Re: [PATCH 0/2] mm: memory-failure: fix HWPoison flag race with non-atomic page flag ops

[David Hildenbrand (Arm)]

[...]

> 
> And again, I'm really not sure fixing a theoretical race when memory
> is failing is worth slowing the world by 0.1-1% for.
> 

Fully agreed. I was hoping RCU was cheaper (I mean, we were once told that RCU
read side locking is essentially for free ... well in some configs :) )

The question if we could optimize it reasonably enough ...

> 
> From what I saw in my testing, if we allocate 4k pages
> it's hidden by the overhead. With hp and thp it's measureably
> worse than rcu on !preempt config.

... for example, by doing the rcu read lock + unlock around the

	for (i = 1; i < (1 << order); i++) {

loop on the alloc path. But I suspect it's not going to make that much of a
difference.

I concluded, similar to Andi, that stop_machine() is too big of a hammer.

I wonder if something could be built out of preempt_disable() and sync SMP
calls. hmm :(

-- 
Cheers,

David