From: David Hildenbrand <dhildenb@redhat.com>
To: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: David Hildenbrand <david@redhat.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, 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: Mon, 13 Jan 2020 04:14:30 -0500 (EST) [thread overview]
Message-ID: <3C3BE5FA-0CFC-4C90-8657-63EF5B680B0B@redhat.com> (raw)
In-Reply-To: <6f0efddc-f124-58ca-28b6-4632469cf992@arm.com>
> Am 13.01.2020 um 10:10 schrieb Anshuman Khandual <anshuman.khandual@arm.com>:
>
>
>
>> On 01/10/2020 02:12 PM, David Hildenbrand wrote:
>>> 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.
>
> Right, it is relevant only for the boot memory on arm64 platform. But this
> new arch callback makes it flexible to reject any given memory range.
>
>>
>> If it's only boot memory, we should disallow offlining instead via a
>> memory notifier - much cleaner.
>
> Dont have much detail understanding of MMU notifier mechanism but from some
> initial reading, it seems like we need to have a mm_struct for a notifier
> to monitor various events on the page table. Just wondering how a physical
> memory range like boot memory can be monitored because it can be used both
> for for kernel (init_mm) or user space process at same time. Is there some
> mechanism we could do this ?
>
>>
>>>
>>> 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
>
> If boot memory pages can be prevented from being offlined for sure, then it
> would indirectly definitely prevent hot remove process as well.
>
>> remember s390x does that with certain areas needed for dumping/kexec.
>
> Could not find any references to mmu_notifier in arch/s390 or any other arch
> for that matter apart from KVM (which has an user space component), could you
> please give some pointers ?
Memory (hotplug) notifier, not MMU notifier :)
Not on my notebook right now, grep for MEM_GOING_OFFLINE, that should be it.
next prev parent reply other threads:[~2020-01-13 9:14 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
2020-01-13 9:11 ` Anshuman Khandual
2020-01-13 9:14 ` David Hildenbrand [this message]
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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