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
