Re: [PATCH] mm: avoid setting up anonymous pages into file mapping

[Boaz Harrosh]

On 07/03/2015 05:07 PM, Kirill A. Shutemov wrote:
> Reading page fault handler code I've noticed that under right
> circumstances kernel would map anonymous pages into file mappings:
> if the VMA doesn't have vm_ops->fault() and the VMA wasn't fully
> populated on ->mmap(), kernel would handle page fault to not populated
> pte with do_anonymous_page().
> 
> There's chance that it was done intentionally, but I don't see good
> justification for this. We just hide bugs in broken drivers.
> 

Have you done a preliminary audit for these broken drivers? If they actually
exist in-tree then this patch is a regression for them.

We need to look for vm_ops without an .fault = . Perhaps define a
map_annonimous() for those to revert to the old behavior, if any
actually exist.

> Let's change page fault handler to use do_anonymous_page() only on
> anonymous VMA (->vm_ops == NULL).
> 
> For file mappings without vm_ops->fault() page fault on pte_none() entry
> would lead to SIGBUS.
> 

Again that could mean a theoretical regression for some in-tree driver,
do you know of any such driver?

Thanks
Boaz

> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> ---
>  mm/memory.c | 15 +++++++++------
>  1 file changed, 9 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 8a2fc9945b46..f3ee782059e3 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3115,6 +3115,9 @@ static int do_fault(struct mm_struct *mm, struct vm_area_struct *vma,
>  			- vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff;
>  
>  	pte_unmap(page_table);
> +
> +	if (unlikely(!vma->vm_ops->fault))
> +		return VM_FAULT_SIGBUS;
>  	if (!(flags & FAULT_FLAG_WRITE))
>  		return do_read_fault(mm, vma, address, pmd, pgoff, flags,
>  				orig_pte);
> @@ -3260,13 +3263,13 @@ static int handle_pte_fault(struct mm_struct *mm,
>  	barrier();
>  	if (!pte_present(entry)) {
>  		if (pte_none(entry)) {
> -			if (vma->vm_ops) {
> -				if (likely(vma->vm_ops->fault))
> -					return do_fault(mm, vma, address, pte,
> -							pmd, flags, entry);
> +			if (!vma->vm_ops) {
> +				return do_anonymous_page(mm, vma, address, pte,
> +						pmd, flags);
> +			} else {
> +				return do_fault(mm, vma, address, pte, pmd,
> +						flags, entry);
>  			}
> -			return do_anonymous_page(mm, vma, address,
> -						 pte, pmd, flags);
>  		}
>  		return do_swap_page(mm, vma, address,
>  					pte, pmd, flags, entry);
> 

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