RE: [PATCH][next] bcache: Use 64-bit arithmetic instead of 32-bit

[David Laight <David.Laight@ACULAB.COM>]

> >  		if (c->gc_stats.in_use <= BCH_WRITEBACK_FRAGMENT_THRESHOLD_MID) {
> > -			fp_term = dc->writeback_rate_fp_term_low *
> > +			fp_term = (int64_t)dc->writeback_rate_fp_term_low *
> >  			(c->gc_stats.in_use - BCH_WRITEBACK_FRAGMENT_THRESHOLD_LOW);
> >  		} else if (c->gc_stats.in_use <= BCH_WRITEBACK_FRAGMENT_THRESHOLD_HIGH) {
> > -			fp_term = dc->writeback_rate_fp_term_mid *
> > +			fp_term = (int64_t)dc->writeback_rate_fp_term_mid *
> >  			(c->gc_stats.in_use - BCH_WRITEBACK_FRAGMENT_THRESHOLD_MID);
> >  		} else {
> > -			fp_term = dc->writeback_rate_fp_term_high *
> > +			fp_term = (int64_t)dc->writeback_rate_fp_term_high *
> >  			(c->gc_stats.in_use - BCH_WRITEBACK_FRAGMENT_THRESHOLD_HIGH);
> >  		}
> >  		fps = div_s64(dirty, dirty_buckets) * fp_term;
> >
> 
> Hmm, should such thing be handled by compiler ?  Otherwise this kind of
> potential overflow issue will be endless time to time.
> 
> I am not a compiler expert, should we have to do such explicit type cast
> all the time ?

We do to get a 64bit product from two 32bit values.
An alternative for the above would be:
		fp_term = c->gc_stats.in_use - BCH_WRITEBACK_FRAGMENT_THRESHOLD_HIGH;
		fp_term *= dc->writeback_rate_fp_term_high;

I hope BCH_WRITEBACK_FRAGMENT_THRESHOLD_LOW is zero :-)

	David

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