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SubjectRe: [PATCH V2] mm/hotplug: Reorder memblock_[free|remove]() calls in try_remove_memory()
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@linux-foundation.org>
> Cc: Oscar Salvador <osalvador@suse.de>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Pavel Tatashin <pasha.tatashin@soleen.com>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>

Acked-by: Michal Hocko <mhocko@suse.com>

> ---
> 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

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