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Date:   Tue, 12 Jan 2021 12:00:28 +0100
From:   Oscar Salvador <osalvador@...e.de>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        David Hildenbrand <david@...hat.com>, vishal.l.verma@...el.com,
        linux-nvdimm@...ts.01.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/5] mm: Teach pfn_to_online_page() about ZONE_DEVICE
 section collisions

On Tue, Jan 12, 2021 at 01:34:53AM -0800, Dan Williams wrote:
> While pfn_to_online_page() is able to determine pfn_valid() at
> subsection granularity it is not able to reliably determine if a given
> pfn is also online if the section is mixes ZONE_{NORMAL,MOVABLE} with
> ZONE_DEVICE. This means that pfn_to_online_page() may return invalid
> @page objects. For example with a memory map like:
> 
> 100000000-1fbffffff : System RAM
>   142000000-143002e16 : Kernel code
>   143200000-143713fff : Kernel rodata
>   143800000-143b15b7f : Kernel data
>   144227000-144ffffff : Kernel bss
> 1fc000000-2fbffffff : Persistent Memory (legacy)
>   1fc000000-2fbffffff : namespace0.0
> 
> This command:
> 
> echo 0x1fc000000 > /sys/devices/system/memory/soft_offline_page
> 
> ...succeeds when it should fail. When it succeeds it touches
> an uninitialized page and may crash or cause other damage (see
> dissolve_free_huge_page()).

[...]
 
> Because the collision case is rare, and for simplicity, the
> SECTION_TAINT_ZONE_DEVICE flag is never cleared once set.
> 
> Fixes: ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Reported-by: Michal Hocko <mhocko@...e.com>
> Reported-by: David Hildenbrand <david@...hat.com>
> Signed-off-by: Dan Williams <dan.j.williams@...el.com>

Reviewed-by: Oscar Salvador <osalvador@...e.de>

-- 
Oscar Salvador
SUSE L3

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