Messages in this thread | | | Date | Thu, 25 Sep 2008 08:42:13 -0700 (PDT) | From | Linus Torvalds <> | Subject | Re: [Bug #11342] Linux 2.6.27-rc3: kernel BUG at mm/vmalloc.c - bisected |
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On Thu, 25 Sep 2008, Rusty Russell wrote: > > This turns out to be awful in practice, mainly due to const. Consider: > > #ifdef CONFIG_CPUMASK_OFFSTACK > typedef unsigned long *cpumask_t; > #else > typedef unsigned long cpumask_t[1]; > #endif > > cpumask_t returns_cpumask(void);
No. That's already broken. You cannot return a cpumask_t, regardless of interface. We must not do it regardless of how we pass those things around, since it generates _yet_ another temporary on the stack for the return slot for any kind of structure.
So all cpumask functions should always return pointers and/or take pointers to be filled in. That's true *regardless* of how we actually are to then allocate them.
So forget returning cpumasks. It's irrelevant.
What _is_ relevant is how we allocate them when we need temporary CPU masks. And _that_ is where my suggestion comes in. For small NR_CPUS, we really do want to allocate them on the stack, because calling kmalloc for a 4- or 8-byte allocation is just _stupid_.
So all your arguments are invalid, because you're looking at the wrong thing. The thing that I was talking about is converting current code that has
random_function(..) { cpumask_t mask;
.. do something with mask ... }
which has to be converted some way. And I think it needs to be converted in a way that does *not* force us to call kmalloc() for idiotically small values.
Linus
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