Re: [PATCH 08/33] readahead: common macros

[Nick Piggin]

Wu Fengguang wrote:

> Define some common used macros for the read-ahead logics.
>
> Signed-off-by: Wu Fengguang <wfg@xxxxxxxxxxxxxxxx>
> ---
>
> mm/readahead.c |   14 ++++++++++++--
> 1 files changed, 12 insertions(+), 2 deletions(-)
>
> --- linux-2.6.17-rc4-mm3.orig/mm/readahead.c
> +++ linux-2.6.17-rc4-mm3/mm/readahead.c
> @@ -5,6 +5,8 @@
>  *
>  * 09Apr2002	akpm@xxxxxxxxxx
>  *		Initial version.
> + * 21May2006	Wu Fengguang <wfg@xxxxxxxxxxxxxxxx>
> + *		Adaptive read-ahead framework.
>  */
>
> #include <linux/kernel.h>
> @@ -14,6 +16,14 @@
> #include <linux/blkdev.h>
> #include <linux/backing-dev.h>
> #include <linux/pagevec.h>
> +#include <linux/writeback.h>
> +#include <linux/nfsd/const.h>

How come you're adding these includes?

> +
> +#define PAGES_BYTE(size) (((size) + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT)
> +#define PAGES_KB(size)	 PAGES_BYTE((size)*1024)
Don't really like the names. Don't think they do anything for clarity, but
if you can come up with something better for PAGES_BYTE I might change my
mind ;) (just forget about PAGES_KB - people know what *1024 means)

Also: the replacements are wrong: if you've defined VM_MAX_READAHEAD to be
4095 bytes, you don't want the _actual_ readahead to be 4096 bytes, do you?
It is saying nothing about minimum, so presumably 0 is the correct choice.


> +
> +#define next_page(pg) (list_entry((pg)->lru.prev, struct page, lru))
> +#define prev_page(pg) (list_entry((pg)->lru.next, struct page, lru))

Again, it is probably easier just to use the expanded version. Then the
reader can immediately say: ah, the next page on the LRU list (rather
than, maybe, the next page in the pagecache).

> void default_unplug_io_fn(struct backing_dev_info *bdi, struct page *page)
> {
> @@ -21,7 +31,7 @@ void default_unplug_io_fn(struct backing
> EXPORT_SYMBOL(default_unplug_io_fn);
>
> struct backing_dev_info default_backing_dev_info = {
> -	.ra_pages	= (VM_MAX_READAHEAD * 1024) / PAGE_CACHE_SIZE,
> +	.ra_pages	= PAGES_KB(VM_MAX_READAHEAD),
> 	.state		= 0,
> 	.capabilities	= BDI_CAP_MAP_COPY,
> 	.unplug_io_fn	= default_unplug_io_fn,
> @@ -50,7 +60,7 @@ static inline unsigned long get_max_read
>
> static inline unsigned long get_min_readahead(struct file_ra_state *ra)
> {
> -	return (VM_MIN_READAHEAD * 1024) / PAGE_CACHE_SIZE;
> +	return PAGES_KB(VM_MIN_READAHEAD);
> }
>
> static inline void reset_ahead_window(struct file_ra_state *ra)
>
> --

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