Re: [PATCH v2 1/4] mm: fix missing page-table destructor in pagetable_free()

[Muchun Song <muchun.song@linux.dev>]

> On Oct 12, 2026, at 21:27, David Hildenbrand (Arm) <david@kernel.org> wrote:
> 
> On 10/10/26 13:24, Muchun Song wrote:
>> Since commit 49f599666420 ("mm: call ctor/dtor for kernel PTEs"),
>> pte_alloc_one_kernel() runs the page-table constructor for kernel PTE
>> tables.
>> 
>> HVO (HugeTLB Vmemmap Optimization) uses pte_alloc_one_kernel() when
>> splitting a PMD. Restoring the vmemmap backing pages does not collapse
>> the PTE table, so a later memory hot-remove eventually frees it through
>> pagetable_free() on x86-64. That releases the page without running the
>> matching destructor and leaves NR_PAGETABLE elevated.
>> 
>> Make pagetable_free() detect constructor-backed pages and run the
>> destructor before releasing their memory to fix the problem.
>> 
>> Fixes: 49f599666420 ("mm: call ctor/dtor for kernel PTEs")
>> Cc: stable@vger.kernel.org
>> Assisted-by: LLM
>> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>> ---
>> v2:
>> - Move destructor handling from x86 into pagetable_free() (suggested by
>>  David Hildenbrand)
>> - Trim and update the commit message (requested by Dave Hansen)
>> ---
>> include/linux/mm.h | 37 ++++++++++++++++++++-----------------
>> 1 file changed, 20 insertions(+), 17 deletions(-)
>> 
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index 5cca90fc1df6..771a7d60b0a5 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -3969,23 +3969,6 @@ static inline void pagetable_free_kernel(struct ptdesc *pt)
>> __pagetable_free(pt);
>> }
>> #endif
>> -/**
>> - * pagetable_free - Free pagetables
>> - * @pt: The page table descriptor
>> - *
>> - * pagetable_free frees the memory of all page tables described by a page
>> - * table descriptor and the memory for the descriptor itself.
>> - */
>> -static inline void pagetable_free(struct ptdesc *pt)
>> -{
>> - if (ptdesc_test_kernel(pt)) {
>> - ptdesc_clear_kernel(pt);
>> - pagetable_free_kernel(pt);
>> - } else {
>> - __pagetable_free(pt);
>> - }
>> -}
>> -
>> #if defined(CONFIG_SPLIT_PTE_PTLOCKS)
>> #if ALLOC_SPLIT_PTLOCKS
>> void __init ptlock_cache_init(void);
>> @@ -4078,6 +4061,26 @@ static inline void pagetable_dtor(struct ptdesc *ptdesc)
>> lruvec_stat_sub_folio(folio, NR_PAGETABLE);
>> }
>> 
>> +/**
>> + * pagetable_free - Free pagetables
>> + * @pt: The page table descriptor
>> + *
>> + * pagetable_free frees the memory of all page tables described by a page
>> + * table descriptor and the memory for the descriptor itself.
>> + */
>> +static inline void pagetable_free(struct ptdesc *pt)
>> +{
>> + if (PageTable(ptdesc_page(pt)))
>> + pagetable_dtor(pt);
> 
> 
> We do have
> 
> static inline void pagetable_dtor_free(struct ptdesc *ptdesc)
> {
> 	pagetable_dtor(ptdesc);
> 	pagetable_free(ptdesc);
> }
> 
> ... can we now get rid of pagetable_dtor_free() and simply use pagetable_free() ?
> (separate cleanup)

You're right, that's exactly my plan.

> 
>> +
>> + 	if (ptdesc_test_kernel(pt)) {
>> + 		ptdesc_clear_kernel(pt);
>> + 		pagetable_free_kernel(pt);
>> + 	} else {
>> + 		__pagetable_free(pt);
>> + 	}
> 
> 
> This goes into the right direction. I'm still confused, but the less freeing
> functions we have the better.
> 
> Acked-by: David Hildenbrand (Arm) <david@kernel.org>

Thanks David.

> 
> -- 
> Cheers,
> 
> David