Re: [PATCH] Fix OOPS in mmap_region() when merging adjacent VM_LOCKED file segments

[Linus Torvalds <torvalds@linux-foundation.org>]

On Mon, 2 Feb 2009, Mel Gorman wrote:
>
> Do not account for address space usage when making hugetlbfs mappings RW
> 
> hugetlbfs accounts for its address space usage separate from the VM
> core. VM_ACCOUNT should not be set for its mappings but it is possible it gets
> set if a user creates a RO hugetlbfs mapping MAP_NORESERVE and then calls
> mprotect(). This patch stops VM_ACCOUNT being set for hugetlbfs mappings
> during mprotect().
> 
> Credit goes to Kosaki Motohiro for spotting this.
> 
> Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> 
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index abe2694..31ddc6a 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -153,7 +153,7 @@ mprotect_fixup(struct vm_area_struct *vma, struct vm_area_struct **pprev,
>  	 * but (without finer accounting) cannot reduce our commit if we
>  	 * make it unwritable again.
>  	 */
> -	if (newflags & VM_WRITE) {
> +	if (newflags & VM_WRITE && !(vma->vm_flags & VM_HUGETLB)) {

Wouldn't it be _much_ nicer to just depend on that whole VM_NORESERVE 
thing?

Those hugetlb mappings _should_ have VM_NORESERVE on them, so the 
following test:

>  		if (!(oldflags & (VM_ACCOUNT|VM_WRITE|
>  						VM_SHARED|VM_NORESERVE))) {
>  			charged = nrpages;

should do it all correctly.

Why make up some ad-hoc testing, when we already have a flag for _exactly_ 
this issue. That's what VM_NORESERVE means: don't apply VM_ACCOUNT.

IOW, I don't see the point of this patch at all.

And if there is some hugetlb path that doesn't set VM_NORESERVE, then fix 
_that_ instead.

			Linus