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

[Will Deacon]

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,
> 
>  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);

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?

> 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.

Will