: 2012-02-23 11:11    [W:3.313 / U:0.000 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site