Re: Memory allocation on speculative fastpaths

[Paul E. McKenney]

On Tue, May 03, 2022 at 06:04:13PM +0200, Michal Hocko wrote:
> On Tue 03-05-22 08:59:13, Paul E. McKenney wrote:
> > Hello!
> > 
> > Just following up from off-list discussions yesterday.
> > 
> > The requirements to allocate on an RCU-protected speculative fastpath
> > seem to be as follows:
> > 
> > 1.	Never sleep.
> > 2.	Never reclaim.
> > 3.	Leave emergency pools alone.
> > 
> > Any others?
> > 
> > If those rules suffice, and if my understanding of the GFP flags is
> > correct (ha!!!), then the following GFP flags should cover this:
> > 
> > 	__GFP_NOMEMALLOC | __GFP_NOWARN
> 
> GFP_NOWAIT | __GFP_NOMEMALLOC | __GFP_NOWARN

Ah, good point on GFP_NOWAIT, thank you!

> > Or is this just a fancy way of always returning NULL or some such?  ;-)
> 
> It could fail quite easily. We would also want to guarantee (by
> documenting I guess) that the page allocator never does anything that
> would depend or invoke rcu_synchronize or something like that.

The GPF_NOWAIT should rule out synchronize_rcu() and similar, correct?

> I believe this is the case currently.

Here is hoping!  ;-)

							Thanx, Paul