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

From: Oleg Nesterov <oleg@redhat.com>
To: Piotr Kwapulinski <kwapulinski.piotr@gmail.com>
Cc: akpm@linux-foundation.org, cmetcalf@ezchip.com, mszeredi@suse.cz,
	viro@zeniv.linux.org.uk, dave@stgolabs.net,
	kirill.shutemov@linux.intel.com, n-horiguchi@ah.jp.nec.com,
	aarcange@redhat.com, mhocko@suse.com, iamjoonsoo.kim@lge.com,
	jack@suse.cz, xiexiuqi@huawei.com, vbabka@suse.cz,
	Vineet.Gupta1@synopsys.com, riel@redhat.com,
	gang.chen.5i5j@gmail.com, linux-kernel@vger.kernel.org,
	linux-mm@kvack.org
Subject: Re: [PATCH] mm: fix incorrect behavior when process virtual address space limit is exceeded
Date: Tue, 17 Nov 2015 17:19:28 +0100	[thread overview]
Message-ID: <20151117161928.GA9611@redhat.com> (raw)
In-Reply-To: <1447695379-14526-1-git-send-email-kwapulinski.piotr@gmail.com>

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.