Re: [PATCH mm-new v3] mm: mincore: use per-vma lock during page table walk

[Kefeng Wang <wangkefeng.wang@huawei.com>]

On 9/16/2026 2:33 PM, David Hildenbrand (Arm) wrote:
> On 9/16/26 06:31, Kefeng Wang wrote:
>> do_mincore() performs a read-only, per-VMA residency query,
>> making it a good candidate for per-VMA locking. Convert it
>> to acquire the per-VMA lock, thereby reducing contention on
>> the per-MM mmap_lock.
>>
>> Reviewed-by: Pedro Falcato <pfalcato@suse.de>
>> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
>> ---
>> v3:
>> - add vma_assert_locked, update changelog/comment, per David
> 
> It likely was Lorenzo :)
> 

Oh, I'm completely blind :)

>> - Add RB
>> v2, (RESEND):
>> - using new vma_start_read_unlocked() API, suggestted by Pedro Falcato
>> v1:
>> - https://lore.kernel.org/linux-mm/20260701144047.3786939-2-wangkefeng.wang@huawei.com/
>>
>>   mm/mincore.c | 29 +++++++++++++++++------------
>>   1 file changed, 17 insertions(+), 12 deletions(-)
>>
>> diff --git a/mm/mincore.c b/mm/mincore.c
>> index c086836bc4bc..0fe50f8a7e62 100644
>> --- a/mm/mincore.c
>> +++ b/mm/mincore.c
>> @@ -235,24 +235,22 @@ static const struct mm_walk_ops mincore_walk_ops = {
>>   	.pmd_entry		= mincore_pte_range,
>>   	.pte_hole		= mincore_unmapped_range,
>>   	.hugetlb_entry		= mincore_hugetlb,
>> -	.walk_lock		= PGWALK_RDLOCK,
>> +	.walk_lock		= PGWALK_VMA_RDLOCK_VERIFY,
>>   };
>>   
>>   /*
>>    * Do a chunk of "sys_mincore()". We've already checked
>> - * all the arguments, we hold the mmap semaphore: we should
>> + * all the arguments, we hold the VMA read lock: we should
>>    * just return the amount of info we're asked for.
>>    */
>> -static long do_mincore(unsigned long addr, unsigned long pages, unsigned char *vec)
>> +static long do_mincore(struct vm_area_struct *vma, unsigned long addr,
>> +		unsigned long pages, unsigned char *vec)
>>   {
>> -	struct vm_area_struct *vma;
>> -	unsigned long end;
>> +	unsigned long end = min(vma->vm_end, addr + (pages << PAGE_SHIFT));
>>   	int err;
>>   
>> -	vma = vma_lookup(current->mm, addr);
>> -	if (!vma)
>> -		return -ENOMEM;
>> -	end = min(vma->vm_end, addr + (pages << PAGE_SHIFT));
>> +	vma_assert_locked(vma);
> 
> I think Lorenzo asked whether we should do that. But the walk_page_vma() further
> below would already verify that due to PGWALK_VMA_RDLOCK_VERIFY (see
> process_vma_walk_lock) so not sure if that's really required here.

I misunderstood Lorenzo's intention, which has already been verified 
through the PGWALK_VMA_RDLOCK_VERIFY check. I don't think it has much 
value, but adding it doesn't do any harm.

> 
> AFAIKS, everything we do in mincore_pte_range() should be compatible with the
> VMA lock, including the swap and pagecache handling.
> 
> Acked-by: David Hildenbrand (Arm) <david@kernel.org>
>