Re: [PATCH v5 v5 2/6] mm/memory_hotplug: Fix incorrect altmap passing in error path

[Muchun Song <muchun.song@linux.dev>]

> On Apr 23, 2026, at 18:38, David Hildenbrand (Arm) <david@kernel.org> wrote:
> 
> On 4/23/26 09:19, Muchun Song wrote:
>> In create_altmaps_and_memory_blocks(), when arch_add_memory() succeeds
>> with memmap_on_memory enabled, the vmemmap pages are allocated from
>> params.altmap. If create_memory_block_devices() subsequently fails, the
>> error path calls arch_remove_memory() with a NULL altmap instead of
>> params.altmap.
>> 
>> This is a bug that could lead to memory corruption. Since altmap is
>> NULL, vmemmap_free() falls back to freeing the vmemmap pages into the
>> system buddy allocator via free_pages() instead of the altmap.
>> arch_remove_memory() then immediately destroys the physical linear
>> mapping for this memory. This injects unowned pages into the buddy
>> allocator, causing machine checks or memory corruption if the system
>> later attempts to allocate and use those freed pages.
>> 
>> Fix this by passing params.altmap to arch_remove_memory() in the error
>> path.
>> 
>> Fixes: 6b8f0798b85a ("mm/memory_hotplug: split memmap_on_memory requests across memblocks")
>> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>> ---
>> mm/memory_hotplug.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>> index 2a943ec57c85..0bad2aed2bde 100644
>> --- a/mm/memory_hotplug.c
>> +++ b/mm/memory_hotplug.c
>> @@ -1468,7 +1468,7 @@ static int create_altmaps_and_memory_blocks(int nid, struct memory_group *group,
>> 	ret = create_memory_block_devices(cur_start, memblock_size, nid,
>>  					params.altmap, group);
>> 	if (ret) {
>> - 		arch_remove_memory(cur_start, memblock_size, NULL);
>> + 		arch_remove_memory(cur_start, memblock_size, params.altmap);
>> 		kfree(params.altmap);
>> 		goto out;
>> 	}
> 
> Yeah, that's nasty. We should CC stable.

Make sense.

> 
> Acked-by: David Hildenbrand (Arm) <david@kernel.org>

Thanks.

> 
> 
> 
> Should we extend the safety checks we already have on the other path?

Better to have.

> 
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 2a943ec57c85..1c304468af08 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1402,6 +1402,12 @@ bool mhp_supports_memmap_on_memory(void)
> }
> EXPORT_SYMBOL_GPL(mhp_supports_memmap_on_memory);
> 
> +static void altmap_free(struct vmemmap_altmap *altmap)
> +{
> +       WARN(altmap->alloc, "Altmap not fully unmapped");

Should we change it to WARN_ONCE?

> +       kfree(altmap);
> +}
> +
> static void remove_memory_blocks_and_altmaps(u64 start, u64 size)
> {
>        unsigned long memblock_size = memory_block_size_bytes();
> @@ -1426,10 +1432,7 @@ static void remove_memory_blocks_and_altmaps(u64 start, u64 size)
>                remove_memory_block_devices(cur_start, memblock_size);
> 
>                arch_remove_memory(cur_start, memblock_size, altmap);
> -
> -               /* Verify that all vmemmap pages have actually been freed. */
> -               WARN(altmap->alloc, "Altmap not fully unmapped");
> -               kfree(altmap);
> +               altmap_free(altmap);
>        }
> }
> 
> @@ -1460,7 +1463,7 @@ static int create_altmaps_and_memory_blocks(int nid, struct memory_group *group,
>                /* call arch's memory hotadd */
>                ret = arch_add_memory(nid, cur_start, memblock_size, &params);
>                if (ret < 0) {
> -                       kfree(params.altmap);
> +                       altmap_free(params.altmap);
>                        goto out;
>                }
> 
> @@ -1469,13 +1472,12 @@ static int create_altmaps_and_memory_blocks(int nid, struct memory_group *group,
>                                                  params.altmap, group);
>                if (ret) {
>                        arch_remove_memory(cur_start, memblock_size, NULL);
> -                       kfree(params.altmap);
> +                       altmap_free(params.altmap);
>                        goto out;
>                }
>        }
> 
>        return 0;
> -out:
>        if (ret && cur_start != start)
>                remove_memory_blocks_and_altmaps(start, cur_start - start);
>        return ret;
> 
> 
> Maybe the helper should even go into altmap code? Not sure.

I think the current changes look great as they are. While I believe this is valuable
as a standalone cleanup, what do you think?

Thanks,
Muchun.

> 
> 
> -- 
> Cheers,
> 
> David