[Intel-gfx] [PATCH 1/2] mm: Report attempts to overwrite PTE from remap_pfn_range()

[Chris Wilson]

On Fri, Jun 13, 2014 at 05:26:17PM +0100, Chris Wilson wrote:
> When using remap_pfn_range() from a fault handler, we are exposed to
> races between concurrent faults. Rather than hitting a BUG, report the
> error back to the caller, like vm_insert_pfn().
> 
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Andrew Morton <akpm at linux-foundation.org>
> Cc: "Kirill A. Shutemov" <kirill.shutemov at linux.intel.com>
> Cc: Peter Zijlstra <peterz at infradead.org>
> Cc: Rik van Riel <riel at redhat.com>
> Cc: Mel Gorman <mgorman at suse.de>
> Cc: Cyrill Gorcunov <gorcunov at gmail.com>
> Cc: Johannes Weiner <hannes at cmpxchg.org>
> Cc: linux-mm at kvack.org
> ---
>  mm/memory.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 037b812a9531..6603a9e6a731 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2306,19 +2306,23 @@ static int remap_pte_range(struct mm_struct *mm, pmd_t *pmd,
>  {
>  	pte_t *pte;
>  	spinlock_t *ptl;
> +	int ret = 0;
>  
>  	pte = pte_alloc_map_lock(mm, pmd, addr, &ptl);
>  	if (!pte)
>  		return -ENOMEM;
>  	arch_enter_lazy_mmu_mode();
>  	do {
> -		BUG_ON(!pte_none(*pte));
> +		if (!pte_none(*pte)) {
> +			ret = -EBUSY;
> +			break;
> +		}
>  		set_pte_at(mm, addr, pte, pte_mkspecial(pfn_pte(pfn, prot)));
>  		pfn++;
>  	} while (pte++, addr += PAGE_SIZE, addr != end);
>  	arch_leave_lazy_mmu_mode();
>  	pte_unmap_unlock(pte - 1, ptl);

Oh. That will want the EBUSY path to increment pte or we will try to
unmap the wrong page.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre