Re: [PATCH 03/10] readahead: combine file_ra_state.prev_index/prev_offset into prev_pos

[Fengguang Wu <wfg@mail.ustc.edu.cn>]

On Mon, Jul 23, 2007 at 09:53:10PM -0700, Andrew Morton wrote:
> On Tue, 24 Jul 2007 12:32:15 +0800 Fengguang Wu <wfg@mail.ustc.edu.cn> 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@mail.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"?
> 
> umm, if one really cared one could do
> 
> 	<expr> == 1 || <expr> == 0

Yeah, I'd prefer this if we are to change it.

> or something.  But whatever - let's leave it as-is.