linux-kernel - Re: [PATCH]fix VM_CAN_NONLINEAR check in sys_remap_file

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Message-ID: <Pine.LNX.4.64.0710081418140.8640@blonde.wat.veritas.com>
Date:	Mon, 8 Oct 2007 14:37:59 +0100 (BST)
From:	Hugh Dickins <hugh@...itas.com>
To:	Yan Zheng <yanzheng@...n.com>
cc:	Nick Piggin <nickpiggin@...oo.com.au>,
	Miklos Szeredi <miklos@...redi.hu>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org
Subject: Re: [PATCH]fix VM_CAN_NONLINEAR check in sys_remap_file_pages

On Mon, 8 Oct 2007, Yan Zheng wrote:
> 
> The test for VM_CAN_NONLINEAR always fails

Good catch indeed.  Though I was puzzled how we do nonlinear at all,
until I realized it's "The test for not VM_CAN_NONLINEAR always fails".

It's not as serious as it appears, since code further down has been
added more recently to simulate nonlinear on non-RAM-backed filesystems,
instead of going the real nonlinear way; so most filesystems are now not
required to do what VM_CAN_NONLINEAR was put in to ensure they could do.

I'm confused as to where that leaves us: is this actually a fix that
needs to go into 2.6.23?  or will it suddenly disable a system call
which has been silently working fine on various filesystems which did
not add VM_CAN_NONLINEAR?  could we just rip out VM_CAN_NONLINEAR?
I hope Nick or Miklos is clearer on what the risks are.

(Apologies for all the "not"s and "non"s here, I'm embarrassed
after just criticizing Ingo's SCHED_NO_NO_OMIT_FRAME_POINTER!)

Hugh

> 
> Signed-off-by: Yan Zheng<yanzheng@...n.com>
> ----
> diff -ur linux-2.6.23-rc9/mm/fremap.c linux/mm/fremap.c
> --- linux-2.6.23-rc9/mm/fremap.c	2007-10-07 15:03:33.000000000 +0800
> +++ linux/mm/fremap.c	2007-10-08 19:33:44.000000000 +0800
> @@ -160,7 +160,7 @@
>  	if (vma->vm_private_data && !(vma->vm_flags & VM_NONLINEAR))
>  		goto out;
> 
> -	if (!vma->vm_flags & VM_CAN_NONLINEAR)
> +	if (!(vma->vm_flags & VM_CAN_NONLINEAR))
>  		goto out;
> 
>  	if (end <= start || start < vma->vm_start || end > vma->vm_end)
> -
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