Re: [GIT PULL v2] Early SLAB fixes for 2.6.31

From: Nick Piggin
Date: Tue Jun 16 2009 - 01:36:25 EST

On Tue, Jun 16, 2009 at 03:28:07PM +1000, Benjamin Herrenschmidt wrote:
> On Tue, 2009-06-16 at 06:57 +0200, Nick Piggin wrote:
> >
> > Yes but that's heavily qualified. As I said, we already require
> > a lot of knowledge of context passed in to it. I have no interest
> > in adding code to make *early boot* code not have to care about
> > that, especially because everybody else certainly has to know
> > whether they are calling the allocator with interrupts disabled
> > or a lock held etc.
> You seem to totally ignore the argument I made to answer this specific
> point in one of my previous emails. Right, they are to some extent
> subjective, but I believe they have some standing. The main one is
> probably that it's a lot less obvious to a lot of code where in the boot
> process it's going to be called, or even if it's going to be called at
> boot, later, both. This is especially true nowadays with all the talks
> about shuffling more of the boot process around.

I didn't ignore that argument, I just don't agree. It only does
not "know" the context it is called from if it does not get that
information passed to it from its caller who does know.

> > To be clear about this: the allocator is fully servicable and
> > no different to normal system running at this point. The
> > difference for example is that code runs with interrupts off
> > but incorrectly uses GFP_KERNEL for allocations. This is a
> > blatent bug in any other kernel code, I don't know why boot
> > code is OK to be horrible and wrong and work around it with
> > the equally horrible system_state (and this gfp mask which is
> > just system_state under another name).
> Because it would be extremely impractical to have to explicitely pass
> the gfp_flags around for anything that can be called at boot time. This
> is as simple as that. A -lot- more impractical than requiring atomic
> call sites to know what they are doing.

We'll see.

> > I just don't want to use this slab fix in question to be a
> > license to throw away and forget all about any context information
> > in the early boot code because someone asserts "it will make the
> > code better". I'm fine with the slab change for now, but let's
> > try to retain context information as well.
> But in many case it's meaningless. Again, what do you define as "boot"
> is a very obscure thing here.

It's not obscure. I'm vague because it doesn't matter *all that much*.

> > If somebody comes up with a patch to remove thousands of lines
> > of boot code by ignoring context, then I might concede the
> > point and we could remove the context annotations.
> No, we don't want to -add- thousands of lines of code :-) And we can

I don't understand. Where would you be adding thousands of lines
of code?

> indeed remove a bunch of the old slab_is_available() tests too indeed.
> And no, they should not -all- be converted to NOWAIT. See vmalloc() as a
> good example, I have a few more like that.

There aren't too many significant code simplifications AFAIKS.
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