linux-kernel - Re: [PATCH v5 1/5] mm,memory_hotplug: Allocate memmap from the added memory range

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Message-ID: <c11ef594-fe9b-cacf-726f-03f00c62c294@redhat.com>
Date:   Fri, 26 Mar 2021 14:36:26 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Oscar Salvador <osalvador@...e.de>
Cc:     Michal Hocko <mhocko@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Pavel Tatashin <pasha.tatashin@...een.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 1/5] mm,memory_hotplug: Allocate memmap from the added
 memory range

>>>
>>> Further a locking rework might be necessary. We hold the device hotplug
>>> lock, but not the memory hotplug lock. E.g., for get_online_mems(). Might
>>> have to move that out online_pages.
> 
> That is a good point.
> I yet have to think about it further, but what about moving those
> mem_hotplug_{begin,done} to memory_block_{offline,online}.
> 
> Something like:
> 
>   static int memory_block_online(...)
>   {
>           int ret;
>   
>           mem_hotplug_begin();
>   
>           if (nr_vmemmap_pages)
>                   vmemmap_hotplug_init();
>   
>           ret = online_pages(...);
>           if (ret)
>   	/*
>   	 * Cleanup stuff
>   	 */
>   
>           mem_hotplug_done();
>   }
> 	
> As you said, you finished cleaning up those users who were calling
> {online,offline}_pages() directly, but is this something that we will
> forbidden in the future too?

Well, I cannot tell what will happen in the future. But at least as long 
as we have memory blocks, I doubt that there are valid use cases for 
online_pages()/offline_pages(). Especially once we have things like 
memmap_on_memory that need special care.

> My question falls within "Are we sure we will not need that locking back
> in those functions because that is something we will not allow to
> happen?"

Who has access to online_pages()/offline_pages() also has access to 
mem_hotplug_begin()/mem_hotplug_done(). It would be nice if we could 
only use online_pages()/offline_pages() internally -- or at least add a 
comment that this is for internal purposes only / requires that locking.

-- 
Thanks,

David / dhildenb