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

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Date:   Thu, 7 Feb 2019 14:36:24 +0100
From:   Oscar Salvador <osalvador@...e.de>
To:     Robin Murphy <robin.murphy@....com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        gregkh@...uxfoundation.org, rafael@...nel.org, mhocko@...nel.org,
        akpm@...ux-foundation.org
Subject: Re: [PATCH] mm/memory-hotplug: Add sysfs hot-remove trigger

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@....com>
> ---
> 
> 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] https://lore.kernel.org/lkml/22d34fe30df0fbacbfceeb47e20cb1184af73585.1511433386.git.ar@linux.vnet.ibm.com/
> 

> +	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:
device_del::device_remove_attrs::device_remove_groups::sysfs_remove_groups
would be enough to remove the dev attributes.
I guess in this case that is not enough, could you explain why?


[1] https://patchwork.kernel.org/patch/10775339/
-- 
Oscar Salvador
SUSE L3