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

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Date:	Wed, 30 Nov 2011 08:37:16 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Jan Kara <jack@...e.cz>
Cc:	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

(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;
        }

Thanks,
Fengguang
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