Re: [PATCH 6/8] hugetlb: add vma based lock for pmd sharing

[Miaohe Lin <linmiaohe@huawei.com>]

On 2022/8/30 6:24, Mike Kravetz wrote:
> On 08/27/22 17:30, Miaohe Lin wrote:
>> On 2022/8/25 1:57, Mike Kravetz wrote:
>>> Allocate a rw semaphore and hang off vm_private_data for
>>> synchronization use by vmas that could be involved in pmd sharing.  Only
>>> add infrastructure for the new lock here.  Actual use will be added in
>>> subsequent patch.
>>>
>>> Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
>>
>> <snip>
>>
>>> +static void hugetlb_vma_lock_free(struct vm_area_struct *vma)
>>> +{
>>> +	/*
>>> +	 * Only present in sharable vmas.  See comment in
>>> +	 * __unmap_hugepage_range_final about the neeed to check both
>>
>> s/neeed/need/
>>
>>> +	 * VM_SHARED and VM_MAYSHARE in free path
>>
>> I think there might be some wrong checks around this patch. As above comment said, we
>> need to check both flags, so we should do something like below instead?
>>
>> 	if (!(vma->vm_flags & (VM_MAYSHARE | VM_SHARED) == (VM_MAYSHARE | VM_SHARED)))
>>
>>> +	 */
> 
> Thanks.  I will update.
> 
>>> +	if (!vma || !(vma->vm_flags & (VM_MAYSHARE | VM_SHARED)))
>>> +		return;
>>> +
>>> +	if (vma->vm_private_data) {
>>> +		kfree(vma->vm_private_data);
>>> +		vma->vm_private_data = NULL;
>>> +	}
>>> +}
>>> +
>>> +static void hugetlb_vma_lock_alloc(struct vm_area_struct *vma)
>>> +{
>>> +	struct rw_semaphore *vma_sema;
>>> +
>>> +	/* Only establish in (flags) sharable vmas */
>>> +	if (!vma || !(vma->vm_flags & VM_MAYSHARE))
>>> +		return;
>>> +
>>> +	/* Should never get here with non-NULL vm_private_data */
>>
>> We can get here with non-NULL vm_private_data when called from hugetlb_vm_op_open during fork?
> 
> Right!
> 
> In fork, We allocate a new semaphore in hugetlb_dup_vma_private, and then
> shortly after call hugetlb_vm_op_open.
> 
> It works as is, and I can update the comment.  However, I wonder if we should
> just clear vm_private_data in hugetlb_dup_vma_private and let hugetlb_vm_op_open
> do the allocation.

I think it's a good idea. We can also avoid allocating memory for vma_lock (via clear_vma_resv_huge_pages()) and
then free the corresponding vma right away (via do_munmap())in move_vma(). But maybe I'm miss something.

Thanks,
Miaohe Lin

> 
>>
>> Also there's one missing change on comment:
>>
>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>> index d0617d64d718..4bc844a1d312 100644
>> --- a/mm/hugetlb.c
>> +++ b/mm/hugetlb.c
>> @@ -863,7 +863,7 @@ __weak unsigned long vma_mmu_pagesize(struct vm_area_struct *vma)
>>   * faults in a MAP_PRIVATE mapping. Only the process that called mmap()
>>   * is guaranteed to have their future faults succeed.
>>   *
>> - * With the exception of reset_vma_resv_huge_pages() which is called at fork(),
>> + * With the exception of hugetlb_dup_vma_private() which is called at fork(),
>>   * the reserve counters are updated with the hugetlb_lock held. It is safe
>>   * to reset the VMA at fork() time as it is not in use yet and there is no
>>   * chance of the global counters getting corrupted as a result of the values.
>>
>>
>> Otherwise this patch looks good to me. Thanks.
> 
> Will update, Thank you!
>