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

[Damian Tometzki]

On Tue, 19. Nov 12:52, 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
> 
> 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>
> ---
> 
> 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;
> +}
> +
Hello David,

what is the meaning of the function ? The return is every time 0. 

I dont understand it ?

Best regards
Damian

>  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
> 
>