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

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Message-ID: <YF3x8BW1+2o50mds@dhcp22.suse.cz>
Date:   Fri, 26 Mar 2021 15:38:40 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Oscar Salvador <osalvador@...e.de>,
        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

On Fri 26-03-21 09:52:58, David Hildenbrand wrote:
[...]
> 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).

Agreed. I do not think we need them. Future might disagree but let's
handle it when we have a clear demand.

> 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()

I think this is similar to the above. Does kasan has to know about
memory which will never be used for anything?

> 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.

Could you be more explicit why this locking is needed? What it would
protect from for vmemmap pages?
-- 
Michal Hocko
SUSE Labs