[PATCH v3] riscv: mm: Avoid spurious fault after hotplugging vmemmap

[Vivian Wang]

On 6/5/26 17:16, David Hildenbrand (Arm) wrote:
> On 6/5/26 07:23, Vivian Wang wrote:
>> section_activate() does not flush TLB after populating new vmemmap
>> pages. On most architectures, this is okay. However it is a problem on
>> RISC-V since there the TLB caching non-present entries is permitted,
>> which causes spurious faults on some hardwares.
>>
>> This seems to be most easily reproduced with DEBUG_VM=y and
>> PAGE_POISONING=y, which causes these newly mapped struct pages to be
>> poisoned i.e. written to immediately after mapping.
>>
>> Add a hook vmemmap_populate_finalize() in __populate_section_memmap()
>> after population, to allow architectures to handle such situations as
>> needed. Then implement it on RISC-V to arrange for the existing
>> exception handler code to deal with these faults if they happen.
>>
>> Signed-off-by: Vivian Wang <wangruikang at iscas.ac.cn>
>> ---
>> Depends on my earlier kfence fixes for mark_new_valid_map() [1].
>>
>> Found while testing AMD_HSA/ZONE_DEVICE on SpacemiT K3. Using
>> ZONE_DEVICE requires another fix [2].
>>
>> [1]: https://lore.kernel.org/linux-riscv/20260303-handle-kfence-protect-spurious-fault-v2-0-f80d8354d79d@iscas.ac.cn
>> [2]: https://lore.kernel.org/linux-riscv/20260309-riscv-sparsemem-vmemmap-limits-v1-2-f40efe18e3cd@iscas.ac.cn
>> ---
>> Changes in v3:
>> - Merged back into one patch (Mike)
>> - (No code changes otherwise.)
>> - Link to v2: https://patch.msgid.link/20260604-mark-after-vmemmap-populate-v2-0-ab6a7d03b434@iscas.ac.cn
>>
>> Changes in v2:
>> - Split patch in two, hook point and riscv hook 
>> - Explain hook necessity in patch 1 message (Mike)
>> - Make hook #define based (Mike)
>> - Call finalize hook only on populate success
>> - Link to v1: https://patch.msgid.link/20260525-mark-after-vmemmap-populate-v1-1-e698d859ba16@iscas.ac.cn
>> ---
>>  arch/riscv/include/asm/pgtable.h | 4 ++++
>>  arch/riscv/mm/init.c             | 6 ++++++
>>  mm/sparse-vmemmap.c              | 8 ++++++++
>>  3 files changed, 18 insertions(+)
>>
>> diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
>> index a1a7c6520a09..aa0f50e3d534 100644
>> --- a/arch/riscv/include/asm/pgtable.h
>> +++ b/arch/riscv/include/asm/pgtable.h
>> @@ -1243,6 +1243,10 @@ static inline pte_t pte_swp_clear_exclusive(pte_t pte)
>>  #define TASK_SIZE	FIXADDR_START
>>  #endif
>>  
>> +/* Needed on SPARSEMEM_VMEMMAP */
>> +#define vmemmap_populate_finalize vmemmap_populate_finalize
>> +void __meminit vmemmap_populate_finalize(void);
>> +
>>  #else /* CONFIG_MMU */
>>  
>>  #define PAGE_SHARED		__pgprot(0)
>> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
>> index 706f43523935..cf9ae4099f82 100644
>> --- a/arch/riscv/mm/init.c
>> +++ b/arch/riscv/mm/init.c
>> @@ -1360,6 +1360,12 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
>>  	 */
>>  	return vmemmap_populate_hugepages(start, end, node, altmap);
>>  }
>> +
>> +void __meminit vmemmap_populate_finalize(void)
>> +{
>> +	/* Avoid faults on cached non-present TLB entries. */
>> +	mark_new_valid_map();
>> +}
>>  #endif
> How does mark_new_valid_map() handle concurrent access?
>
> There is some comment in there, but I am not sure if that actually implies that
> mark_new_valid_map() can be called concurrently from arbitrary context?

AFAICT, yes, it is intended to be used concurrently from arbitrary
context, and this is why I used them.

I had not originally written the code, so some riscv maintainer can
maybe chime in on this, but as I understand it the idea is that we only
ever set bits in the bitmap in mark_new_valid_map() while "allocating",
whereas the assembly code in handle_exception in
arch/riscv/kernel/entry.S only ever clears individual bits. So at least
the access to the bitmap itself should be fine.

This was added in commit 503638e0babf ("riscv: Stop emitting preventive
sfence.vma for new vmalloc mappings").

Do you think the mark_new_valid_map() function needs extra
synchronization like smp_wmb()?

Regards,
Vivian "dramforever" Wang