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

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Message-ID: <e958d731-67d4-a56b-aa1d-a8054cf232f2@oracle.com>
Date:   Tue, 6 Apr 2021 17:53:59 -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 4/2/21 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.
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
never want to indicate reservations are available.  The ternary makes
sure a positive value is never returned.

-- 
Mike Kravetz

> -		return ret < 0 ? ret : 0;
> +		return ret;
>  }
>  
>  static long vma_needs_reservation(struct hstate *h,
>