linux-kernel - Re: [PATCH] mm/memory_hotplug.c: don't fail hot unplug quite so eagerly

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Date:   Wed, 21 Jun 2023 19:22:24 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     David Hildenbrand <david@...hat.com>,
        Oscar Salvador <osalvador@...e.de>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>
Subject: Re: [PATCH] mm/memory_hotplug.c: don't fail hot unplug quite so
 eagerly

On 6/21/23 01:11, David Hildenbrand wrote:
>> ...what about discerning between "user initiated offline_pages" and
>> "offline pages as part of a driver shutdown/unload"?
> 
> Makes sense to me.
> 
> There are two ways for triggering it directly from user space:
> 
> 1) drivers/base/core.c:online_store()
> 2) drivers/base/memory.c:state_store()
> 
> We cannot easily hook into 2) to indicate "we're offlining directly
> from user space". SO we might have to do it the other way around.
> 
> 
> Something along the following lines should do the trick (expect whitespace damage):
> 
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 53ee7654f009..acd4b739505a 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -152,6 +152,13 @@ void put_online_mems(void)
> 
>   bool movable_node_enabled = false;
> 
> +/*
> + * Protected by the device hotplug lock. Indicates whether device offlining
> + * is triggered from try_offline_memory_block() such that we don't fail memory
> + * offlining if a signal is pending.
> + */
> +static bool mhp_in_try_offline_memory_block;
> +
>   #ifndef CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE
>   int mhp_default_online_type = MMOP_OFFLINE;
>   #else
> @@ -1860,7 +1867,8 @@ int __ref offline_pages(unsigned long start_pfn, unsigned long nr_pages,
>          do {
>                  pfn = start_pfn;
>                  do {
> -                       if (signal_pending(current)) {
> +                       if (!mhp_in_try_offline_memory_block &&
> +                           signal_pending(current)) {
>                                  ret = -EINTR;
>                                  reason = "signal backoff";
>                                  goto failed_removal_isolated;
> @@ -2177,7 +2185,9 @@ static int try_offline_memory_block(struct memory_block *mem, void *arg)
>          if (page && zone_idx(page_zone(page)) == ZONE_MOVABLE)
>                  online_type = MMOP_ONLINE_MOVABLE;
> 
> +       mhp_in_try_offline_memory_block = true;
>          rc = device_offline(&mem->dev);
> +       mhp_in_try_offline_memory_block = false;
>          /*
>           * Default is MMOP_OFFLINE - change it only if offlining succeeded,
>           * so try_reonline_memory_block() can do the right thing.
> 
> 
> 
> There is still arch/powerpc/platforms/pseries/hotplug-memory.c that calls
> device_offline() and would fail on signals (not sure if relevant, like for virtio-mem it
> shouldn't be that relevant).
> 
> I guess dlpar_remove_lmb() can now simply call offline_and_remove_memory().
> [I might craft a patch later]
> 

This direction looks good to me, I'd love to see a patch if you
put something together.


thanks,
-- 
John Hubbard
NVIDIA