Hello Sudarshan,
On 10/17/2020 07:32 AM, Sudarshan Rajagopalan wrote:
> In the patch that enables memory hot-remove (commit bbd6ec605c0f ("arm64/mm:
> Enable memory hot remove")) for arm64, there’s a notifier put in place that
> prevents boot memory from being offlined and removed. The commit text
> mentions that boot memory on arm64 cannot be removed. But x86 and other archs
> doesn’t seem to do this prevention.
>
> The current logic is that only “new” memory blocks which are hot-added can
> later be offlined and removed. The memory that system booted up with cannot
> be offlined and removed. But there could be many usercases such as inter-VM
> memory sharing where a primary VM could offline and hot-remove a
> block/section of memory and lend it to secondary VM where it could hot-add
> it. And after usecase is done, the reverse happens where secondary VM
> hot-removes and gives it back to primary which can hot-add it back. In such
> cases, the present logic for arm64 doesn’t allow this hot-remove in primary
> to happen.
>
> Also, on systems with movable zone that sort of guarantees pages to be
> migrated and isolated so that blocks can be offlined, this logic also defeats
> the purpose of having a movable zone which system can rely on memory
> hot-plugging, which say virt-io mem also relies on for fully plugged memory
> blocks.
>
> This patch tries to solve by introducing a new section mem map sit
> 'SECTION_MARK_HOTPLUGGABLE' which allows the concerned module drivers be able
> to mark requried sections as "hotpluggable" by setting this bit. Also this
> marking is only allowed for sections which are in movable zone and have
> unmovable pages. The arm64 mmu code on receiving the MEM_GOING_OFFLINE
> notification, we disallow offlining of any boot memory by checking if
> section_early or not. With the introduction of SECTION_MARK_HOTPLUGGABLE, we
> allow boot mem sections that are marked as hotpluggable with this bit set to
> be offlined and removed. Thereby allowing required bootmem sections to be
> offlinable.
This series was posted right after another thread you initiated in this regard
but without even waiting for it to conclude in any manner.
https://lore.kernel.org/linux-arm-kernel/de8388df2fbc5a6a33aab95831ba7...@codeaurora.org/
Inter-VM memory migration could be solved in other methods as David has
mentioned.
Boot memory cannot be removed and hence offlined on arm64 due to multiple
reasons
including making kexec non-functional afterwards. Besides these intrusive core
MM
changes are not really required.
- Anshuman
