On Mon, Jul 17, 2023 at 07:51:50PM +0800, Wupeng Ma wrote:
> From: Ma Wupeng <mawupeng1@xxxxxxxxxx>
>
> 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
