Re: [PATCH v4 6/8] mm: Introduce deferred freeing for kernel page tables

[Baolu Lu <baolu.lu@linux.intel.com>]

On 9/12/25 16:14, Tian, Kevin wrote:
>> From: Lu Baolu <baolu.lu@linux.intel.com>
>> Sent: Friday, September 5, 2025 1:51 PM
>>
>> From: Dave Hansen <dave.hansen@linux.intel.com>
>>
>> On x86 and other architectures that map the kernel's virtual address space
>> into the upper portion of every process's page table, the IOMMU's paging
>> structure caches can become stale when the CPU page table is shared with
>> IOMMU in the Shared Virtual Address (SVA) context. This occurs when a page
>> used for the kernel's page tables is freed and reused without the IOMMU
>> being notified.
>>
>> While the IOMMU driver is notified of changes to user virtual address
>> mappings, there is no similar notification mechanism for kernel page
>> table changes. This can lead to data corruption or system instability
>> when Shared Virtual Address (SVA) is enabled, as the IOMMU's internal
>> caches may retain stale entries for kernel virtual addresses.
> 
> above could be saved to the last patch.

Yes.

> 
>>
>> This introduces a conditional asynchronous mechanism, enabled by
>> CONFIG_ASYNC_PGTABLE_FREE. When enabled, this mechanism defers the
>> freeing
>> of pages that are used as page tables for kernel address mappings. These
>> pages are now queued to a work struct instead of being freed immediately.
>>
>> This deferred freeing provides a safe context for a future patch to add
>> an IOMMU-specific callback, which might be expensive on large-scale
>> systems. This ensures the necessary IOMMU cache invalidation is performed
>> before the page is finally returned to the page allocator outside of any
>> critical, non-sleepable path.
>>
>> In the current kernel, some page table pages are allocated with an
>> associated struct ptdesc, while others are not. Those without a ptdesc are
>> freed using free_pages() and its variants, which bypasses the destructor
>> that pagetable_dtor_free() would run. While the long-term plan is to
>> convert all page table pages to use struct ptdesc, this uses a temporary
>> flag within ptdesc to indicate whether a page needs a destructor,
>> considering that this aims to fix a potential security issue in IOMMU SVA.
>> The flag and its associated logic can be removed once the conversion is
>> complete.
> 
> stale comment?

Yes. Fixed.

> 
>>
>> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
>> Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
> 
> Reviewed-by: Kevin Tian <kevin.tian@intel.com>

Thanks,
baolu