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

[Ilias Apalodimas <ilias.apalodimas@linaro.org>]

[...]
> > > +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.

Note here that we can only use this because dma_addr is not aliased to
compound page anymore (after the initial page_pool recycling patchset). 
We must keep this in mind if we even restructure struct page.

Can we do something more radical for this? The 64/32 bit dance is only
there for 32 bit systems with 64 bit dma.  Since the last time we asked
about this no one seemed to care about these, and I really doubt we'll get
an ethernet driver for them (that needs recycling....), can we *only* support 
frag allocation and recycling for 'normal' systems? We could always just r
e-purpose dma_addr[1] for those.

Regards
/Ilias

> 
> 
> > > +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
> > > 
> > 
>