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

From: Oscar Salvador <osalvador@suse.de>
To: David Hildenbrand <david@redhat.com>
Cc: Michal Hocko <mhocko@suse.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Anshuman Khandual <anshuman.khandual@arm.com>,
	Vlastimil Babka <vbabka@suse.cz>,
	Pavel Tatashin <pasha.tatashin@soleen.com>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v5 1/5] mm,memory_hotplug: Allocate memmap from the added memory range
Date: Fri, 26 Mar 2021 09:57:43 +0100	[thread overview]
Message-ID: <YF2iBxueewnKIG3V@localhost.localdomain> (raw)
In-Reply-To: <5be95091-b4ac-8e05-4694-ac5c65f790a4@redhat.com>

On Fri, Mar 26, 2021 at 09:52:58AM +0100, David Hildenbrand wrote:
> Might have to set fully spanned section online. (vmemmap >= SECTION_SIZE)

Hi David,

could you elaborate on this a bit?

> Something else to note:
> 
> 
> We'll not call the memory notifier (e.g., MEM_ONLINE) for the vmemmap. The
> result is that
> 
> 1. We won't allocate extended struct pages for the range. Don't think this
> is really problematic (pages are never allocated/freed, so I guess we don't
> care - like ZONE_DEVICE code).
> 
> 2. We won't allocate kasan shadow memory. We most probably have to do it
> explicitly via kasan_add_zero_shadow()/kasan_remove_zero_shadow(), see
> mm/memremap.c:pagemap_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.

I will have a look and see how it goes.

-- 
Oscar Salvador
SUSE L3