On Wed, Jan 29, 2025 at 12:54:03PM +0100, David Hildenbrand wrote:
> Let's do it just like mprotect write-upgrade or during NUMA-hinting
> faults on PROT_NONE PTEs: detect if the PTE can be writable by using
> can_change_pte_writable().
>
> Set the PTE only dirty if the folio is dirty: we might not
> necessarily have a write access, and setting the PTE writable doesn't
> require setting the PTE dirty.
Not sure whether there's much difference in practice, since a device
exclusive access means a write, so the folio better be dirty (unless we
aborted halfway through). But then I couldn't find the code in nouveau to
do that, so now I'm confused.
-Sima
> With this change in place, there is no need to have separate
> readable and writable device-exclusive entry types, and we'll merge
> them next separately.
>
> Note that, during fork(), we first convert the device-exclusive entries
> back to ordinary PTEs, and we only ever allow conversion of writable
> PTEs to device-exclusive -- only mprotect can currently change them to
> readable-device-exclusive. Consequently, we always expect
> PageAnonExclusive(page)==true and can_change_pte_writable()==true,
> unless we are dealing with soft-dirty tracking or uffd-wp. But reusing
> can_change_pte_writable() for now is cleaner.
>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> ---
> mm/memory.c | 11 +++++++----
> 1 file changed, 7 insertions(+), 4 deletions(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 03efeeef895a..db38d6ae4e74 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -725,18 +725,21 @@ static void restore_exclusive_pte(struct vm_area_struct *vma,
> struct folio *folio = page_folio(page);
> pte_t orig_pte;
> pte_t pte;
> - swp_entry_t entry;
>
> orig_pte = ptep_get(ptep);
> pte = pte_mkold(mk_pte(page, READ_ONCE(vma->vm_page_prot)));
> if (pte_swp_soft_dirty(orig_pte))
> pte = pte_mksoft_dirty(pte);
>
> - entry = pte_to_swp_entry(orig_pte);
> if (pte_swp_uffd_wp(orig_pte))
> pte = pte_mkuffd_wp(pte);
> - else if (is_writable_device_exclusive_entry(entry))
> - pte = maybe_mkwrite(pte_mkdirty(pte), vma);
> +
> + if ((vma->vm_flags & VM_WRITE) &&
> + can_change_pte_writable(vma, address, pte)) {
> + if (folio_test_dirty(folio))
> + pte = pte_mkdirty(pte);
> + pte = pte_mkwrite(pte, vma);
> + }
>
> VM_BUG_ON_FOLIO(pte_write(pte) && (!folio_test_anon(folio) &&
> PageAnonExclusive(page)), folio);
> --
> 2.48.1
>
--
Simona Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
