Re: [PATCH v7 11/12] zsmalloc: page migration support

[Vlastimil Babka]

On 06/02/2016 02:25 AM, Minchan Kim wrote:
> On Wed, Jun 01, 2016 at 04:09:26PM +0200, Vlastimil Babka wrote:
> > On 06/01/2016 01:21 AM, Minchan Kim wrote:
> > > +	reset_page(page);
> > > +	put_page(page);
> > > +	page = newpage;
> > > +
> > > +	ret = 0;
> > > +unpin_objects:
> > > +	for (addr = s_addr + offset; addr < s_addr + pos;
> > > +						addr += class->size) {
> > > +		head = obj_to_head(page, addr);
> > > +		if (head & OBJ_ALLOCATED_TAG) {
> > > +			handle = head & ~OBJ_ALLOCATED_TAG;
> > > +			if (!testpin_tag(handle))
> > > +				BUG();
> > > +			unpin_tag(handle);
> > > +		}
> > > +	}
> > > +	kunmap_atomic(s_addr);
> >
> > The above seems suspicious to me. In the success case, page points to
> > newpage, but s_addr is still the original one?
>
> s_addr is virtual adress of old page by kmap_atomic so page pointer of
> new page doesn't matter.

Hmm, I see. The value (head address/handle) it reads from the old page 
should be the same as the one in the newpage. And this value doesn't get 
changed in the process. So it works, it's just subtle :)