Re: [PATCH 16/20] mm: Provide helper for finishing mkwrite faults

[Jan Kara <jack@suse.cz>]

On Wed 19-10-16 11:21:52, Ross Zwisler wrote:
> On Wed, Oct 19, 2016 at 09:16:00AM +0200, Jan Kara wrote:
> > > > @@ -2315,26 +2335,17 @@ static int wp_page_shared(struct vm_fault *vmf)
> > > >  			put_page(vmf->page);
> > > >  			return tmp;
> > > >  		}
> > > > -		/*
> > > > -		 * Since we dropped the lock we need to revalidate
> > > > -		 * the PTE as someone else may have changed it.  If
> > > > -		 * they did, we just return, as we can count on the
> > > > -		 * MMU to tell us if they didn't also make it writable.
> > > > -		 */
> > > > -		vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
> > > > -						vmf->address, &vmf->ptl);
> > > > -		if (!pte_same(*vmf->pte, vmf->orig_pte)) {
> > > > +		tmp = finish_mkwrite_fault(vmf);
> > > > +		if (unlikely(!tmp || (tmp &
> > > > +				      (VM_FAULT_ERROR | VM_FAULT_NOPAGE)))) {
> > > 
> > > The 'tmp' return from finish_mkwrite_fault() can only be 0 or VM_FAULT_WRITE.
> > > I think this test should just be 
> > > 
> > > 		if (unlikely(!tmp)) {
> > 
> > Right, finish_mkwrite_fault() cannot currently throw other errors than
> > "retry needed" which is indicated by tmp == 0. However I'd prefer to keep
> > symmetry with finish_fault() handler which can throw other errors and
> > better be prepared to handle them from finish_mkwrite_fault() as well.
> 
> Fair enough.  You can add:
> 
> Reviewed-by: Ross Zwisler <ross.zwisler@linux.intel.com>

Thanks. Actually, your question made me have a harder look at return values
from finish_mkwrite_fault() and I've added one more commit switching the
return values so that finish_mkwrite_fault() returns 0 on success and
VM_FAULT_NOPAGE if PTE changed. That is less confusing and even more
consistent with what finish_fault() returns.

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

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