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

[Matthew Wilcox]

On Tue, May 11, 2021 at 05:11:13PM +0300, Ilias Apalodimas wrote:
> 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?

I'd rather you didn't use page_private ... Any reason not to use:

			unsigned long pp_magic;
			struct page_pool *pp;
			unsigned long _pp_mapping_pad;
			unsigned long dma_addr[2];

?