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Subject: Re: [PATCH] mm: allow unmapped hole at head side of mbind range
To:     Li Xinhai <xinhai.li@outlook.com>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>
References: <TY2PR04MB29753892EBAD17D9E8FECBAEE86A0@TY2PR04MB2975.apcprd04.prod.outlook.com>
From:   Vlastimil Babka <vbabka@suse.cz>
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Message-ID: <c90d668a-9abc-df1b-8b9c-70a6e6afd94a@suse.cz>
Date:   Thu, 24 Oct 2019 12:48:48 +0200
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+ linux-api

On 10/24/19 9:35 AM, Li Xinhai wrote:
> From: Li Xinhai  <xinhai.li@outlook.com>
> 
> mbind_range silently ignore unmapped hole at middle and tail of the 
> specified range, but report EFAULT if hole at head side.


Hmm that's unfortunate. mbind() manpage says:

EFAULT Part or all of the memory range specified by nodemask and maxnode
points outside your accessible address space.  Or, there was an unmapped
hole in  the  specified  memory range specified by addr and len.

That sounds like any hole inside the specified range should return
EFAULT. But perhaps it can be also interpreted as you suggest, that the
whole range is an unmapped hole. There's some risk of breaking existing
userspace if we change it either way.

> It is more reasonable to support silently ignore holes at any part of 
> the range, only report EFAULT if the whole range is in hole.
> 
> Signed-off-by: Li Xinhai <xinhai.li@outlook.com>
> ---
> 
>  mm/mempolicy.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> 
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 4ae967bcf954..ae160d9936d9 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -738,7 +738,7 @@ static int mbind_range(struct mm_struct *mm, unsigned long start,
>         unsigned long vmend;
>  
>         vma = find_vma(mm, start);
> -       if (!vma || vma->vm_start > start)
> +       if (!vma || vma->vm_start >= end)
>                 return -EFAULT;
>  
>         prev = vma->vm_prev;
>