linux-kernel - Re: [patch 05/11] syslets: core code

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Message-ID: <20070213222443.GH22104@elte.hu>
Date:	Tue, 13 Feb 2007 23:24:43 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@....com.au>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Ulrich Drepper <drepper@...hat.com>,
	Zach Brown <zach.brown@...cle.com>,
	Evgeniy Polyakov <johnpol@....mipt.ru>,
	"David S. Miller" <davem@...emloft.net>,
	Benjamin LaHaise <bcrl@...ck.org>,
	Suparna Bhattacharya <suparna@...ibm.com>,
	Davide Libenzi <davidel@...ilserver.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [patch 05/11] syslets: core code


* Andi Kleen <andi@...stfloor.org> wrote:

> Ingo Molnar <mingo@...e.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.

	Ingo
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