From: David Hildenbrand <david@redhat.com>
To: Anshuman Khandual <anshuman.khandual@arm.com>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
linux-arm-kernel@lists.infradead.org, akpm@linux-foundation.org,
catalin.marinas@arm.com, will@kernel.org
Cc: mark.rutland@arm.com, cai@lca.pw, logang@deltatee.com,
cpandya@codeaurora.org, arunks@codeaurora.org,
dan.j.williams@intel.com, mgorman@techsingularity.net,
osalvador@suse.de, ard.biesheuvel@arm.com, steve.capper@arm.com,
broonie@kernel.org, valentin.schneider@arm.com,
Robin.Murphy@arm.com, steven.price@arm.com,
suzuki.poulose@arm.com, ira.weiny@intel.com
Subject: Re: [PATCH V11 1/5] mm/hotplug: Introduce arch callback validating the hot remove range
Date: Fri, 10 Jan 2020 09:42:21 +0100 [thread overview]
Message-ID: <80ab5f55-77ef-4719-52fc-2b23c0ecb866@redhat.com> (raw)
In-Reply-To: <1578625755-11792-2-git-send-email-anshuman.khandual@arm.com>
On 10.01.20 04:09, Anshuman Khandual wrote:
> Currently there are two interfaces to initiate memory range hot removal i.e
> remove_memory() and __remove_memory() which then calls try_remove_memory().
> Platform gets called with arch_remove_memory() to tear down required kernel
> page tables and other arch specific procedures. But there are platforms
> like arm64 which might want to prevent removal of certain specific memory
> ranges irrespective of their present usage or movability properties.
Why? Is this only relevant for boot memory? I hope so, otherwise the
arch code needs fixing IMHO.
If it's only boot memory, we should disallow offlining instead via a
memory notifier - much cleaner.
>
> Current arch call back arch_remove_memory() is too late in the process to
> abort memory hot removal as memory block devices and firmware memory map
> entries would have already been removed. Platforms should be able to abort
> the process before taking the mem_hotplug_lock with mem_hotplug_begin().
> This essentially requires a new arch callback for memory range validation.
I somewhat dislike this very much. Memory removal should never fail if
used sanely. See e.g., __remove_memory(), it will BUG() whenever
something like that would strike.
>
> This differentiates memory range validation between memory hot add and hot
> remove paths before carving out a new helper check_hotremove_memory_range()
> which incorporates a new arch callback. This call back provides platforms
> an opportunity to refuse memory removal at the very onset. In future the
> same principle can be extended for memory hot add path if required.
>
> Platforms can choose to override this callback in order to reject specific
> memory ranges from removal or can just fallback to a default implementation
> which allows removal of all memory ranges.
I suspect we want really want to disallow offlining instead. E.g., I
remember s390x does that with certain areas needed for dumping/kexec.
Somebody who added memory via add_memory() should always be able to
remove the memory via remove_memory() again. Only boot memory can be
treated in a special way, but boot memory is initially always online.
--
Thanks,
David / dhildenb
next prev parent reply other threads:[~2020-01-10 8:42 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-10 3:09 [PATCH V11 0/5] arm64/mm: Enable memory hot remove Anshuman Khandual
2020-01-10 3:09 ` [PATCH V11 1/5] mm/hotplug: Introduce arch callback validating the hot remove range Anshuman Khandual
2020-01-10 8:42 ` David Hildenbrand [this message]
2020-01-13 9:11 ` Anshuman Khandual
2020-01-13 9:14 ` David Hildenbrand
2020-01-13 9:50 ` Anshuman Khandual
2020-01-13 10:37 ` David Hildenbrand
2020-01-14 2:13 ` Anshuman Khandual
2020-01-14 11:09 ` Anshuman Khandual
2020-01-14 12:30 ` David Hildenbrand
2020-01-11 14:11 ` kbuild test robot
2020-01-13 4:06 ` Anshuman Khandual
2020-01-11 19:49 ` kbuild test robot
2020-01-10 3:09 ` [PATCH V11 2/5] mm/memblock: Introduce MEMBLOCK_BOOT flag Anshuman Khandual
2020-01-13 7:37 ` Mike Rapoport
2020-01-13 8:43 ` Anshuman Khandual
2020-01-13 8:57 ` David Hildenbrand
2020-01-10 3:09 ` [PATCH V11 3/5] of/fdt: Mark boot memory with MEMBLOCK_BOOT Anshuman Khandual
2020-01-10 3:09 ` [PATCH V11 4/5] arm64/mm: Hold memory hotplug lock while walking for kernel page table dump Anshuman Khandual
2020-01-10 3:09 ` [PATCH V11 5/5] arm64/mm: Enable memory hot remove Anshuman Khandual
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