On Sat, Jan 30, 2021 at 11:54 PM Kevin Hao <haoke...@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 <haoke...@gmail.com>
> Acked-by: Vlastimil Babka <vba...@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.
