Re: [PATCH 2/4] mm/hugeltb: simplify the return code of __vma_reservation_common()

[Miaohe Lin <linmiaohe@huawei.com>]

On 2021/4/8 5:23, Mike Kravetz wrote:
> On 4/6/21 8:09 PM, Miaohe Lin wrote:
>> On 2021/4/7 10:37, Mike Kravetz wrote:
>>> On 4/6/21 7:05 PM, Miaohe Lin wrote:
>>>> Hi:
>>>> On 2021/4/7 8:53, Mike Kravetz wrote:
>>>>> On 4/2/21 2:32 AM, Miaohe Lin wrote:
>>>>>> It's guaranteed that the vma is associated with a resv_map, i.e. either
>>>>>> VM_MAYSHARE or HPAGE_RESV_OWNER, when the code reaches here or we would
>>>>>> have returned via !resv check above. So ret must be less than 0 in the
>>>>>> 'else' case. Simplify the return code to make this clear.
>>>>>
>>>>> I believe we still neeed that ternary operator in the return statement.
>>>>> Why?
>>>>>
>>>>> There are two basic types of mappings to be concerned with:
>>>>> shared and private.
>>>>> For private mappings, a task can 'own' the mapping as indicated by
>>>>> HPAGE_RESV_OWNER.  Or, it may not own the mapping.  The most common way
>>>>> to create a non-owner private mapping is to have a task with a private
>>>>> mapping fork.  The parent process will have HPAGE_RESV_OWNER set, the
>>>>> child process will not.  The idea is that since the child has a COW copy
>>>>> of the mapping it should not consume reservations made by the parent.
>>>>
>>>> The child process will not have HPAGE_RESV_OWNER set because at fork time, we do:
>>>> 		/*
>>>> 		 * Clear hugetlb-related page reserves for children. This only
>>>> 		 * affects MAP_PRIVATE mappings. Faults generated by the child
>>>> 		 * are not guaranteed to succeed, even if read-only
>>>> 		 */
>>>> 		if (is_vm_hugetlb_page(tmp))
>>>> 			reset_vma_resv_huge_pages(tmp);
>>>> i.e. we have vma->vm_private_data = (void *)0; for child process and vma_resv_map() will
>>>> return NULL in this case.
>>>> Or am I missed something?
>>>>
>>>>> Only the parent (HPAGE_RESV_OWNER) is allowed to consume the
>>>>> reservations.
>>>>> Hope that makens sense?
>>>>>
>>>>>>
>>>>>> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
>>>>>> ---
>>>>>>  mm/hugetlb.c | 2 +-
>>>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>>
>>>>>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>>>>>> index a03a50b7c410..b7864abded3d 100644
>>>>>> --- a/mm/hugetlb.c
>>>>>> +++ b/mm/hugetlb.c
>>>>>> @@ -2183,7 +2183,7 @@ static long __vma_reservation_common(struct hstate *h,
>>>>>>  			return 1;
>>>>>>  	}
>>>>>>  	else
>>>>>
>>>>> This else also handles the case !HPAGE_RESV_OWNER.  In this case, we
>>>>
>>>> IMO, for the case !HPAGE_RESV_OWNER, we won't reach here. What do you think?
>>>>
>>>
>>> I think you are correct.
>>>
>>> However, if this is true we should be able to simply the code even
>>> further.  There is no need to check for HPAGE_RESV_OWNER because we know
>>> it must be set.  Correct?  If so, the code could look something like:
>>>
>>> 	if (vma->vm_flags & VM_MAYSHARE)
>>> 		return ret;
>>>
>>> 	/* We know private mapping with HPAGE_RESV_OWNER */
>>> 	 * ...						 *
>>> 	 * Add that existing comment                     */
>>>
>>> 	if (ret > 0)
>>> 		return 0;
>>> 	if (ret == 0)
>>> 		return 1;
>>> 	return ret;
>>>
>>
>> Many thanks for good suggestion! What do you mean is this ?
> 
> I think the below changes would work fine.
> 
> However, this patch/discussion has made me ask the question.  Do we need
> the HPAGE_RESV_OWNER flag?  Is the followng true?
> !(vm_flags & VM_MAYSHARE) && vma_resv_map()  ===> HPAGE_RESV_OWNER
> !(vm_flags & VM_MAYSHARE) && !vma_resv_map() ===> !HPAGE_RESV_OWNER
> 

I agree with you.

HPAGE_RESV_OWNER is set in hugetlb_reserve_pages() and there's no way to clear it
in the owner process. The child process can not inherit both HPAGE_RESV_OWNER and
resv_map. So for !HPAGE_RESV_OWNER vma, it knows nothing about resv_map.

IMO, in !(vm_flags & VM_MAYSHARE) case, we must have:
	!!vma_resv_map() == !!HPAGE_RESV_OWNER

> I am not suggesting we eliminate the flag and make corresponding
> changes.  Just curious if you believe we 'could' remove the flag and
> depend on the above conditions.
> 
> One reason for NOT removing the flag is that that flag itself and
> supporting code and commnets help explain what happens with hugetlb
> reserves for COW mappings.  That code is hard to understand and the
> existing code and coments around HPAGE_RESV_OWNER help with
> understanding.

Agree. These codes took me several days to understand...

> 

Thanks.