Messages in this thread | | | Date | Tue, 13 Feb 2007 23:24:43 +0100 | From | Ingo Molnar <> | Subject | Re: [patch 05/11] syslets: core code |
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* Andi Kleen <andi@firstfloor.org> wrote:
> Ingo Molnar <mingo@elte.hu> writes: > > > + > > +static struct async_thread * > > +pick_ready_cachemiss_thread(struct async_head *ah) > > The cachemiss names are confusing. I assume that's just a left over > from Tux?
yeah. Although 'stuff goes async' is quite similar to a cachemiss. We didnt have some resource available right now so the syscall has to block == i.e. some cache was not available.
> > + > > + memset(atom->args, 0, sizeof(atom->args)); > > + > > + ret |= __get_user(arg_ptr, &uatom->arg_ptr[0]); > > + if (!arg_ptr) > > + return ret; > > + if (!access_ok(VERIFY_WRITE, arg_ptr, sizeof(*arg_ptr))) > > + return -EFAULT; > > It's a little unclear why you do that many individual access_ok()s. > And why is the target constant sized anyways?
each indirect pointer has to be checked separately, before dereferencing it. (Andrew pointed out that they should be VERIFY_READ, i fixed that in my tree)
it looks a bit scary in C but the assembly code is very fast and quite straightforward.
> + /* > + * Lock down the ring. Note: user-space should not munlock() this, > + * because if the ring pages get swapped out then the async > + * completion code might return a -EFAULT instead of the expected > + * completion. (the kernel safely handles that case too, so this > + * isnt a security problem.) > + * > + * mlock() is better here because it gets resource-accounted > + * properly, and even unprivileged userspace has a few pages > + * of mlock-able memory available. (which is more than enough > + * for the completion-pointers ringbuffer) > + */ > > If it's only a few pages you don't need any resource accounting. If > it's more then it's nasty to steal the users quota. I think plain > gup() would be better.
get_user_pages() would have to be limited in some way - and i didnt want to add yet another wacky limit thing - so i just used the already existing mlock() infrastructure for this. If Oracle wants to set up a 10 MB ringbuffer, they can set the PAM resource limits to 11 MB and still have enough stuff left. And i dont really expect GPG to start using syslets - just yet ;-)
a single page is enough for 1024 completion pointers - that's more than enough for most purposes - and the default mlock limit is 40K.
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