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

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Message-ID: <20070724043215.GA6317@mail.ustc.edu.cn>
Date:	Tue, 24 Jul 2007 12:32:15 +0800
From:	Fengguang Wu <wfg@...l.ustc.edu.cn>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	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 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"?

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