[RFC PATCH] arm64: mm: Fix kernel page tables incorrectly deleted during memory removal

[David Hildenbrand]

On 24.07.23 07:54, Anshuman Khandual wrote:
> 
> 
> On 7/24/23 06:55, mawupeng wrote:
>>
>> On 2023/7/21 18:36, Will Deacon wrote:
>>> On Mon, Jul 17, 2023 at 07:51:50PM +0800, Wupeng Ma wrote:
>>>> From: Ma Wupeng <mawupeng1 at huawei.com>
>>>>
>>>> During our test, we found that kernel page table may be unexpectedly
>>>> cleared with rodata off. The root cause is that the kernel page is
>>>> initialized with pud size(1G block mapping) while offline is memory
>>>> block size(MIN_MEMORY_BLOCK_SIZE 128M), eg, if 2G memory is hot-added,
>>>> when offline a memory block, the call trace is shown below,

Is someone adding memory in 2 GiB granularity and then removing parts of 
it in 128 MiB granularity? That would be against what we support using 
the add_memory() / offline_and_remove_memory() API and that driver 
should be fixed instead.

Or does this trigger only when a hotplugged memory block falls into the 
same 2 GiB area as boot memory?

>>>>
>>>>   offline_and_remove_memory
>>>>      try_remove_memory
>>>>        arch_remove_memory
>>>>         __remove_pgd_mapping
>>>>           unmap_hotplug_range
>>>>             unmap_hotplug_p4d_range
>>>>               unmap_hotplug_pud_range
>>>>                 if (pud_sect(pud))
>>>>                   pud_clear(pudp);

Which drivers triggers that? In-tree is only virtio-mem and dax/kmem. 
Both add and remove memory in the same granularity.

For example, virtio-mem will only call add_memory(memory_block_size()) 
to then offline_and_remove_memory(memory_block_size()).

Could that trigger it as well?

>>> Sorry, but I'm struggling to understand the problem here. If we're adding
>>> and removing a 2G memory region, why _wouldn't_ we want to use large 1GiB
>>> mappings?
>>
>>> Or are you saying that only a subset of the memory is removed,
>>> but we then accidentally unmap the whole thing?
>> Yes, umap a subset but the whole thing page table entry is removed.
>>

Can we have some more details about the user and how to trigger it?

>>>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>>>> index 95d360805f8a..44c724ce4f70 100644
>>>> --- a/arch/arm64/mm/mmu.c
>>>> +++ b/arch/arm64/mm/mmu.c
>>>> @@ -44,6 +44,7 @@
>>>>   #define NO_BLOCK_MAPPINGS	BIT(0)
>>>>   #define NO_CONT_MAPPINGS	BIT(1)
>>>>   #define NO_EXEC_MAPPINGS	BIT(2)	/* assumes FEAT_HPDS is not used */
>>>> +#define NO_PUD_MAPPINGS		BIT(3)
>>>>   
>>>>   int idmap_t0sz __ro_after_init;
>>>>   
>>>> @@ -344,7 +345,7 @@ static void alloc_init_pud(pgd_t *pgdp, unsigned long addr, unsigned long end,
>>>>   		 */
>>>>   		if (pud_sect_supported() &&
>>>>   		   ((addr | next | phys) & ~PUD_MASK) == 0 &&
>>>> -		    (flags & NO_BLOCK_MAPPINGS) == 0) {
>>>> +		    (flags & (NO_BLOCK_MAPPINGS | NO_PUD_MAPPINGS)) == 0) {
>>>>   			pud_set_huge(pudp, phys, prot);
>>>>   
>>>>   			/*
>>>> @@ -1305,7 +1306,7 @@ struct range arch_get_mappable_range(void)
>>>>   int arch_add_memory(int nid, u64 start, u64 size,
>>>>   		    struct mhp_params *params)
>>>>   {
>>>> -	int ret, flags = NO_EXEC_MAPPINGS;
>>>> +	int ret, flags = NO_EXEC_MAPPINGS | NO_PUD_MAPPINGS;
>>> I think we should allow large mappings here and instead prevent partial
>>> removal of the block, if that's what is causing the issue.
>> This could solve this problem.
>> Or we can prevent  partial removal? Or rebulid page table entry which is not removed?
> 
> + David Hildenbrand <david at redhat.com>
> 
> Splitting the block mapping and rebuilding page table entry to reflect non-removed
> areas will require additional information such as flags and pgtable alloc function
> as in __create_pgd_mapping(), which need to be passed along, depending on whether
> it's tearing down vmemmap (would not have PUD block map) or linear mapping. But I
> am just wondering if we have to go in that direction at all or just prevent partial
> memory block removal as suggested by Will.
> 
> - arch_remove_memory() does not have return type, core MM hotremove would not fail
>    because arch_remove_memory() failed or warned
> 
> - core MM hotremove does check_hotplug_memory_range() which ensures the range and
>    start address are memory_block_size_bytes() aligned
> 
> - Default memory_block_size_bytes() is dependent on SECTION_SIZE_BITS which on arm64
>    now can be less than PUD_SIZE triggering this problem.
> 
> 	#define MIN_MEMORY_BLOCK_SIZE     (1UL << SECTION_SIZE_BITS)
> 
> 	unsigned long __weak memory_block_size_bytes(void)
> 	{
>          	return MIN_MEMORY_BLOCK_SIZE;
> 	}
> 	EXPORT_SYMBOL_GPL(memory_block_size_bytes);
> 
> - We would need to override memory_block_size_bytes() on arm64 to accommodate such
>    scenarios here
> 
> Something like this might work (built but not tested)
> 
> commit 2eb8dc0d08dfe0b2a3bb71df93b12f7bf74a2ca6 (HEAD)
> Author: Anshuman Khandual <anshuman.khandual at arm.com>
> Date:   Mon Jul 24 06:45:34 2023 +0100
> 
>      arm64/mm: Define memory_block_size_bytes()
>      
>      Define memory_block_size_bytes() on arm64 platforms to set minimum hot plug
>      and remove granularity as PUD_SIZE in case where MIN_MEMORY_BLOCK_SIZE just
>      falls below PUD_SIZE. Otherwise a complete PUD block mapping will be teared
>      down while unmapping MIN_MEMORY_BLOCK_SIZE range.
>      
>      Signed-off-by: Anshuman Khandual <anshuman.khandual at arm.com>
> 
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 95d360805f8a..1918459b3460 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -1157,6 +1157,17 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
>   }
>   
>   #ifdef CONFIG_MEMORY_HOTPLUG
> +unsigned long memory_block_size_bytes(void)
> +{
> +       /*
> +        * Linear mappings might include PUD based block mappings which
> +        * cannot be teared down in part during memory hotremove. Hence
> +        * PUD_SIZE needs to be the minimum granularity, for memory hot
> +        * removal in case MIN_MEMORY_BLOCK_SIZE falls below.
> +        */
> +       return max_t(unsigned long, MIN_MEMORY_BLOCK_SIZE, PUD_SIZE);
> +}
> +
>   void vmemmap_free(unsigned long start, unsigned long end,
>                  struct vmem_altmap *altmap)
>   {
> 

OH god no. That would seriously degrade memory hotplug capabilities in 
virtual environments (especially, virtio-mem and DIMMS).

If someone adds memory in 128 MiB chunks and removes memory in 128 MiB 
chunks, that has to be working.

Removing boot memory is blocked via 
register_memory_notifier(&prevent_bootmem_remove_nb);

-- 
Cheers,

David / dhildenb