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

[Mike Kravetz]

On 4/7/21 7:44 PM, Miaohe Lin wrote:
> 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@xxxxxxxxxx>
>>>>>>> ---
>>>>>>>  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...
> 

Please prepare v2 with the changes to remove the HPAGE_RESV_OWNER check
and move the large comment.


I would prefer to leave other places that mention HPAGE_RESV_OWNER
unchanged.

Thanks,
-- 
Mike Kravetz