linux-kernel - Re: [PATCH v4 3/5] mm: Teach pfn_to_online_page() about ZONE

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Message-ID: <20210120103007.GH9371@dhcp22.suse.cz>
Date:   Wed, 20 Jan 2021 11:30:07 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     akpm@...ux-foundation.org, David Hildenbrand <david@...hat.com>,
        Oscar Salvador <osalvador@...e.de>, linux-mm@...ck.org,
        linux-nvdimm@...ts.01.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 3/5] mm: Teach pfn_to_online_page() about ZONE_DEVICE
 section collisions

On Wed 13-01-21 16:43:26, 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()).
> 
> While the memory map above is contrived via the memmap=ss!nn kernel
> command line option, the collision happens in practice on shipping
> platforms. The memory controller resources that decode spans of
> physical address space are a limited resource. One technique
> platform-firmware uses to conserve those resources is to share a decoder
> across 2 devices to keep the address range contiguous. Unfortunately the
> unit of operation of a decoder is 64MiB while the Linux section size is
> 128MiB. This results in situations where, without subsection hotplug
> memory mappings with different lifetimes collide into one object that
> can only express one lifetime.

Thank you this is a very useful insight to have in the changelog.

> Update move_pfn_range_to_zone() to flag (SECTION_TAINT_ZONE_DEVICE) a
> section that mixes ZONE_DEVICE pfns with other online pfns. With
> SECTION_TAINT_ZONE_DEVICE to delineate, pfn_to_online_page() can fall
> back to a slow-path check for ZONE_DEVICE pfns in an online section. In
> the fast path online_section() for a full ZONE_DEVICE section returns
> false.
> 
> 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>
> Reviewed-by: David Hildenbrand <david@...hat.com>
> Reviewed-by: Oscar Salvador <osalvador@...e.de>
> Signed-off-by: Dan Williams <dan.j.williams@...el.com>

Acked-by: Michal Hocko <mhocko@...e.com>

I do not want to bikeshed but online_device_section is quite confusing.
device_mixed_section would sound like a better name to me.
-- 
Michal Hocko
SUSE Labs