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

[Kevin Hao <haokexin@gmail.com>]

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On Tue, Feb 02, 2021 at 08:19:54AM -0800, Alexander Duyck wrote:
> 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.

On arm64:

       if (align)
               offset = ALIGN_DOWN(offset, align);

	4b1503e2        neg     w2, w21
	710002bf        cmp     w21, #0x0
	0a020082        and     w2, w4, w2
	1a841044        csel    w4, w2, w4, ne  // ne = any


	offset &= align_mask

	0a0402a4        and     w4, w21, w4


Yes, we do cut 3 instructions by using align mask.

> 
> 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.

Will do.

Thanks,
Kevin


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