Re: [PATCH v2] mm/memory_hotplug: Don't allow to online/offline memory blocks with holes

[Michal Hocko]

On Tue 19-11-19 12:52:37, David Hildenbrand wrote:
> Our onlining/offlining code is unnecessarily complicated. Only memory
> blocks added during boot can have holes (a range that is not
> IORESOURCE_SYSTEM_RAM). Hotplugged memory never has holes (e.g., see
> add_memory_resource()). All memory blocks that belong to boot memory are
> already online.
> 
> Note that boot memory can have holes and the memmap of the holes is marked
> PG_reserved. However, also memory allocated early during boot is
> PG_reserved - basically every page of boot memory that is not given to the
> buddy is PG_reserved.
> 
> Therefore, when we stop allowing to offline memory blocks with holes, we
> implicitly no longer have to deal with onlining memory blocks with holes.
> E.g., online_pages() will do a
> walk_system_ram_range(..., online_pages_range), whereby
> online_pages_range() will effectively only free the memory holes not
> falling into a hole to the buddy. The other pages (holes) are kept
> PG_reserved (via move_pfn_range_to_zone()->memmap_init_zone()).
> 
> This allows to simplify the code. For example, we no longer have to
> worry about marking pages that fall into memory holes PG_reserved when
> onlining memory. We can stop setting pages PG_reserved completely in
> memmap_init_zone().
> 
> Offlining memory blocks added during boot is usually not guaranteed to work
> either way (unmovable data might have easily ended up on that memory during
> boot). So stopping to do that should not really hurt. Also, people are not
> even aware of a setup where onlining/offlining of memory blocks with
> holes used to work reliably (see [1] and [2] especially regarding the
> hotplug path) - I doubt it worked reliably.
> 
> For the use case of offlining memory to unplug DIMMs, we should see no
> change. (holes on DIMMs would be weird).
> 
> Please note that hardware errors (PG_hwpoison) are not memory holes and
> are not affected by this change when offlining.
> 
> [1] https://lkml.org/lkml/2019/10/22/135
> [2] https://lkml.org/lkml/2019/8/14/1365

Please do not use lkml.org links, they tend to break longterm. Use
http://lkml.kernel.org/r/$msg_id instead.

> Reviewed-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Oscar Salvador <osalvador@xxxxxxx>
> Cc: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> Cc: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>

yes this looks sensible. We already do restrict offlining memry blocks
which span multiple zones (e.g. when NUMA nodes are interleaved through
a memblock boundary) so this is not the first restriction like that.
If that allows future changes then let's just try it out and see whether
there are real usecases that needs to handle boottime memory with holes
hotremove.

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

> ---
> 
> This patch was part of:
> 	[PATCH v1 00/10] mm: Don't mark hotplugged pages PG_reserved
> 	(including ZONE_DEVICE)
> 	-> https://www.spinics.net/lists/linux-driver-devel/msg130042.html
> 
> However, before we can perform the PG_reserved changes, we have to fix
> pfn_to_online_page() in special scenarios first (bootmem and devmem falling
> into a single section). Dan is working on that.
> 
> I propose to give this patch a churn in -next so we can identify if this
> change would break any existing setup. I will then follow up with cleanups
> and the PG_reserved changes later.
> 
> ---
>  mm/memory_hotplug.c | 28 ++++++++++++++++++++++++++--
>  1 file changed, 26 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 46b2e056a43f..fc617ad6f035 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1455,10 +1455,19 @@ static void node_states_clear_node(int node, struct memory_notify *arg)
>  		node_clear_state(node, N_MEMORY);
>  }
>  
> +static int count_system_ram_pages_cb(unsigned long start_pfn,
> +				     unsigned long nr_pages, void *data)
> +{
> +	unsigned long *nr_system_ram_pages = data;
> +
> +	*nr_system_ram_pages += nr_pages;
> +	return 0;
> +}
> +
>  static int __ref __offline_pages(unsigned long start_pfn,
>  		  unsigned long end_pfn)
>  {
> -	unsigned long pfn, nr_pages;
> +	unsigned long pfn, nr_pages = 0;
>  	unsigned long offlined_pages = 0;
>  	int ret, node, nr_isolate_pageblock;
>  	unsigned long flags;
> @@ -1469,6 +1478,22 @@ static int __ref __offline_pages(unsigned long start_pfn,
>  
>  	mem_hotplug_begin();
>  
> +	/*
> +	 * Don't allow to offline memory blocks that contain holes.
> +	 * Consequently, memory blocks with holes can never get onlined
> +	 * via the hotplug path - online_pages() - as hotplugged memory has
> +	 * no holes. This way, we e.g., don't have to worry about marking
> +	 * memory holes PG_reserved, don't need pfn_valid() checks, and can
> +	 * avoid using walk_system_ram_range() later.
> +	 */
> +	walk_system_ram_range(start_pfn, end_pfn - start_pfn, &nr_pages,
> +			      count_system_ram_pages_cb);
> +	if (nr_pages != end_pfn - start_pfn) {
> +		ret = -EINVAL;
> +		reason = "memory holes";
> +		goto failed_removal;
> +	}
> +
>  	/* This makes hotplug much easier...and readable.
>  	   we assume this for now. .*/
>  	if (!test_pages_in_a_zone(start_pfn, end_pfn, &valid_start,
> @@ -1480,7 +1505,6 @@ static int __ref __offline_pages(unsigned long start_pfn,
>  
>  	zone = page_zone(pfn_to_page(valid_start));
>  	node = zone_to_nid(zone);
> -	nr_pages = end_pfn - start_pfn;
>  
>  	/* set above range as isolated */
>  	ret = start_isolate_page_range(start_pfn, end_pfn,
> -- 
> 2.21.0

-- 
Michal Hocko
SUSE Labs