Re: [PATCH]fix VM_CAN_NONLINEAR check in sys_remap_file_pages

[Nick Piggin <nickpiggin@yahoo.com.au>]

On Monday 08 October 2007 23:37, Hugh Dickins wrote:
> 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.

Well, I think all filesystems can do VM_CAN_NONLINEAR anyway. Device
drivers and "weird" things tend to have trouble...


> 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?

We probably should keep VM_CAN_NONLINEAR for the moment, I think.
But now that we have the fallback path, we _could_ use that instead of
failing. I doubt anybody will be using nonlinear mappings on anything but
regular files for the time being, but as a trivial fix, I think this probably
should go into 2.6.23.

Thanks for spotting this problem
Acked-by: Nick Piggin <npiggin@suse.de>

> 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@21cn.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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