linux-kernel - Re: [PATCH 03/10] readahead: combine file_ra_state.prev_index/prev_offset into prev

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Message-ID: <20070724043708.GA6627@mail.ustc.edu.cn>
Date:	Tue, 24 Jul 2007 12:37:08 +0800
From:	Fengguang Wu <wfg@...l.ustc.edu.cn>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 03/10] readahead: combine file_ra_state.prev_index/prev_offset into prev_pos

On Tue, Jul 24, 2007 at 12:32:15PM +0800, Fengguang Wu wrote:
> On Mon, Jul 23, 2007 at 08:55:35PM -0700, Andrew Morton wrote:
> > On Tue, 24 Jul 2007 10:00:12 +0800 Fengguang Wu <wfg@...l.ustc.edu.cn> wrote:
> > 
> > > @@ -342,11 +342,9 @@ ondemand_readahead(struct address_space 
> > >  		   bool hit_readahead_marker, pgoff_t offset,
> > >  		   unsigned long req_size)
> > >  {
> > > -	int max;	/* max readahead pages */
> > > -	int sequential;
> > > -
> > > -	max = ra->ra_pages;
> > > -	sequential = (offset - ra->prev_index <= 1UL) || (req_size > max);
> > > +	int	max = ra->ra_pages;	/* max readahead pages */
> > > +	pgoff_t prev_offset;
> > > +	int	sequential;
> > >  
> > >  	/*
> > >  	 * It's the expected callback offset, assume sequential access.
> > > @@ -360,6 +358,9 @@ ondemand_readahead(struct address_space 
> > >  		goto readit;
> > >  	}
> > >  
> > > +	prev_offset = ra->prev_pos >> PAGE_CACHE_SHIFT;
> > > +	sequential = offset - prev_offset <= 1UL || req_size > max;
> > 
> > It's a bit pointless using an opaque type for prev_offset here, and then
> > encoding the knowledge that it is implemented as "unsigned long".
> > 
> > It's a minor thing, but perhaps just "<= 1" would make more sense here.
> 
> Yeah, "<= 1" is OK.  But the expression still requires pgoff_t to be
> 'unsigned' to work correctly.
> 
> So what about "<= 1U"?

I wrote a test case and find that if pgoff_t is 'signed long',
"<= 1U" still yields the wrong result. Only "<= 1UL" does the trick :(

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