Re: [PATCH 15/22] mm: only initialise pt_share_count for user pgtables

On 21/07/2026 17:12, David Hildenbrand (Arm) wrote:
> On 7/14/26 16:04, Kevin Brodsky wrote:
>> ptdesc_pmd_pts_init() initialises the pt_share_count ptdesc field.
>> That field is only used for hugetlb tracking purposes, so there is
>> no need to initialise it for kernel page tables. Skip the call for
>> kernel page tables, like pmd_ptlock_init().
>>
>> From now on pagetable_*_ctor() and pagetable_dtor() do nothing for
>> kernel page tables.
>>
>> Signed-off-by: Kevin Brodsky <[email protected]>
>> ---
>>  include/linux/mm.h | 9 ++++++---
>>  1 file changed, 6 insertions(+), 3 deletions(-)
>>
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index 94f0fb1c662a..b217ccbed5a3 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -3905,9 +3905,12 @@ static inline spinlock_t *pmd_lock(struct mm_struct *mm, pmd_t *pmd)
>>  static inline bool pagetable_pmd_ctor(struct mm_struct *mm,
>>  				      struct ptdesc *ptdesc)
>>  {
>> -	if (!mm_is_kernel(mm) && !pmd_ptlock_init(ptdesc))
>> -		return false;
>> -	ptdesc_pmd_pts_init(ptdesc);
>> +	if (!mm_is_kernel(mm)) {
>> +		if (!pmd_ptlock_init(ptdesc))
>> +			return false;
>> +		ptdesc_pmd_pts_init(ptdesc);
>> +	}
>> +
> I'd have said "the initialization is cheap, why bother", but we already do have
> the mm_is_kernel() check in place, so

This isn't mainly about the overhead, but rather the fact that from here
on the ctor/dtor do nothing for kernel page tables. Patch 20-22 rely on
this, and in part 2 I would go further and only ever call the ctor/dtor
for user page tables, so that the mm_is_kernel() check can be dropped.

> Acked-by: David Hildenbrand (Arm) <[email protected]>

Thanks for the reviews!

- Kevin