linux-kernel - Re: [PATCH v3] mm/mmap.c: remove incorrect MAP_FIXED flag comparison from mmap

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Message-ID: <20151127052738.GA25042@hori1.linux.bs1.fc.nec.co.jp>
Date:	Fri, 27 Nov 2015 05:27:39 +0000
From:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:	Piotr Kwapulinski <kwapulinski.piotr@...il.com>
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	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH v3] mm/mmap.c: remove incorrect MAP_FIXED flag
 comparison from mmap_region

On Mon, Nov 23, 2015 at 06:36:42PM +0100, Piotr Kwapulinski wrote:
> The following flag comparison in mmap_region makes no sense:
> 
> if (!(vm_flags & MAP_FIXED))
>     return -ENOMEM;
> 
> The condition is always false and thus the above "return -ENOMEM" is never
> executed. The vm_flags must not be compared with MAP_FIXED flag.
> The vm_flags may only be compared with VM_* flags.
> MAP_FIXED has the same value as VM_MAYREAD.
> Hitting the rlimit is a slow path and find_vma_intersection should realize
> that there is no overlapping VMA for !MAP_FIXED case pretty quickly.
> 
> Remove the code that makes no sense.
> 
> Signed-off-by: Piotr Kwapulinski <kwapulinski.piotr@...il.com>

Looks good to me. Thank you.

Reviewed-by: Naoya Horiguchi <n-horiguchi@...jp.nec.com>

> ---
>  mm/mmap.c | 3 ---
>  1 file changed, 3 deletions(-)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 2ce04a6..42a8259 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1551,9 +1551,6 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>  		 * MAP_FIXED may remove pages of mappings that intersects with
>  		 * requested mapping. Account for the pages it would unmap.
>  		 */
> -		if (!(vm_flags & MAP_FIXED))
> -			return -ENOMEM;
> -
>  		nr_pages = count_vma_pages_range(mm, addr, addr + len);
>  
>  		if (!may_expand_vm(mm, (len >> PAGE_SHIFT) - nr_pages))
> -- 
> 2.6.2
> --
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