On 9/19/23 06:42, Matteo Rizzo wrote:
> On Mon, 18 Sept 2023 at 19:39, Ingo Molnar <mi...@kernel.org> wrote:
>> What's the split of the increase in overhead due to SLAB_VIRTUAL=y, between
>> user-space execution and kernel-space execution?
> Same benchmark as before (compiling a kernel on a system running the patched
> kernel):

Thanks for running those.  One more situation that comes to mind is how
this will act under memory pressure.  Will some memory pressure make
contention on 'slub_kworker_lock' visible or make the global TLB flushes
less bearable?

In any case, none of this looks _catastrophic_.  It's surely a cost that
some folks will pay.

But I really do think it needs to be more dynamic.  There are a _couple_
of reasons for this.  If it's only a compile-time option, it's never
going to get turned on except for maybe ChromeOS and the datacenter
folks that are paranoid.  I suspect the distros will never turn it on.

A lot of questions get easier if you can disable/enable it at runtime.
For instance, what do you do if the virtual area fills up?  You _could_
just go back to handing out direct map addresses.  Less secure?  Yep.
But better than crashing (for some folks).

It also opens up the door to do this per-slab.  That alone would be a
handy debugging option.

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