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

[Ilias Apalodimas]

On Tue, 11 May 2021 at 17:19, Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> 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];
>
> ?

Nope not at all, either would work. we'll switch to that