Re: [PATCH net-next v2 1/4] mm: page_frag: Introduce page_frag_alloc_align()

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

On Sat, Jan 30, 2021 at 11:54 PM Kevin Hao <haokexin@gmail.com> wrote:
>
> In the current implementation of page_frag_alloc(), it doesn't have
> any align guarantee for the returned buffer address. But for some
> hardwares they do require the DMA buffer to be aligned correctly,
> so we would have to use some workarounds like below if the buffers
> allocated by the page_frag_alloc() are used by these hardwares for
> DMA.
>     buf = page_frag_alloc(really_needed_size + align);
>     buf = PTR_ALIGN(buf, align);
>
> These codes seems ugly and would waste a lot of memories if the buffers
> are used in a network driver for the TX/RX. So introduce
> page_frag_alloc_align() to make sure that an aligned buffer address is
> returned.
>
> Signed-off-by: Kevin Hao <haokexin@gmail.com>
> Acked-by: Vlastimil Babka <vbabka@suse.cz>
> ---
> v2:
>   - Inline page_frag_alloc()
>   - Adopt Vlastimil's suggestion and add his Acked-by
>
>  include/linux/gfp.h | 12 ++++++++++--
>  mm/page_alloc.c     |  8 +++++---
>  2 files changed, 15 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> index 6e479e9c48ce..39f4b3070d09 100644
> --- a/include/linux/gfp.h
> +++ b/include/linux/gfp.h
> @@ -583,8 +583,16 @@ extern void free_pages(unsigned long addr, unsigned int order);
>
>  struct page_frag_cache;
>  extern void __page_frag_cache_drain(struct page *page, unsigned int count);
> -extern void *page_frag_alloc(struct page_frag_cache *nc,
> -                            unsigned int fragsz, gfp_t gfp_mask);
> +extern void *page_frag_alloc_align(struct page_frag_cache *nc,
> +                                  unsigned int fragsz, gfp_t gfp_mask,
> +                                  int align);
> +
> +static inline void *page_frag_alloc(struct page_frag_cache *nc,
> +                            unsigned int fragsz, gfp_t gfp_mask)
> +{
> +       return page_frag_alloc_align(nc, fragsz, gfp_mask, 0);
> +}
> +
>  extern void page_frag_free(void *addr);
>
>  #define __free_page(page) __free_pages((page), 0)
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 519a60d5b6f7..4667e7b6993b 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5137,8 +5137,8 @@ void __page_frag_cache_drain(struct page *page, unsigned int count)
>  }
>  EXPORT_SYMBOL(__page_frag_cache_drain);
>
> -void *page_frag_alloc(struct page_frag_cache *nc,
> -                     unsigned int fragsz, gfp_t gfp_mask)
> +void *page_frag_alloc_align(struct page_frag_cache *nc,
> +                     unsigned int fragsz, gfp_t gfp_mask, int align)

I would make "align" unsigned since really we are using it as a mask.
Actually passing it as a mask might be even better. More on that
below.

>  {
>         unsigned int size = PAGE_SIZE;
>         struct page *page;
> @@ -5190,11 +5190,13 @@ void *page_frag_alloc(struct page_frag_cache *nc,
>         }
>
>         nc->pagecnt_bias--;
> +       if (align)
> +               offset = ALIGN_DOWN(offset, align);
>         nc->offset = offset;
>
>         return nc->va + offset;
>  }
> -EXPORT_SYMBOL(page_frag_alloc);
> +EXPORT_SYMBOL(page_frag_alloc_align);
>
>  /*
>   * Frees a page fragment allocated out of either a compound or order 0 page.

Rather than using the conditional branch it might be better to just do
"offset &= align_mask". Then you would be adding at most 1 instruction
which can likely occur in parallel with the other work that is going
on versus the conditional branch which requires a test, jump, and then
the 3 alignment instructions to do the subtraction, inversion, and
AND.

However it would ripple through the other patches as you would also
need to update you other patches to assume ~0 in the unaligned case,
however with your masked cases you could just use the negative
alignment value to generate your mask which would likely be taken care
of by the compiler.