Re: [PATCH rfc v4 2/4] page_pool: add interface to manipulate bias in page pool

[Alexander Duyck <alexander.duyck@gmail.com>]

On Tue, Jul 13, 2021 at 2:25 AM Yunsheng Lin <linyunsheng@huawei.com> 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 bias for frag page support
> in page pool for 32 bit systems with 64 bit dma, which
> should be rare those days.
>
> For normal system, the dma_addr[1] in 'struct page' is not
> used, so we can reuse the dma_addr[1] for storing bias.
>
> The PAGE_POOP_USE_DMA_ADDR_1 macro is used to decide where
> to store the bias, as the "sizeof(dma_addr_t) > sizeof(
> unsigned long)" is false for normal system, so hopefully the
> compiler will optimize out the unused code for those system.

I assume the name is a typo and you meant PAGE_POOL_USE_DMA_ADDR_1?

> The newly added page_pool_set_bias() should be called before
> the page is passed to any user. Otherwise, call the newly
> added page_pool_atomic_sub_bias_return().
>
> Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
> ---
>  include/net/page_pool.h | 70 ++++++++++++++++++++++++++++++++++++++++++++++---
>  net/core/page_pool.c    | 10 +++++++
>  2 files changed, 77 insertions(+), 3 deletions(-)
>
> diff --git a/include/net/page_pool.h b/include/net/page_pool.h
> index 8d7744d..315b9f2 100644
> --- a/include/net/page_pool.h
> +++ b/include/net/page_pool.h
> @@ -198,21 +198,85 @@ static inline void page_pool_recycle_direct(struct page_pool *pool,
>         page_pool_put_full_page(pool, page, true);
>  }
>
> +#define PAGE_POOP_USE_DMA_ADDR_1       (sizeof(dma_addr_t) > sizeof(unsigned long))
> +
>  static inline dma_addr_t page_pool_get_dma_addr(struct page *page)
>  {
> -       dma_addr_t ret = page->dma_addr[0];
> -       if (sizeof(dma_addr_t) > sizeof(unsigned long))
> +       dma_addr_t ret;
> +
> +       if (PAGE_POOP_USE_DMA_ADDR_1) {
> +               ret = READ_ONCE(page->dma_addr[0]) & PAGE_MASK;
>                 ret |= (dma_addr_t)page->dma_addr[1] << 16 << 16;

Alternatively we could change things a bit and rename things so we
have the MSB of dma_addr where dma_addr[1] is and we rename
dma_addr[0] to pp_frag_count we could have it also contain the lower
bits and handle it like so:
    ret = page->dma_addr;
    if (PAGE_POOL_DMA_USE_PP_FRAG_COUNT) {
        ret <<= 32;
        ret |= atomic_long_read(&page->pp_frag_count) & PAGE_MASK;
    }

> +       } else {
> +               ret = page->dma_addr[0];
> +       }
> +
>         return ret;
>  }
>
>  static inline void page_pool_set_dma_addr(struct page *page, dma_addr_t addr)
>  {
>         page->dma_addr[0] = addr;
> -       if (sizeof(dma_addr_t) > sizeof(unsigned long))
> +       if (PAGE_POOP_USE_DMA_ADDR_1)
>                 page->dma_addr[1] = upper_32_bits(addr);

So assuming similar logic to above we could do something like:
    if (PAGE_POOL_DMA_USE_PP_FRAG_COUNT) {
        atomic_long_set(&page->pp_frag_count, addr & PAGE_MASK);
        addr >>= 32;
    }
    pp->dma_addr = addr;

>  }
>
> +static inline int page_pool_atomic_sub_bias_return(struct page *page, int nr)
> +{
> +       int bias;
> +
> +       if (PAGE_POOP_USE_DMA_ADDR_1) {
> +               unsigned long *bias_ptr = &page->dma_addr[0];
> +               unsigned long old_bias = READ_ONCE(*bias_ptr);
> +               unsigned long new_bias;
> +
> +               do {
> +                       bias = (int)(old_bias & ~PAGE_MASK);
> +
> +                       /* Warn when page_pool_dev_alloc_pages() is called
> +                        * with PP_FLAG_PAGE_FRAG flag in driver.
> +                        */
> +                       WARN_ON(!bias);
> +
> +                       /* already the last user */
> +                       if (!(bias - nr))
> +                               return 0;
> +
> +                       new_bias = old_bias - nr;
> +               } while (!try_cmpxchg(bias_ptr, &old_bias, new_bias));
> +
> +               WARN_ON((new_bias & PAGE_MASK) != (old_bias & PAGE_MASK));
> +
> +               bias = new_bias & ~PAGE_MASK;
> +       } else {
> +               atomic_t *v = (atomic_t *)&page->dma_addr[1];

The problem with casting like this is that it makes assumptions about
byte ordering in the case that atomic_t is a 32b value and dma_addr is
a long value.

> +
> +               if (atomic_read(v) == nr)
> +                       return 0;
> +
> +               bias = atomic_sub_return(nr, v);
> +               WARN_ON(bias < 0);
> +       }

Rather than have 2 versions of this function it might work better to
just use the atomic_long version of these functions instead. Then you
shouldn't need to have two versions of the code.

You could just modify the block on the end to check for new_frag_count
vs old_frag_count if PAGE_POOL_USE_PP_FRAG_COUNT is true, or
new_frag_count < 0 if false.

> +
> +       return bias;
> +}
> +
> +static inline void page_pool_set_bias(struct page *page, int bias)
> +{
> +       if (PAGE_POOP_USE_DMA_ADDR_1) {
> +               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);
> +       } else {
> +               atomic_t *v = (atomic_t *)&page->dma_addr[1];
> +
> +               atomic_set(v, bias);
> +       }

Similarly here you could just update bias to include the dma_addr in
the if case, and then use atomic_long_set for both cases.

> +}
> +
>  static inline bool is_page_pool_compiled_in(void)
>  {
>  #ifdef CONFIG_PAGE_POOL
> diff --git a/net/core/page_pool.c b/net/core/page_pool.c
> index 78838c6..6ac5b00 100644
> --- a/net/core/page_pool.c
> +++ b/net/core/page_pool.c
> @@ -198,6 +198,16 @@ static bool page_pool_dma_map(struct page_pool *pool, struct page *page)
>         if (dma_mapping_error(pool->p.dev, dma))
>                 return false;
>
> +       if (PAGE_POOP_USE_DMA_ADDR_1 &&
> +           WARN_ON(pool->p.flags & PP_FLAG_PAGE_FRAG &&
> +                   dma & ~PAGE_MASK)) {
> +               dma_unmap_page_attrs(pool->p.dev, dma,
> +                                    PAGE_SIZE << pool->p.order,
> +                                    pool->p.dma_dir,
> +                                    DMA_ATTR_SKIP_CPU_SYNC);
> +               return false;
> +       }
> +
>         page_pool_set_dma_addr(page, dma);
>
>         if (pool->p.flags & PP_FLAG_DMA_SYNC_DEV)
> --
> 2.7.4
>