linux-kernel - Re: [PATCH v9 3/8] mm,memory_hotplug: Factor out adjusting present pages into adjust_present_page

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Date:   Wed, 21 Apr 2021 10:06:33 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Oscar Salvador <osalvador@...e.de>, Michal Hocko <mhocko@...e.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v9 3/8] mm,memory_hotplug: Factor out adjusting present
 pages into adjust_present_page_count()

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@...hat.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@...hat.com>
>>> Signed-off-by: Oscar Salvador <osalvador@...e.de>
>>> Reviewed-by: Oscar Salvador <osalvador@...e.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@...e.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.

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