linux-kernel - Re: [PATCH 8/9] readahead: basic support for backwards prefetching

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Date:	Wed, 30 Nov 2011 12:21:46 +0100
From:	Jan Kara <jack@...e.cz>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Jan Kara <jack@...e.cz>, Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	"Li, Shaohua" <shaohua.li@...el.com>,
	Linux Memory Management List <linux-mm@...ck.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 8/9] readahead: basic support for backwards prefetching

On Wed 30-11-11 08:37:16, Wu Fengguang wrote:
> (snip)
> > > @@ -676,6 +677,20 @@ ondemand_readahead(struct address_space 
> > >  	}
> > >  
> > >  	/*
> > > +	 * backwards reading
> > > +	 */
> > > +	if (offset < ra->start && offset + req_size >= ra->start) {
> > > +		ra->pattern = RA_PATTERN_BACKWARDS;
> > > +		ra->size = get_next_ra_size(ra, max);
> > > +		max = ra->start;
> > > +		if (ra->size > max)
> > > +			ra->size = max;
> > > +		ra->async_size = 0;
> > > +		ra->start -= ra->size;
> >   IMHO much more obvious way to write this is:
> > ra->size = get_next_ra_size(ra, max);
> > if (ra->size > ra->start) {
> >   ra->size = ra->start;
> >   ra->start = 0;
> > } else
> >   ra->start -= ra->size;
> 
> Good idea! Here is the updated code:
> 
>         /*
>          * backwards reading
>          */
>         if (offset < ra->start && offset + req_size >= ra->start) {
>                 ra->pattern = RA_PATTERN_BACKWARDS;
>                 ra->size = get_next_ra_size(ra, max);
>                 if (ra->size > ra->start) {
>                         /*
>                          * ra->start may be concurrently set to some huge
>                          * value, the min() at least avoids submitting huge IO
>                          * in this race condition
>                          */
>                         ra->size = min(ra->start, max);
>                         ra->start = 0;
>                 } else
>                         ra->start -= ra->size;
>                 ra->async_size = 0;
>                 goto readit;
>         }
  Looks good. You can add:
Acked-by: Jan Kara <jack@...e.cz>
  to the patch.

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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