Re: [PATCH v9 3/8] mm,memory_hotplug: Factor out adjusting present pages into adjust_present_page_count()

[David Hildenbrand]

On 21.04.21 10:00, Oscar Salvador wrote:
> On Tue, Apr 20, 2021 at 11:45:55AM +0200, Michal Hocko wrote:
> > On Fri 16-04-21 13:24:06, Oscar Salvador wrote:
> > > From: David Hildenbrand <david@xxxxxxxxxx>
> > >
> > > Let's have a single place (inspired by adjust_managed_page_count()) where
> > > we adjust present pages.
> > > In contrast to adjust_managed_page_count(), only memory onlining/offlining
> > > is allowed to modify the number of present pages.
> > >
> > > Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> > > Signed-off-by: Oscar Salvador <osalvador@xxxxxxx>
> > > Reviewed-by: Oscar Salvador <osalvador@xxxxxxx>
> >
> > Not sure self review counts ;)
>
> Uhm, the original author is David, I just added my signed-off-by as a deliverer.
> I thought that in that case was ok to stick my Reviewed-by.
> Or maybe my signed-off-by carries that implicitly.
>
> > Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> >
> > Btw. I strongly suspect the resize lock is quite pointless here.
> > Something for a follow up patch.
>
> What makes you think that?
> I have been thinking about this, let us ignore this patch for a moment.
>
> If I poked the code correctly, node_size_lock is taken in:
>
> remove_pfn_range_from_zone()
> move_pfn_range_to_zone()
>
> both of them handling {zone,node}->spanned_pages
>
> Then we take it in {offline,online}_pages() for {zone,node}->present_pages.
>
> The other places where we take it are __init functions, so not of interest.
>
> Given that {offline,online}_pages() is serialized by the memory_hotplug lock,
> I would say that {node,zone}->{spanned,present}_pages is, at any time, stable?
> So, no need for the lock even without considering this patch?
>
> Now, getting back to this patch.
> adjust_present_page_count() will be called from memory_block_online(), which
> is not holding the memory_hotplug lock yet.
> But, we only fiddle with present pages out of {online,offline}_pages() if
> we have vmemmap pages, and since that operates on the same memory block,
> its lock should serialize that.
>
> I think I went down a rabbit hole, I am slightly confused now.

We always hold the device hotplug lock when onlining/offlining memory.

I agree that the lock might be unnecessary (had the same thoughts a 
while ago), we can look into that in the future.

--
Thanks,

David / dhildenb