From: Michal Hocko <mhocko@suse.com>
To: Dan Williams <dan.j.williams@intel.com>
Cc: akpm@linux-foundation.org, David Hildenbrand <david@redhat.com>,
Oscar Salvador <osalvador@suse.de>,
linux-mm@kvack.org, linux-nvdimm@lists.01.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4 3/5] mm: Teach pfn_to_online_page() about ZONE_DEVICE section collisions
Date: Wed, 20 Jan 2021 11:30:07 +0100 [thread overview]
Message-ID: <20210120103007.GH9371@dhcp22.suse.cz> (raw)
In-Reply-To: <161058500675.1840162.7887862152161279354.stgit@dwillia2-desk3.amr.corp.intel.com>
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@linux-foundation.org>
> Reported-by: Michal Hocko <mhocko@suse.com>
> Reported-by: David Hildenbrand <david@redhat.com>
> Reviewed-by: David Hildenbrand <david@redhat.com>
> Reviewed-by: Oscar Salvador <osalvador@suse.de>
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Acked-by: Michal Hocko <mhocko@suse.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
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next prev parent reply other threads:[~2021-01-20 10:30 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-14 0:43 [PATCH v4 0/5] mm: Fix pfn_to_online_page() with respect to ZONE_DEVICE Dan Williams
2021-01-14 0:43 ` [PATCH v4 1/5] mm: Move pfn_to_online_page() out of line Dan Williams
2021-01-20 10:11 ` Michal Hocko
2021-01-14 0:43 ` [PATCH v4 2/5] mm: Teach pfn_to_online_page() to consider subsection validity Dan Williams
2021-01-20 10:24 ` Michal Hocko
2021-01-14 0:43 ` [PATCH v4 3/5] mm: Teach pfn_to_online_page() about ZONE_DEVICE section collisions Dan Williams
2021-01-20 10:30 ` Michal Hocko [this message]
2021-01-14 0:43 ` [PATCH v4 4/5] mm: Fix page reference leak in soft_offline_page() Dan Williams
2021-01-14 1:49 ` HORIGUCHI NAOYA(堀口 直也)
2021-01-14 6:18 ` Dan Williams
2021-01-14 6:30 ` Oscar Salvador
2021-01-14 7:10 ` HORIGUCHI NAOYA(堀口 直也)
2021-01-17 22:01 ` Andrew Morton
2021-01-17 22:35 ` Dan Williams
2021-01-14 0:43 ` [PATCH v4 5/5] mm: Fix memory_failure() handling of dax-namespace metadata Dan Williams
2021-01-14 2:25 ` HORIGUCHI NAOYA(堀口 直也)
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