linux-kernel - Re: [PATCH 2/4] mm/hugeltb: simplify the return code of __vma_reservation

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Message-ID: <aaea15d4-c8e0-ee37-8ceb-35326b7ad1ae@oracle.com>
Date:   Tue, 6 Apr 2021 19:37:41 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Miaohe Lin <linmiaohe@...wei.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 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;

-- 
Mike Kravetz