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

From: Oscar Salvador <osalvador@suse.de>
To: Michal Hocko <mhocko@suse.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	David Hildenbrand <david@redhat.com>,
	Anshuman Khandual <anshuman.khandual@arm.com>,
	Pavel Tatashin <pasha.tatashin@soleen.com>,
	Vlastimil Babka <vbabka@suse.cz>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v9 3/8] mm,memory_hotplug: Factor out adjusting present pages into adjust_present_page_count()
Date: Wed, 21 Apr 2021 10:00:36 +0200	[thread overview]
Message-ID: <20210421080036.GC22456@linux> (raw)
In-Reply-To: <YH6i09ieDte+xog8@dhcp22.suse.cz>

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@redhat.com>
> > 
> > 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@redhat.com>
> > Signed-off-by: Oscar Salvador <osalvador@suse.de>
> > Reviewed-by: Oscar Salvador <osalvador@suse.de>
> 
> 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@suse.com>
> 
> 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.

-- 
Oscar Salvador
SUSE L3