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

["David Hildenbrand (Arm)" <david@kernel.org>]

On 6/30/26 08:30, David Hildenbrand (Arm) wrote:
> On 6/29/26 23:50, Michael S. Tsirkin wrote:
>> On Mon, Jun 29, 2026 at 11:22:11PM +0200, David Hildenbrand (Arm) wrote:
>>> [...]
>>>
>>>
>>> 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 ...
>>>
>>>
>>> ... for example, by doing the rcu read lock + unlock around the
>>>
>>> 	for (i = 1; i < (1 << order); i++) {
>>>
>>> loop on the alloc path.
>>
>> Is this different from what this patch is doing?
> 
> Ah, I missed that we batch this already. We could make it include the
> 
> page_cpupid_reset_last(page);
> page->flags.f &= ~PAGE_FLAGS_CHECK_AT_PREP;
> 
> As well, to reduce from 3 to 1 locks.
> 
> So I guess there is potential for optimization.
> 
> [...]
> 
>>
>>> 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 :(
>>
>> rcu_lock is basically same as preempt_disable if rcu is non preemptible,
>> no?
> 
> Yes. See my other mail, I learned that preempt_disable() should likely just do
> for our use case. So the preemptible RCU case would not matter.
> 
> I assume that's as good as it gets.
> 
> 1) Use preempt_disable/preempt_enable to protect
> 2) Batch as good as possible in the page allocator
> 
> If the overhead is then still noticeable, there is not a lot we can do to handle
> this cleanly I'm afraid.

FWIW, there seems to be preempt_enable_no_resched() that is a bit more
lightweight. I'm sure there is some downside, but it might be worth a try.

-- 
Cheers,

David