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

[...]

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+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.

quoted

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

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+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.

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-       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