Re: [PATCH v5 1/5] mm: add vm_insert_mixed_mkwrite()

[Kirill A. Shutemov]

On Mon, Jul 24, 2017 at 11:06:12AM -0600, Ross Zwisler wrote:
> @@ -1658,14 +1658,35 @@ static int insert_pfn(struct vm_area_struct *vma, unsigned long addr,
>  	if (!pte)
>  		goto out;
>  	retval = -EBUSY;
> -	if (!pte_none(*pte))
> -		goto out_unlock;
> +	if (!pte_none(*pte)) {
> +		if (mkwrite) {
> +			/*
> +			 * For read faults on private mappings the PFN passed
> +			 * in may not match the PFN we have mapped if the
> +			 * mapped PFN is a writeable COW page.  In the mkwrite
> +			 * case we are creating a writable PTE for a shared
> +			 * mapping and we expect the PFNs to match.
> +			 */

Can we?

I guess it's up to filesystem if it wants to reuse the same spot to write
data or not. I think your assumptions works for ext4 and xfs. I wouldn't
be that sure for btrfs or other filesystems with CoW support.

-- 
 Kirill A. Shutemov