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

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:

quoted

@@ -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