linux-kernel - Re: [PATCH v1 1/2] mm/page_alloc.c: Don't set pages PageReserved() when offlining

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Message-ID: <20191021154712.GW9379@dhcp22.suse.cz>
Date:   Mon, 21 Oct 2019 17:47:12 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Oscar Salvador <osalvador@...e.de>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Wei Yang <richard.weiyang@...il.com>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Pavel Tatashin <pavel.tatashin@...rosoft.com>
Subject: Re: [PATCH v1 1/2] mm/page_alloc.c: Don't set pages PageReserved()
 when offlining

On Mon 21-10-19 17:39:36, David Hildenbrand wrote:
> On 21.10.19 16:43, Michal Hocko wrote:
[...]
> > We still set PageReserved before onlining pages and that one should be
> > good to go as well (memmap_init_zone).
> > Thanks!
> 
> memmap_init_zone() is called when onlining memory. There, set all pages to
> reserved right now (on context == MEMMAP_HOTPLUG). We clear PG_reserved when
> onlining a page to the buddy (e.g., generic_online_page). If we would online
> a memory block with holes, we would want to keep all such pages
> (!pfn_valid()) set to reserved. Also, there might be other side effects.

Isn't it sufficient to have those pages in a poisoned state? They are
not onlined so their state is basically undefined anyway. I do not see
how PageReserved makes this any better.

Also is the hole inside a hotplugable memory something we really have to
care about. Has anybody actually seen a platform to require that?
-- 
Michal Hocko
SUSE Labs