Date:   Fri, 9 Apr 2021 10:52:02 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>, <akpm@...ux-foundation.org>
CC:     <n-horiguchi@...jp.nec.com>, <hillf.zj@...baba-inc.com>,
        <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>
Subject: Re: [PATCH 2/4] mm/hugeltb: simplify the return code of
 __vma_reservation_common()

On 2021/4/9 6:40, Mike Kravetz wrote:
> 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@...wei.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...
>>
> 
> Please prepare v2 with the changes to remove the HPAGE_RESV_OWNER check
> and move the large comment.
> 

Sure. Will do. Thanks.

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

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