Re: [PATCH net-next v7 1/6] page_pool: frag API support for 32-bit arch with 64-bit DMA

[Ilias Apalodimas]

On Mon, 21 Aug 2023 at 11:38, Ilias Apalodimas
<ilias.apalodimas@xxxxxxxxxx> wrote:
>
> resending for the mailing list apologies for the noise.
>
>
> On Sat, 19 Aug 2023 at 00:51, Jakub Kicinski <kuba@xxxxxxxxxx> wrote:
> >
> > On Fri, 18 Aug 2023 09:12:09 +0300 Ilias Apalodimas wrote:
> > > > Right, IIUC we don't have enough space to fit dma_addr_t and the
> > > > refcount, but if we store the dma addr on a shifted u32 instead
> > > > of using dma_addr_t explicitly - the refcount should fit?
> > >
> > > struct page looks like this:
> > >
> > > unsigned long dma_addr;
> > > union {
> > >       unsigned long dma_addr_upper;
> > >       atomic_long_t pp_frag_count;
> > > };
> >
> > I could be completely misunderstanding the problem.
>
> You aren't!
>
> > Let me show you the diff of what I was thinking more or less.
> >
> > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> > index 5e74ce4a28cd..58ffa8dc745f 100644
> > --- a/include/linux/mm_types.h
> > +++ b/include/linux/mm_types.h
> > @@ -126,11 +126,6 @@ struct page {
> >                         unsigned long _pp_mapping_pad;
> >                         unsigned long dma_addr;
> >                         union {
> > -                               /**
> > -                                * dma_addr_upper: might require a 64-bit
> > -                                * value on 32-bit architectures.
> > -                                */
> > -                               unsigned long dma_addr_upper;
> >                                 /**
> >                                  * For frag page support, not supported in
> >                                  * 32-bit architectures with 64-bit DMA.
> > diff --git a/include/net/page_pool/helpers.h b/include/net/page_pool/helpers.h
> > index 94231533a369..6f87a0fa2178 100644
> > --- a/include/net/page_pool/helpers.h
> > +++ b/include/net/page_pool/helpers.h
> > @@ -212,16 +212,24 @@ static inline dma_addr_t page_pool_get_dma_addr(struct page *page)
> >         dma_addr_t ret = page->dma_addr;
> >
> >         if (PAGE_POOL_DMA_USE_PP_FRAG_COUNT)
> > -               ret |= (dma_addr_t)page->dma_addr_upper << 16 << 16;
> > +               ret <<= PAGE_SHIFT;
> >
> >         return ret;
> >  }
> >
> > -static inline void page_pool_set_dma_addr(struct page *page, dma_addr_t addr)
> > +static inline bool page_pool_set_dma_addr(struct page *page, dma_addr_t addr)
> >  {
> > +       bool failed = false;
> > +
> >         page->dma_addr = addr;
> > -       if (PAGE_POOL_DMA_USE_PP_FRAG_COUNT)
> > -               page->dma_addr_upper = upper_32_bits(addr);
> > +       if (PAGE_POOL_DMA_USE_PP_FRAG_COUNT) {
> > +               page->dma_addr >>= PAGE_SHIFT;
> > +               /* We assume page alignment to shave off bottom bits,
> > +                * if this "compression" doesn't work we need to drop.
> > +                */
> > +               failed = addr != page->dma_addr << PAGE_SHIFT;
> > +       }
> > +       return failed;
> >  }
> >
> >  static inline bool page_pool_put(struct page_pool *pool)
> > diff --git a/net/core/page_pool.c b/net/core/page_pool.c
> > index 77cb75e63aca..9ea42e242a89 100644
> > --- a/net/core/page_pool.c
> > +++ b/net/core/page_pool.c
> > @@ -211,10 +211,6 @@ static int page_pool_init(struct page_pool *pool,
> >                  */
> >         }
> >
> > -       if (PAGE_POOL_DMA_USE_PP_FRAG_COUNT &&
> > -           pool->p.flags & PP_FLAG_PAGE_FRAG)
> > -               return -EINVAL;
> > -
> >  #ifdef CONFIG_PAGE_POOL_STATS
> >         pool->recycle_stats = alloc_percpu(struct page_pool_recycle_stats);
> >         if (!pool->recycle_stats)
> > @@ -359,12 +355,19 @@ static bool page_pool_dma_map(struct page_pool *pool, struct page *page)
> >         if (dma_mapping_error(pool->p.dev, dma))
> >                 return false;
> >
> > -       page_pool_set_dma_addr(page, dma);
> > +       if (page_pool_set_dma_addr(page, dma))
> > +               goto unmap_failed;
> >
> >         if (pool->p.flags & PP_FLAG_DMA_SYNC_DEV)
> >                 page_pool_dma_sync_for_device(pool, page, pool->p.max_len);
> >
> >         return true;
> > +
> > +unmap_failed:
> > +       dma_unmap_page_attrs(pool->p.dev, dma,
> > +                            PAGE_SIZE << pool->p.order, pool->p.dma_dir,
> > +                            DMA_ATTR_SKIP_CPU_SYNC | DMA_ATTR_WEAK_ORDERING);
> > +       return false;
> >  }
>
> That seems reasonable and would work for pages > 4k as well. But is
> 16TB enough?  I am more familiar with embedded than large servers,
> which do tend to scale that high.

Right never respond before coffee .... I think this is reasonable overall.

Thanks
/Ilias
>
> Regards
> /Ilias
> >
> >  static void page_pool_set_pp_info(struct page_pool *pool,