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

[Hugh Dickins <hughd@google.com>]

On Mon, 12 Apr 2021, Peter Xu wrote:
> 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.

I did not mean to imply that it's wrong to have pte_dirty without
pte_write: no, I agree with you, I believe that there are accepted
and generic ways in which we can have pte_dirty without pte_write
(and we could each probably add a warning somewhere which would
very quickly prove that - but those would not prove that there
are not BUG_ONs on some other path, which had been my fear).

I wanted now to demonstrate that by pointing to change_pte_range() in
mm/mprotect.c, showing that it does not clear pte_dirty when it clears
pte_write. But alarmingly found rather the reverse: that it appears to
set pte_write when it finds pte_dirty - if dirty_accountable.

That looks very wrong, but if I spent long enough following up
dirty_accountable in detail, I think I would be reassured to find that
it is only adding the pte_write there when it had removed it from the
prot passed down, for dirty accounting reasons (which apply !VM_SHARED
protections in the VM_SHARED case, so that page_mkwrite() is called
and dirty accounting done when necessary).

What I did mean to imply is that changing set_page_dirty to pte_mkdirty,
to make that userfaultfd code block look nicer, is not a change to be
done lightly: by all means try it out, test it, and send a patch after
Axel's series is in, but please do not ask Axel to make that change as
a part of his series - it would be taking a risk, just for a cleanup.

Now, I have also looked up the mail exchange with Andrea which led to
his dcf7fe9d8976 ("userfaultfd: shmem: UFFDIO_COPY: set the page dirty
if VM_WRITE is not set") - it had to be off-list at the time.  And he
was rather led to that set_page_dirty by following old patterns left
over in shmem_getpage_gfp(); but when I said "or it could be done with
pte_mkdirty without pte_mkwrite", he answered "I explicitly avoided
that because pte_dirty then has side effects on mprotect to decide
pte_write. It looks safer to do set_page_dirty and not set dirty bits
in not writable ptes unnecessarily".

Haha: I think Andrea is referring to exactly the dirty_accountable code
in change_pte_protection() which worried me above. Now, I think that
will turn out okay (shmem does not have a page_mkwrite(), and does not
participate in dirty accounting), but you will have to do some work to
assure us all of that, before sending in a cleanup patch.

Hugh