On Tue, 26 Jun 2012 11:17:49 +0400
Glauber Costa [off-list ref] wrote:

On 06/26/2012 03:27 AM, Andrew Morton wrote:

quoted

On Mon, 25 Jun 2012 18:15:17 +0400
Glauber Costa [off-list ref] wrote:

quoted

What I am proposing with this series is a stripped down version of the
kmem controller for memcg that would allow us to merge significant parts
of the infrastructure, while leaving out, for now, the polemic bits about
the slab while it is being reworked by Cristoph.

Me reasoning for that is that after the last change to introduce a gfp
flag to mark kernel allocations, it became clear to me that tracking other
resources like the stack would then follow extremely naturaly. I figured
that at some point we'd have to solve the issue pointed by David, and avoid
testing the Slab flag in the page allocator, since it would soon be made
more generic. I do that by having the callers to explicit mark it.

So to demonstrate how it would work, I am introducing a stack tracker here,
that is already a functionality per-se: it successfully stops fork bombs to
happen. (Sorry for doing all your work, Frederic =p ). Note that after all
memcg infrastructure is deployed, it becomes very easy to track anything.
The last patch of this series is extremely simple.

The infrastructure is exactly the same we had in memcg, but stripped down
of the slab parts. And because what we have after those patches is a feature
per-se, I think it could be considered for merging.

hm.  None of this new code makes the kernel smaller, faster, easier to
understand or more fun to read!

Not sure if this is a general comment - in case I agree - or if targeted 
to my statement that this is "stripped down". If so, it is of course 
smaller relative to my previous slab accounting patches.

It's a general comment.  The patch adds overhead: runtime costs and
maintenance costs.  Do its benefits justify that cost?

The infrastructure is largely common, but I realized that a future user,
tracking the stack, would be a lot simpler and could be done first.

quoted

Presumably we're getting some benefit for all the downside.  When the
time is appropriate, please do put some time into explaining that
benefit, so that others can agree that it is a worthwhile tradeoff.

Well, for one thing, we stop fork bombs for processes inside cgroups.

"inside cgroups" is a significant limitation!  Is this capability
important enough to justify adding the new code?  That's unobvious
(to me).

Are there any other user-facing things which we can do with this
feature?  Present, or planned?

I can't speak for everybody here, but AFAIK, tracking the stack through
the memory it used, therefore using my proposed kmem controller, was an
idea that good quite a bit of traction with the memcg/memory people. 
So here you have something that people already asked a lot for, in a
shape and interface that seem to be acceptable.

mm, maybe.  Kernel developers tend to look at code from the point of
view "does it work as designed", "is it clean", "is it efficient", "do
I understand it", etc.  We often forget to step back and really
consider whether or not it should be merged at all.

I mean, unless the code is an explicit simplification, we should have
a very strong bias towards "don't merge".