Message-ID: <42308073.CEA0DF75@tv-sign.ru>
Date: Thu, 10 Mar 2005 20:14:27 +0300
From: Oleg Nesterov <oleg@tv-sign.ru>
MIME-Version: 1.0
To: Ram <linuxram@us.ibm.com>
Cc: linux-kernel@vger.kernel.org, Steven Pratt <slpratt@austin.ibm.com>,
       Andrew Morton <akpm@osdl.org>
Subject: Re: [PATCH 2/2] readahead: improve sequential read detection
References: <42260F30.BE15B4DA@tv-sign.ru> <1110412324.4816.89.camel@localhost>
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Ram wrote:
>
> On Wed, 2005-03-02 at 11:08, Oleg Nesterov wrote:
> >
> >  out:
> > - return newsize;
> > + return ra->prev_page + 1;
>
> This change introduces one key behavioural change in
> page_cache_readahead(). Instead of returning the number-of-pages
> successfully read, it now returns the next-page-index which is yet to be
> read. Was this essential?

The problem is that with this change:
+       if (offset == ra->prev_page && --req_size)
+               ++offset;
we can't just return newsize.

Because the caller of page_cache_readahead(offset, req_size) expects
that returned value is the number-of-pages successfully read from
this original offset.

Consider do_generic_mapping_read() reading two pages at offset 10,
with ra->prev_page == 10.

1st page, index == 10:
	ret_size = page_cache_readahead(10, 2);		// returns 1
	next_index += ret_size;				// next_index == 11

2nd page, index == 11:
	if (index == next_index)			// Yes
		page_cache_readahead(11, 1);		// BOGUS!

It can be solved without behavioural change of course, but it will be
more complex.

> At least, a comment towards this effect at the top of the function is
> worth adding.

Yes, it's my fault. I should have added comment in changelog at least.

Oleg.
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