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

Ilias Apalodimas
2021-05-11T14:11:24Z

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