Re: [PATCH net-next v4 1/4] mm: add a signature in struct page

[Ilias Apalodimas]

Hi Matthew,

On Tue, May 11, 2021 at 02:45:32PM +0100, Matthew Wilcox wrote:
> On Tue, May 11, 2021 at 03:31:15PM +0200, Matteo Croce wrote:
> > @@ -101,6 +101,7 @@ struct page {
> >  			 * 32-bit architectures.
> >  			 */
> >  			unsigned long dma_addr[2];
> > +			unsigned long signature;
> >  		};
> >  		struct {	/* slab, slob and slub */
> >  			union {
> 
> No.  Signature now aliases with page->mapping, which is going to go
> badly wrong for drivers which map this page into userspace.
> 
> I had this as:
> 
> +                       unsigned long pp_magic;
> +                       unsigned long xmi;
> +                       unsigned long _pp_mapping_pad;
>                         unsigned long dma_addr[2];
> 
> and pp_magic needs to be set to something with bits 0&1 clear and
> clearly isn't a pointer.  I went with POISON_POINTER_DELTA + 0x40.

Regardless to the changes required, there's another thing we'd like your
opinion on.
There was a change wrt to the previous patchset. We used to store the
struct xdp_mem_info into page->private.  On the new version we store the
page_pool ptr address in page->private (there's an explanation why on the
mail thread, but the tl;dr is that we can get some more speed and keeping
xdp_mem_info is not that crucial). So since we can just store the page_pool
address directly, should we keep using page->private or it's better to
do: 

+                       unsigned long pp_magic;
+                       unsigned long pp_ptr;
+                       unsigned long _pp_mapping_pad;
                        unsigned long dma_addr[2];
and use pp_ptr?

Thanks
/Ilias