linux-kernel - Re: [PATCH] mm: fix incorrect behavior when process virtual address space limit is exceeded

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Date:	Tue, 17 Nov 2015 17:19:28 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Piotr Kwapulinski <kwapulinski.piotr@...il.com>
Cc:	akpm@...ux-foundation.org, cmetcalf@...hip.com, mszeredi@...e.cz,
	viro@...iv.linux.org.uk, dave@...olabs.net,
	kirill.shutemov@...ux.intel.com, n-horiguchi@...jp.nec.com,
	aarcange@...hat.com, mhocko@...e.com, iamjoonsoo.kim@....com,
	jack@...e.cz, xiexiuqi@...wei.com, vbabka@...e.cz,
	Vineet.Gupta1@...opsys.com, riel@...hat.com,
	gang.chen.5i5j@...il.com, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH] mm: fix incorrect behavior when process virtual
	address space limit is exceeded

On 11/16, Piotr Kwapulinski wrote:
>
> @@ -1551,7 +1552,7 @@ 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))
> +		if (!(flags & MAP_FIXED))
>  			return -ENOMEM;

Agree, "vm_flags & MAP_FIXED" makes no sense and just wrong...

Can't we simply remove this check? Afaics it only helps to avoid
count_vma_pages_range() in the unlikely case when may_expand_vm() fails.
And without MAP_FIXED count_vma_pages_range() should be cheap,
find_vma_intersection() should fail.

And afaics arch/tile/mm/elf.c can use do_mmap(MAP_FIXED ...) rather than
mmap_region(), it can be changed by a separate patch. In this case we can
unexport mmap_region().


OTOH, I won't insist, this patch looks fine to me.

Oleg.

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