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

[Jan Kara <jack-AlSwsSmVLrQ@public.gmane.org>]

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
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