Re: [for-416 PATCH] bcache: fix writeback target calc on large devices

Tang Junhui--

Thank you for the feedback.

On Mon, Jan 1, 2018 at 10:33 PM,  [off-list ref] wrote:

From: Tang Junhui <redacted>

This patch is useful for preventing the overflow of the expression
(cache_dirty_target * bdev_sectors(dc->bdev)), but it also
lead into a calc error, for example, when there is a 1G and
100*164G cached device, it would cause the "target" value to
be aways zero of the 1G device, which would cause write-back
threshold losing efficacy.

Maybe at first we can judge if it overflows or not of the expression
(cache_dirty_target * bdev_sectors(dc->bdev)), if it overflows,
We can calc the value of target as the patch, otherwise,
we calc it as old way.

Maybe it'd be preferable just to ensure that share always >=1.  It
seems like a pretty narrow set of cases where the current math works
and the new math doesn't work, though, as I expect that it's
relatively rare to have such a variation in sizes, and even so 16.4TB
uses up 35 bits of the 64 bit quantity.

I don't really like special cases or trying two different ways to do
the math, because then it's very difficult to test.

What do you think?

Mike