Re: [PATCH v2] x86/mm/pat: allocate split page tables as kernel page tables

On Tue, Jul 21, 2026 at 01:14:52PM +0100, Lorenzo Stoakes (ARM) wrote:
> When splitting a large page in CPA in __split_large_page() we allocate a
> PTE directly without going through the standard page table allocation
> routines such as pte_alloc_one_kernel().
> 
> This means the page table constructor is never called nor is the page table
> marked as a kernel page table.
> 
> The former results in the folio associated with the page table not being
> marked as a page table (__pagetable_ctor() is never called thus neither is
> __folio_set_pgtable()) nor are statistics updated to reflect
> it (lruvec_stat_add_folio() is never called).
> 
> The latter issue of failing to mark the page table as a kernel page
> table (ptdesc_set_kernel() is never called) is far more problematic.
> 
> Since commit 5ba2f0a15564 ("mm: introduce deferred freeing for kernel page
> tables") kernel page table freeing has been batched and since the
> subsequent commit e37d5a2d60a3 ("iommu/sva: invalidate stale IOTLB entries
> for kernel address space") IOTLB cache entries for kernel page tables have
> been invalidated upon being freed.
> 
> Since split page tables are freed without this invalidation, the IOTLB can
> contain stale entries for them.
> 
> Resolve the issue by using the ordinary PTE allocation API at split time.
> 
> This results in these kernel page tables invoking a page table constructor,
> and thus requires a page table destructor.
> 
> Since we cannot assume one is always present (early allocated direct map
> page tables are not marked as such), we conditionally call
> pagetable_dtor_free() if the PG_table folio flag for the ptdesc is set,
> otherwise we free the page table via pagetable_free().
> 
> Regardless of which path is taken page tables marked as kernel page tables,
> which now includes split page tables, take the correct route through
> pagetable_free_kernel().
> 
> There is a user-visible side effect in that split page tables will appear
> in nr_page_table_pages in /proc/vmstat (as do other kernel page tables
> allocated after early boot), however this is a positive change.
> 
> This issue started being markedly problematic after commit
> 5ba2f0a15564 ("mm: introduce deferred freeing for kernel page tables") so
> choose this as the Fixes target.
> 
> Fixes: 5ba2f0a15564 ("mm: introduce deferred freeing for kernel page tables")
> Cc: [email protected]
> Signed-off-by: Lorenzo Stoakes (ARM) <[email protected]>

Acked-by: Vishal Moola <[email protected]>