linux-kernel - Re: [RFC] splice() and readahead interaction

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Date:	Mon, 7 May 2007 14:54:12 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Fengguang Wu" <fengguang.wu@...il.com>
Cc:	"Eric Dumazet" <dada1@...mosbay.com>,
	"Andi Kleen" <andi@...stfloor.org>,
	"Oleg Nesterov" <oleg@...sign.ru>,
	"Steven Pratt" <slpratt@...tin.ibm.com>,
	"Ram Pai" <linuxram@...ibm.com>, linux-kernel@...r.kernel.org,
	"Ingo Molnar" <mingo@...e.hu>
Subject: Re: [RFC] splice() and readahead interaction

On Sat, 5 May 2007 05:04:29 -0400
"Fengguang Wu" <fengguang.wu@...il.com> wrote:

> Readahead logic somehow fails to populate the page range with data.
> It can be because
> 1) the readahead routine is not always called in the following lines of
> fs/splice.c:
>         if (!loff || nr_pages > 1)
>                 page_cache_readahead(mapping, &in->f_ra, in, index,
> nr_pages);
> 2) even called, page_cache_readahead() wont guarantee the pages are there.
> It wont submit readahead I/O for pages already in the radix tree, or when
> (ra_pages == 0), or after 256 cache hits.
> 
> In your case, it should be because of the retried reads, which lead to
> excessive cache hits, and disables readahead at some time.
> 
> And that _one_ failure of readahead blocks the whole read process.
> The application receives EAGAIN and retries the read, but
> __generic_file_splice_read() refuse to make progress:
> - in the previous invocation, it has allocated a blank page and inserted it
> into the radix tree, but never has the chance to start I/O for it: the test
> of SPLICE_F_NONBLOCK goes before that.
> - in the retried invocation, the readahead code will neither get out of the
> cache hit mode, nor will it submit I/O for an already existing page.
> 
> The attached patch should fix the critical splice bug. Sorry for not being
> able to test it locally for now - I'm at home and running knoppix. And the
> readahead bug will be fixed by the upcoming on-demand readahead patch. I
> should be back and submit it after a week.
> 
> Thank you,
> Fengguang Wu
> 
> 
> [splice-nonblock-fix.patch  text/x-patch (506B)]
> --- linux-2.6.21.1/fs/splice.c.old	2007-05-05 04:40:38.000000000 -0400
> +++ linux-2.6.21.1/fs/splice.c	2007-05-05 04:41:59.000000000 -0400
> @@ -378,10 +378,11 @@
>  			 * If in nonblock mode then dont block on waiting
>  			 * for an in-flight io page
>  			 */
> -			if (flags & SPLICE_F_NONBLOCK)
> -				break;
> -
> -			lock_page(page);
> +			if (flags & SPLICE_F_NONBLOCK) {
> +				if (TestSetPageLocked(page))
> +					break;
> +			} else
> +				lock_page(page);
>  
>  			/*
>  			 * page was truncated, stop here. if this isn't the

So.. afaik we're awaiting testing results for this change?

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