Please note I have not tested this version yet: I'm just putting it out
for review.

Mike

On 01/02/2018 11:00 AM, Michael Lyle wrote:

This fixes bcache writeback target calculation, for defects relating to
large cache sets and calculation errors when there are multiple backing
devices.

Bcache needs to scale the dirty data in the cache over the multiple
backing disks in order to calculate writeback rates for each.
The previous code did this by multiplying the target number of dirty
sectors by the backing device size, and expected it to fit into a
uint64_t; this blows up on relatively small backing devices.

The new approach figures out the bdev's share in 16384ths of the overall
cached data.  This is chosen to cope well when bdevs drastically vary in
size and to ensure that bcache can cross the petabyte boundary for each
backing device.

Per review, we ensure that every device gets at least 1/16384th of the
writeback share.  This also differs from v1 of the patchset in scaling
the error, which corrects previous problems with writeback rate
calculation across multiple devices.

Reported-by: Jack Douglas <redacted>
Signed-off-by: Michael Lyle <redacted>