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

Kirill A. Shutemov
2017-07-24T22:14:06Z

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