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

David Hildenbrand
2025-05-08T10:45:10Z

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