Re: [PATCH rfc v3 2/4] page_pool: add interface for getting and setting pagecnt_bias

[Jesper Dangaard Brouer]

On 12/07/2021 18.02, Alexander Duyck wrote:
> On Mon, Jul 12, 2021 at 5:17 AM Yunsheng Lin <linyunsheng@xxxxxxxxxx> wrote:
> > As suggested by Alexander, "A DMA mapping should be page
> > aligned anyway so the lower 12 bits would be reserved 0",
> > so it might make more sense to repurpose the lower 12 bits
> > of the dma address to store the pagecnt_bias for frag page
> > support in page pool.
> >
> > As newly added page_pool_get_pagecnt_bias() may be called
> > outside of the softirq context, so annotate the access to
> > page->dma_addr[0] with READ_ONCE() and WRITE_ONCE().
> >
> > And page_pool_get_pagecnt_bias_ptr() is added to implement
> > the pagecnt_bias atomic updating when a page is passsed to
> > the user.
> >
> > Other three interfaces using page->dma_addr[0] is only called
> > in the softirq context during normal rx processing, hopefully
> > the barrier in the rx processing will ensure the correct order
> > between getting and setting pagecnt_bias.
> >
> > Signed-off-by: Yunsheng Lin <linyunsheng@xxxxxxxxxx>
> > ---
> >   include/net/page_pool.h | 29 +++++++++++++++++++++++++++--
> >   net/core/page_pool.c    |  8 +++++++-
> >   2 files changed, 34 insertions(+), 3 deletions(-)
> >
> > diff --git a/include/net/page_pool.h b/include/net/page_pool.h
> > index 8d7744d..84cd972 100644
> > --- a/include/net/page_pool.h
> > +++ b/include/net/page_pool.h
> > @@ -200,17 +200,42 @@ static inline void page_pool_recycle_direct(struct page_pool *pool,
> >
> >   static inline dma_addr_t page_pool_get_dma_addr(struct page *page)
> >   {
> > -       dma_addr_t ret = page->dma_addr[0];
> > +       dma_addr_t ret = READ_ONCE(page->dma_addr[0]) & PAGE_MASK;
> >          if (sizeof(dma_addr_t) > sizeof(unsigned long))
> >                  ret |= (dma_addr_t)page->dma_addr[1] << 16 << 16;
> >          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)
> >   {
> > +       if (WARN_ON(addr & ~PAGE_MASK))
> > +               return false;
> > +
> >          page->dma_addr[0] = addr;
> >          if (sizeof(dma_addr_t) > sizeof(unsigned long))
> >                  page->dma_addr[1] = upper_32_bits(addr);
> > +
> > +       return true;
> > +}
> > +
>
> Rather than making this a part of the check here it might make more
> sense to pull this out and perform the WARN_ON after the check for
> dma_mapping_error.

I need to point out that I don't like WARN_ON and BUG_ON code in 
fast-path code, because compiler adds 'ud2' assembler instructions that 
influences the instruction-cache fetching in the CPU.  Yes, I have seen 
a measuresable impact from this before.


> Also it occurs to me that we only really have to do this in the case
> where dma_addr_t is larger than the size of a long. Otherwise we could
> just have the code split things so that dma_addr[0] is the dma_addr
> and dma_addr[1] is our pagecnt_bias value in which case we could
> probably just skip the check.

The dance to get 64-bit DMA addr on 32-bit systems is rather ugly and 
confusing, sadly.  We could take advantage of this, I just hope this 
will not make it uglier.


> > +static inline int page_pool_get_pagecnt_bias(struct page *page)
> > +{
> > +       return READ_ONCE(page->dma_addr[0]) & ~PAGE_MASK;
> > +}
> > +
> > +static inline unsigned long *page_pool_pagecnt_bias_ptr(struct page *page)
> > +{
> > +       return page->dma_addr;
> > +}
> > +
> > +static inline void page_pool_set_pagecnt_bias(struct page *page, int bias)
> > +{
> > +       unsigned long dma_addr_0 = READ_ONCE(page->dma_addr[0]);
> > +
> > +       dma_addr_0 &= PAGE_MASK;
> > +       dma_addr_0 |= bias;
> > +
> > +       WRITE_ONCE(page->dma_addr[0], dma_addr_0);
> >   }
> >
> >   static inline bool is_page_pool_compiled_in(void)
> > diff --git a/net/core/page_pool.c b/net/core/page_pool.c
> > index 78838c6..1abefc6 100644
> > --- a/net/core/page_pool.c
> > +++ b/net/core/page_pool.c
> > @@ -198,7 +198,13 @@ static bool page_pool_dma_map(struct page_pool *pool, struct page *page)
> >          if (dma_mapping_error(pool->p.dev, dma))
> >                  return false;
>
> So instead of adding to the function below you could just add your
> WARN_ON check here with the unmapping call.
>
> > -       page_pool_set_dma_addr(page, dma);
> > +       if (unlikely(!page_pool_set_dma_addr(page, dma))) {
> > +               dma_unmap_page_attrs(pool->p.dev, dma,
> > +                                    PAGE_SIZE << pool->p.order,
> > +                                    pool->p.dma_dir,
> > +                                    DMA_ATTR_SKIP_CPU_SYNC);
> > +               return false;
> > +       }
> >
> >          if (pool->p.flags & PP_FLAG_DMA_SYNC_DEV)
> >                  page_pool_dma_sync_for_device(pool, page, pool->p.max_len);
> > --
> > 2.7.4