Re: [RFC-PATCH 2/4] mm: Add __rcu_alloc_page_lockless() func.

[Michal Hocko]

On Fri 18-09-20 21:48:15, Uladzislau Rezki (Sony) wrote:
[...]
> Proposal
> ========
> Introduce a lock-free function that obtain a page from the per-cpu-lists
> on current CPU. It returns NULL rather than acquiring any non-raw spinlock.

I was not happy about this solution when we have discussed this
last time and I have to say I am still not happy. This is exposing
an internal allocator optimization and allows a hard to estimate
consumption of pcp free pages. IIUC this run on pcp cache can be
controled directly from userspace (close(open) loop IIRC) which makes it
even bigger no-no.

I strongly agree with Thomas http://lkml.kernel.org/r/87tux4kefm.fsf@xxxxxxxxxxxxxxxxxxxxxxx
that this optimization is not aiming at reasonable workloads. Really, go
with pre-allocated buffer and fallback to whatever slow path you have
already. Exposing more internals of the allocator is not going to do any
good for long term maintainability.
-- 
Michal Hocko
SUSE Labs