linux-kernel - Re: [PATCH v4 1/5] mm,memory_hotplug: Allocate memmap from the added memory range

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Message-ID: <YFMPBFSJPq2VEOk9@localhost.localdomain>
Date:   Thu, 18 Mar 2021 09:27:48 +0100
From:   Oscar Salvador <osalvador@...e.de>
To:     David Hildenbrand <david@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.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 v4 1/5] mm,memory_hotplug: Allocate memmap from the added
 memory range

On Wed, Mar 17, 2021 at 03:35:41PM +0100, David Hildenbrand wrote:
> Just assume you have two ranges
> 
> [ ZONE_DEVICE 0 ][ ZONE_DEVICE 1]
> 
> If the vmemmap of ZONE_DEVICE 1 could be taken from the altmap of
> ZONE_DEVICE 0, we could be in trouble, as both parts can be
> removed/repurposed independently ...

I have to say my knowledge about ZONE_DEVICE and its intrinsencs tend to
0, that is why I thought it might not matter, but I agree that this is
only asking for trouble.

> If we check for
> 
> IS_ALIGNED(nr_vmemmap_pages, PMD_SIZE), please add a proper TODO comment
> that this is most probably the wrong place to take care of this.

Sure, I will stuff the check in there and place a big TODO comment so we
do not forget about addressing this issue the right way.

I will prepare a v5 (hopefully the last one) and do some more testing
before sending it out.

Thanks David!

-- 
Oscar Salvador
SUSE L3