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

[Paul E. McKenney]

On Thu, Sep 24, 2020 at 01:19:07PM +0200, Peter Zijlstra wrote:
> On Thu, Sep 24, 2020 at 10:16:14AM +0200, Uladzislau Rezki wrote:
> > The key point is "enough". We need pages to make a) fast progress b) support
> > single argument of kvfree_rcu(one_arg). Not vice versa. That "enough" depends
> > on scheduler latency and vague pre-allocated number of pages, it might
> > be not enough what would require to refill it more and more or we can overshoot
> > that would lead to memory overhead. So we have here timing issues and
> > not accurate model. IMHO.
> 
> I'm firmly opposed to the single argument kvfree_rcu() idea, that's
> requiring memory to free memory.

Not quite.

First, there is a fallback when memory allocation fails.  Second,
in heavy-use situations, there is only one allocation per about
500 kvfree_rcu() calls on 64-bit systems.  Third, there are other
long-standing situations that require allocating memory in order to
free memory.

So I agree that it is a good general rule of thumb to avoid allocating
on free paths, but there are exceptions.  This is one of them.

						Thanx, Paul