Re: [Xen-devel] [PATCH RFC v1 01/12] mm/memory_hotplug: Don't allow to online/offline memory blocks with holes

[Anshuman Khandual <anshuman.khandual@xxxxxxx>]

On 10/22/2019 10:42 PM, David Hildenbrand wrote:
> Our onlining/offlining code is unnecessarily complicated. Only memory
> blocks added during boot can have holes. Hotplugged memory never has
> holes. That memory is already online.

Why hot plugged memory at runtime cannot have holes (e.g a semi bad DIMM).
Currently, do we just abort adding that memory block if there are holes ?

> 
> When we stop allowing to offline memory blocks with holes, we implicitly
> stop to online memory blocks with holes.

Reducing hotplug support for memory blocks with holes just to simplify
the code. Is it worth ?

> 
> 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.

Could not there be any other way of tracking these holes if not the page
reserved bit. In the memory section itself and corresponding struct pages
just remained poisoned ? Just wondering, might be all wrong here.

> 
> Offlining memory blocks added during boot is usually not guranteed to work
> either way. So stopping to do that (if anybody really used and tested

That guarantee does not exist right now because how boot memory could have
been used after boot not from a limitation of the memory hot remove itself.

> this over the years) should not really hurt. For the use case of
> offlining memory to unplug DIMMs, we should see no change. (holes on
> DIMMs would be weird)

Holes on DIMM could be due to HW errors affecting only parts of it. By not
allowing such DIMM's hot add and remove, we are definitely reducing the
scope of overall hotplug functionality. Is code simplification in itself
is worth this reduction in functionality ?

> 
> 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>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> ---
>  mm/memory_hotplug.c | 26 ++++++++++++++++++++++++--
>  1 file changed, 24 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 561371ead39a..7210f4375279 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1447,10 +1447,19 @@ 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;
> @@ -1461,6 +1470,20 @@ static int __ref __offline_pages(unsigned long 
> start_pfn,
>  
>       mem_hotplug_begin();
>  
> +     /*
> +      * We don't allow to offline memory blocks that contain holes
> +      * and consecuently don't allow to online memory blocks that contain
> +      * holes. This allows to simplify the code quite a lot and we don't
> +      * have to mess with PG_reserved pages for memory holes.
> +      */
> +     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,
> @@ -1472,7 +1495,6 @@ 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,
> 

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