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

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Jun 29, 2026 at 11:22:11PM +0200, David Hildenbrand (Arm) wrote:
> [...]
> 
> > 
> > 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.

Is this different from what this patch is doing?

        if (unlikely(order)) {
                int i;

                hwpoison_rcu_lock();
                if (compound) {
                        page[1].flags.f &= ~PAGE_FLAGS_SECOND;
  #ifdef NR_PAGES_IN_LARGE_FOLIO
                        folio->_nr_pages = 0;
  #endif
                }
                for (i = 1; i < (1 << order); i++) {
                        if (compound)
                                bad += free_tail_page_prepare(page, page + i);
                        if (is_check_pages_enabled()) {
                                if (free_page_is_bad(page + i)) {
                                        bad++;
                                        continue;
                                }
                        }
                        (page + i)->flags.f &= ~PAGE_FLAGS_CHECK_AT_PREP;
                }
                hwpoison_rcu_unlock();
        }
  


> But I suspect it's not going to make that much of a
> difference.

It makes a difference! Just not enough to make it completely unmeasureable.


> 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?


> -- 
> Cheers,
> 
> David