linux-kernel - Re: [PATCH] readahead:add blk_run_backing

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Date:	Wed, 27 May 2009 13:06:12 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	kosaki.motohiro@...fujitsu.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hisashi Hifumi <hifumi.hisashi@....ntt.co.jp>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"jens.axboe@...cle.com" <jens.axboe@...cle.com>
Subject: Re: [PATCH] readahead:add blk_run_backing_dev

> > Ah.  So it's likely to be some strange interaction with the RAID setup.
> 
> The normal case is, if page N become uptodate at time T(N), then
> T(N) <= T(N+1) holds. But for RAID, the data arrival time depends on
> runtime status of individual disks, which breaks that formula. So
> in do_generic_file_read(), just after submitting the async readahead IO
> request, the current page may well be uptodate, so the page won't be locked,
> and the block device won't be implicitly unplugged:

Hifumi-san, Can you get blktrace data and confirm Wu's assumption?


> 
>                if (PageReadahead(page))
>                         page_cache_async_readahead()
>                 if (!PageUptodate(page))
>                                 goto page_not_up_to_date;
>                 //...
> page_not_up_to_date:
>                 lock_page_killable(page);
> 
> 
> Therefore explicit unplugging can help, so
> 
>         Acked-by: Wu Fengguang <fengguang.wu@...el.com> 
> 
> The only question is, shall we avoid the double unplug by doing this?
> 
> ---
>  mm/readahead.c |   10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> --- linux.orig/mm/readahead.c
> +++ linux/mm/readahead.c
> @@ -490,5 +490,15 @@ page_cache_async_readahead(struct addres
>  
>  	/* do read-ahead */
>  	ondemand_readahead(mapping, ra, filp, true, offset, req_size);
> +
> +	/*
> +	 * Normally the current page is !uptodate and lock_page() will be
> +	 * immediately called to implicitly unplug the device. However this
> +	 * is not always true for RAID conifgurations, where data arrives
> +	 * not strictly in their submission order. In this case we need to
> +	 * explicitly kick off the IO.
> +	 */
> +	if (PageUptodate(page))
> +		blk_run_backing_dev(mapping->backing_dev_info, NULL);
>  }
>  EXPORT_SYMBOL_GPL(page_cache_async_readahead);



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