Re: replace "memset(...,0,PAGE_SIZE)" calls with "clear_page()"?

[Robert P. J. Day]

On Mon, 1 Jan 2007, Folkert van Heusden wrote:

> > > regarding alignment that don't allow clear_page() to be used
> > > copy_page() in the memcpy() case), but it's going to need a lot of
>
> Maybe these optimalisations should be in the coding style docs?

i was thinking of submitting the following as a new "chapter" for the
doc -- it would address *some* of these issues:


Chapter XX:  Page-related memory management

  The following functions and macro definitions are available via
include/linux/gfp.h for page-based memory management:

  struct page *alloc_pages(gfp_mask, order);
  unsigned long __get_free_pages(gfp_mask, order);
  unsigned long get_zeroed_page(gfp_mask);
  void __free_pages(struct page *page, order);
  void free_pages(unsigned long addr, order);

  #define alloc_page(gfp_mask) alloc_pages(gfp_mask, 0)
  #define __get_free_page(gfp_mask) __get_free_pages((gfp_mask),0)
  #define __free_page(page) __free_pages((page), 0)
  #define free_page(addr) free_pages((addr),0)

  #define __get_dma_pages(gfp_mask, order) \
                __get_free_pages((gfp_mask) | GFP_DMA,(order))

  Given the above, some basic suggestions for page-based memory management:

 (a) If you need to allocate or free a single page, use the single page
     version of the routine/macro, rather than calling the multi-page
     version with an order value of zero, such as:

	alloc_pages(gfp_mask, 0);	/* no */
	alloc_page(gfp_mask);		/* better */

 (b) If you need to allocate a single zeroed page by logical address,
     use get_zeroed_page(), rather than __get_free_page() followed
     by a call to memset() to clear that page.

 (c) If you need to specifically allocate some DMA pages, use the
     __get_dma_pages() macro, as in:

	__get_free_pages(GFP_KERNEL|GFP_DMA, order)	/* no */
	__get_dma_pages(GFP_KERNEL, order)		/* better */

 (d) If you need to clear (zero) a page, be aware that every
     architecture defines a clear_page() routine, either as a macro
     in include/<arch>/page.h or as an assembler routine.

     You should check if it's appropriate to use that routine/macro,
     rather than making an explicit call to memset(...,0, PAGE_SIZE).
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