Re: [PATCH 2/5] mm/readahead: Change the condition for SetPageReadahead

[Fengguang Wu <fengguang.wu@intel.com>]

On Sat, Sep 22, 2012 at 04:03:11PM +0530, raghu.prabhu13@gmail.com wrote:
> From: Raghavendra D Prabhu <rprabhu@wnohang.net>
> 
> If page lookup from radix_tree_lookup is successful and its index page_idx ==
> nr_to_read - lookahead_size, then SetPageReadahead never gets called, so this
> fixes that.

NAK. Sorry. It's actually an intentional behavior, so that for the
common cases of many cached files that are accessed frequently, no
PG_readahead will be set at all to pointlessly trap into the readahead
routines once and again.

Perhaps we need a patch for commenting that case. :)

Thanks,
Fengguang

> Signed-off-by: Raghavendra D Prabhu <rprabhu@wnohang.net>
> ---
>  mm/readahead.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/readahead.c b/mm/readahead.c
> index 461fcc0..fec726c 100644
> --- a/mm/readahead.c
> +++ b/mm/readahead.c
> @@ -189,8 +189,10 @@ __do_page_cache_readahead(struct address_space *mapping, struct file *filp,
>  			break;
>  		page->index = page_offset;
>  		list_add(&page->lru, &page_pool);
> -		if (page_idx == nr_to_read - lookahead_size)
> +		if (page_idx >= nr_to_read - lookahead_size) {
>  			SetPageReadahead(page);
> +			lookahead_size = 0;
> +		}
>  		ret++;
>  	}
>  
> -- 
> 1.7.12.1

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