Re: [PATCH] mm/memory-hotplug: Add sysfs hot-remove trigger

From: Oscar Salvador
Date: Thu Feb 07 2019 - 08:36:31 EST

On Wed, Feb 06, 2019 at 05:03:53PM +0000, Robin Murphy wrote:
> ARCH_MEMORY_PROBE is a useful thing for testing and debugging hotplug,
> but being able to exercise the (arguably trickier) hot-remove path would
> be even more useful. Extend the feature to allow removal of offline
> sections to be triggered manually to aid development.
> Signed-off-by: Robin Murphy <robin.murphy@xxxxxxx>
> ---
> This is inspired by a previous proposal[1], but in coming up with a
> more robust interface I ended up rewriting the whole thing from
> scratch. The lack of documentation is semi-deliberate, since I don't
> like the idea of anyone actually relying on this interface as ABI, but
> as a handy tool it felt useful enough to be worth sharing :)

Hi Robin,

I think this might come in handy, especially when trying to test hot-remove
on arch's that do not have any means to hot-remove memory, or even on virtual
platforms that do not have yet support for hot-remove depending on the platform,
like qemu/arm64.

I could have used this while testing hot-remove on other archs for [1]

> Robin.
> [1]

> + if (mem->state != MEM_OFFLINE)
> + return -EBUSY;

We do have the helper "is_memblock_offlined()", although it is only used in one place now.
So, I would rather use it here as well.

> +
> + ret = lock_device_hotplug_sysfs();
> + if (ret)
> + return ret;
> +
> + if (device_remove_file_self(dev, attr)) {
> + __remove_memory(pfn_to_nid(start_pfn), PFN_PHYS(start_pfn),
> + MIN_MEMORY_BLOCK_SIZE * sections_per_block);

Sorry, I am not into sysfs inners, but I thought that:
would be enough to remove the dev attributes.
I guess in this case that is not enough, could you explain why?

Oscar Salvador