Messages in this thread | | | Subject | Re: [PATCH 0/2 x86] fix some page faults in nmi if kmemcheck is enabled | From | Li Zhong <> | Date | Tue, 28 Feb 2012 10:45:41 +0800 |
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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 ...
Thanks, Zhong
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