Re: [PATCH V2] mm/hotplug: Reorder memblock_[free|remove]() calls in try_remove_memory()

[Michal Hocko]

On Wed 25-09-19 08:27:53, Anshuman Khandual wrote:
> Currently during memory hot add procedure, memory gets into memblock before
> calling arch_add_memory() which creates it's linear mapping.
> 
> add_memory_resource() {
> 	..................
> 	memblock_add_node()
> 	..................
> 	arch_add_memory()
> 	..................
> }
> 
> But during memory hot remove procedure, removal from memblock happens first
> before it's linear mapping gets teared down with arch_remove_memory() which
> is not consistent. Resource removal should happen in reverse order as they
> were added. However this does not pose any problem for now, unless there is
> an assumption regarding linear mapping. One example was a subtle failure on
> arm64 platform [1]. Though this has now found a different solution.
> 
> try_remove_memory() {
> 	..................
> 	memblock_free()
> 	memblock_remove()
> 	..................
> 	arch_remove_memory()
> 	..................
> }
> 
> This changes the sequence of resource removal including memblock and linear
> mapping tear down during memory hot remove which will now be the reverse
> order in which they were added during memory hot add. The changed removal
> order looks like the following.
> 
> try_remove_memory() {
> 	..................
> 	arch_remove_memory()
> 	..................
> 	memblock_free()
> 	memblock_remove()
> 	..................
> }
> 
> [1] https://patchwork.kernel.org/patch/11127623/
> 
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Oscar Salvador <osalvador@xxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: David Hildenbrand <david@xxxxxxxxxx>
> Cc: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

> ---
> Changes in V2:
> 
> - Changed the commit message as per Michal and David 
> 
> Changed in V1: https://patchwork.kernel.org/patch/11146361/
> 
> Original patch https://lkml.org/lkml/2019/9/3/327
> 
> Memory hot remove now works on arm64 without this because a recent commit
> 60bb462fc7ad ("drivers/base/node.c: simplify unregister_memory_block_under_nodes()").
> 
> David mentioned that re-ordering should still make sense for consistency
> purpose (removing stuff in the reverse order they were added). This patch
> is now detached from arm64 hot-remove series.
> 
> https://lkml.org/lkml/2019/9/3/326
> 
>  mm/memory_hotplug.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 49f7bf91c25a..4f7d426a84d0 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1763,13 +1763,13 @@ static int __ref try_remove_memory(int nid, u64 start, u64 size)
>  
>  	/* remove memmap entry */
>  	firmware_map_remove(start, start + size, "System RAM");
> -	memblock_free(start, size);
> -	memblock_remove(start, size);
>  
>  	/* remove memory block devices before removing memory */
>  	remove_memory_block_devices(start, size);
>  
>  	arch_remove_memory(nid, start, size, NULL);
> +	memblock_free(start, size);
> +	memblock_remove(start, size);
>  	__release_memory_resource(start, size);
>  
>  	try_offline_node(nid);
> -- 
> 2.20.1

-- 
Michal Hocko
SUSE Labs