linux-kernel - Re: [PATCH 0/4] mm,memory_hotplug: allocate memmap from hotadded memory

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Message-ID: <20190403104602.GJ15605@dhcp22.suse.cz>
Date:   Wed, 3 Apr 2019 12:46:02 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Oscar Salvador <osalvador@...e.de>
Cc:     David Hildenbrand <david@...hat.com>, akpm@...ux-foundation.org,
        dan.j.williams@...el.com, Jonathan.Cameron@...wei.com,
        anshuman.khandual@....com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH 0/4] mm,memory_hotplug: allocate memmap from hotadded
 memory

On Wed 03-04-19 11:40:54, Oscar Salvador wrote:
> On Wed, Apr 03, 2019 at 10:37:57AM +0200, Michal Hocko wrote:
> > That being said it should be the caller of the hotplug code to tell
> > the vmemmap allocation strategy. For starter, I would only pack vmemmaps
> > for "regular" kernel zone memory. Movable zones should be more careful.
> > We can always re-evaluate later when there is a strong demand for huge
> > pages on movable zones but this is not the case now because those pages
> > are not really movable in practice.
> 
> I agree that makes sense to let the caller specify if it wants to allocate
> vmemmaps per memblock or per memory-range, so we are more flexible when it
> comes to granularity in hot-add/hot-remove operations.

And just to be more specific. The api shouldn't really care about this
implementation detail. So ideally the caller of add_pages just picks up
the proper allocator and the rest is completely transparent.

> But the thing is that the zones are picked at onling stage, while
> vmemmaps are created at hot-add stage, so I am not sure we can define
> the strategy depending on the zone.

THis is a good point. We do have means to tell a default zone for the
regular memory hotplug so this can be reused based on the pfn range
(default_zone_for_pfn).
-- 
Michal Hocko
SUSE Labs