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

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Date:   Wed, 7 Apr 2021 10:05:12 +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()

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?

> never want to indicate reservations are available.  The ternary makes
> sure a positive value is never returned.
> 

Many thanks for review and reply! :)