Re: [PATCH v4 1/5] mm: add mkwrite param to vm_insert_mixed()

Jan Kara
2017-07-24T15:59:12Z

On Mon 24-07-17 09:23:57, Ross Zwisler wrote:
> On Mon, Jul 24, 2017 at 01:25:30PM +0200, Jan Kara wrote:
> > > @@ -1658,14 +1658,28 @@ 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) {
> > > + if (WARN_ON_ONCE(pte_pfn(*pte) != pfn_t_to_pfn(pfn)))
> >
> > Is the WARN_ON_ONCE() really appropriate here? Your testcase with private
> > mappings has triggered this situation if I'm right...
>
> Yep, I think this WARN_ON_ONCE() is correct. The test with private mappings
> had collisions between read-only DAX mappings which were being faulted in via
> insert_pfn(), and read/write COW page cache mappings which were being faulted
> in by wp_page_copy().
>
> I was hitting a false-positive warning when I had the WARN_ON_ONCE() in
> insert_pfn() outside of the mkwrite case, i.e.:
>
> if (!pte_none(*pte)) {
> if (WARN_ON_ONCE(pte_pfn(*pte) != pfn_t_to_pfn(pfn)))
> goto out_unlock;
> if (mkwrite) {
> entry = *pte;
> goto out_mkwrite;
> } else
> goto out_unlock;
> }
>
> This was triggering when one thread was faulting in a read-only DAX mapping
> when another thread had already faulted in a read-write COW page cache page.
>
> The patches I sent out have the warning in the mkwrite case, which would mean
> that we were getting a fault for a read/write PTE in insert_pfn() and the PFN
> didn't match what was already in the PTE.
>
> This can't ever happen in the private mapping case because we will never
> install a read/write PTE for normal storage, only for COW page cache pages.
> Essentially I don't think we should ever be able to hit this warning, and if
> we do I'd like to get the bug report so that I can track down how it was
> happening and make sure that it's safe. It is in the mkwrite path of
> insert_pfn() which is currently only used by the DAX code.
>
> Does that make sense to you, or would you recommend leaving it out? (If so,
> why?)

Ah, OK, makes sense. So feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR