Re: [hugepage] Fix unmap_and_free_vma backout path

['David Gibson' <david@gibson.dropbear.id.au>]

On Sun, Nov 12, 2006 at 09:29:48PM -0800, Christoph Lameter wrote:
> On Mon, 13 Nov 2006, 'David Gibson' wrote:
> 
> > This may not be all we want.  Even with this patch, performing such a
> > failing map on to of an existing mapping will clobber (unmap) that
> > pre-existing mapping.  This is in contrast to the analogous situation
> > with normal page mappings - mapping on top with a misaligned offset
> > will fail early enough not to clobber the pre-existing mapping.
> 
> Then it is best to check the huge page alignment at the 
> same place as regular alignment.

Probably, yes, although it's yet another "if (hugepage)
specialcase()".  But I still think we want the above patch as well.
It will make sure we correctly back out from any other possible
failure cases in hugetlbfs_file_mmap() - ones I haven't thought of, or
which get added later.

As far as I can tell, there is in general no guarantee that a failing
MAP_FIXED() mmap() *won't* clobber what was there before.  I believe
there are (admittedly rare) possible late failure cases in pure
normalpage paths which will result in a failed mmap() after clobbering
the prior mapping.  Any failure from the filesystem or device's
f_ops->mmap callback will do this, for example.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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