Re: [PATCH v2 RESEND 3/3] mm: memblock: synchronize debugfs reads with memory hotplug

[Mike Rapoport]

Hi,

On Fri, Aug 21, 2026 at 10:09:10AM +0800, Meijing Zhao wrote:
> From: Meijing Zhao <zhaomeijing@xxxxxxxxxxx>
> 
> memblock_debug_show() walks a memblock region array without
> synchronization. With CONFIG_ARCH_KEEP_MEMBLOCK, memory hotplug can
> concurrently add a region. If the array has to grow,
> memblock_double_array() replaces type->regions and frees the old
> allocation while the debugfs reader may still be using it.
> 
> Hold mem_hotplug_lock in read mode while producing the debugfs output.
> Memory hotplug updates already hold the write side of this lock, so the
> region array remains stable throughout the walk. The helpers are no-ops
> when CONFIG_MEMORY_HOTPLUG is disabled.

Hmm, I'm not sure it's the right lock to take here. With MEMORY_HOTPLUG and
ARCH_KEEP_MEMBLOCK there potentially other races in memblock and we'd
better to design proper locking for them.
 
> Fixes: f9126ab9241f ("memory-hotplug: fix wrong edge when hot add a new node")
> Signed-off-by: Meijing Zhao <zhaomeijing@xxxxxxxxxxx>
> ---
>  mm/memblock.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/mm/memblock.c b/mm/memblock.c
> index 36a8d2a9378d..f1ddbcce47ac 100644
> --- a/mm/memblock.c
> +++ b/mm/memblock.c
> @@ -14,6 +14,7 @@
>  #include <linux/pfn.h>
>  #include <linux/debugfs.h>
>  #include <linux/kmemleak.h>
> +#include <linux/memory_hotplug.h>
>  #include <linux/seq_file.h>
>  #include <linux/memblock.h>
>  #include <linux/mutex.h>
> @@ -2899,6 +2900,7 @@ static int memblock_debug_show(struct seq_file *m, void *private)
>  	phys_addr_t end;
>  	bool first;
>  
> +	get_online_mems();
>  	for (i = 0; i < type->cnt; i++) {
>  		reg = &type->regions[i];
>  		end = reg->base + reg->size - 1;
> @@ -2927,6 +2929,7 @@ static int memblock_debug_show(struct seq_file *m, void *private)
>  			seq_puts(m, "NONE\n");
>  		}
>  	}
> +	put_online_mems();
>  	return 0;
>  }
>  DEFINE_SHOW_ATTRIBUTE(memblock_debug);
> -- 
> 2.25.1
> 

-- 
Sincerely yours,
Mike.