netdev - Re: [PATCH net-next v2 1/4] mm: page_frag: Introduce page_frag_alloc

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Date:   Tue, 2 Feb 2021 11:36:18 +0000
From:   Ioana Ciornei <ioana.ciornei@....com>
To:     Kevin Hao <haokexin@...il.com>
CC:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net-next v2 1/4] mm: page_frag: Introduce
 page_frag_alloc_align()

On Sun, Jan 31, 2021 at 03:44:23PM +0800, Kevin Hao 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.

Isn't the memory wasted even with this change?

I am not familiar with the frag allocator so I might be missing
something, but from what I understood each page_frag_cache keeps only
the offset inside the current page being allocated, offset which you
ALIGN_DOWN() to match the alignment requirement. I don't see how that
memory between the non-aligned and aligned offset is going to be used
again before the entire page is freed.

> So introduce
> page_frag_alloc_align() to make sure that an aligned buffer address is
> returned.
> 
> Signed-off-by: Kevin Hao <haokexin@...il.com>
> Acked-by: Vlastimil Babka <vbabka@...e.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)
>  {
>  	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.
> -- 
> 2.29.2
>