Re: [PATCH v4] userfaultfd/shmem: fix MCOPY_ATOMIC_CONTINUE behavior

[Peter Xu]

Hi, Hugh,

On Tue, Apr 06, 2021 at 11:14:30PM -0700, Hugh Dickins wrote:
> > +static int mcopy_atomic_install_ptes(struct mm_struct *dst_mm, pmd_t *dst_pmd,
> > +				     struct vm_area_struct *dst_vma,
> > +				     unsigned long dst_addr, struct page *page,
> > +				     enum mcopy_atomic_mode mode, bool wp_copy)
> > +{

[...]

> > +	if (writable) {
> > +		_dst_pte = pte_mkdirty(_dst_pte);
> > +		if (wp_copy)
> > +			_dst_pte = pte_mkuffd_wp(_dst_pte);
> > +		else
> > +			_dst_pte = pte_mkwrite(_dst_pte);
> > +	} else if (vm_shared) {
> > +		/*
> > +		 * Since we didn't pte_mkdirty(), mark the page dirty or it
> > +		 * could be freed from under us. We could do this
> > +		 * unconditionally, but doing it only if !writable is faster.
> > +		 */
> > +		set_page_dirty(page);
> 
> I do not remember why Andrea or I preferred set_page_dirty() here to
> pte_mkdirty(); but I suppose there might somewhere be a BUG_ON(pte_dirty)
> which this would avoid.  Risky to change it, though it does look odd.

Is any of the possible BUG_ON(pte_dirty) going to trigger because the pte has
write bit cleared?  That's one question I was not very sure, e.g., whether one
pte is allowed to be "dirty" if it's not writable.

To me it's okay, it's actually very suitable for UFFDIO_COPY case, where it is
definitely dirty data (so we must never drop it) even if it's installed as RO,
however to achieve that we can still set the dirty on the page rather than the
pte as what we do here.  It's just a bit awkward as you said.

Meanwhile today I just noticed this in arm64 code:

static inline pte_t pte_wrprotect(pte_t pte)
{
	/*
	 * If hardware-dirty (PTE_WRITE/DBM bit set and PTE_RDONLY
	 * clear), set the PTE_DIRTY bit.
	 */
	if (pte_hw_dirty(pte))
		pte = pte_mkdirty(pte);

	pte = clear_pte_bit(pte, __pgprot(PTE_WRITE));
	pte = set_pte_bit(pte, __pgprot(PTE_RDONLY));
	return pte;
}

So arm64 will explicitly set the dirty bit (from the HW dirty bit) when
wr-protect.  It seems to prove that at least for arm64 it's very valid to have
!write && dirty pte.

Thanks,

-- 
Peter Xu