Re: Memory hotplug locking issue: Useless (?) zone span seqlock

[David Hildenbrand <david@redhat.com>]

On 07.03.25 21:22, Mathieu Desnoyers wrote:
> I'm currently perfecting my understanding of the mm code and reviewing
> pieces of it as I go, and stumbled on this:
> 
> commit 27cacaad16c5 ("mm,memory_hotplug: drop unneeded locking")
> 
> This commit removes all users of zone_span_writelock(), thus making
> the inline useless, but leaves the now useless
> zone_span_seqbegin()/zone_span_seqretry() in place within
> page_outside_zone_boundaries().
> 
> So I'm confused. What's going on ?
> 
> And if this commit got things very wrong when removing the
> seqlock, I wonder if there are cases where its partial
> pgdat_resize_lock() removal can be an issue as well.

I stumbled over that myself recently as well. I think I mentioned in the 
past that we should just store

start_pfn + end_pfn

instead of

start_pfn + nr_pages


Then, concurrent resizing could happen (and we could atomically read 
start_pfn / end_pfn).

Right now, when adjusting start_pfn, we always also have to adjust 
nr_pages. A concurrent reader calculating end_pfn manually could see 
some crappy result.

Having that said, I am not aware of issues in that area, but it all 
looks like only a partial cleanup to me.

-- 
Cheers,

David / dhildenb