Tang Junhui-- On Fri, Jan 5, 2018 at 11:29 PM, <tang.junhui@xxxxxxxxxx> wrote: > From: Tang Junhui <tang.junhui@xxxxxxxxxx> > > Hello Mike, > > I thought twice, and feel this patch is a little complex and still not very accurate for > small backend devices. I think we can resolve it like this: > > uint64_t cache_dirty_target = > div_u64(cache_sectors * dc->writeback_percent, 100); > > - int64_t target = div64_u64(cache_dirty_target * bdev_sectors(dc->bdev), > - c->cached_dev_sectors); > + int64_t target = div64_u64(cache_dirty_target, > + div64_u64(c->cached_dev_sectors, bdev_sectors(dc->bdev))); > > c->cached_dev_sectors is always bigger than bdev_sectors(dc->bdev), so > div64_u64(c->cached_dev_sectors, bdev_sectors(dc->bdev) would be never smaller than 1, > and this expression has no multiplication operation, so we do not need to worry > about overflows. I thought about this, but as I think you've noticed below it's very inaccurate for typical cases. e.g. if you have 1TB and 750GB backing volumes, you get (1750/1000) vs (1750/750) and the 750GB is allowed half as much dirty space, and you end up storing 150% of the desired info. Other rounding approaches aren't good, either. > we also can multiply a value(2^10) to avoid loss of accuracy in division like bellow: > (it would support all device smaller than 2^54 BYTE). > + int64_t target = div64_u64(cache_dirty_target<<10, > + div64_u64(c->cached_dev_sectors<<10, bdev_sectors(dc->bdev))); > > How do you think about? This is a bit better. It trades a per-backing-volume limit of 1PB in my code for a total of 9PB cached limit. Really all of these approaches are "wrong" because they don't allow the total amount of dirty to be used if you have quiet volumes and should be replaced with something better. I am just trying to get something temporary in to 4.16 because this is a real bug that is preventing real users from using bcache. I'm pretty sure the new code works in all the cases the old code failed. Simultaneously having a 1GB and 16TB cached volume is a bit of a silly case, and what the code does in this silly case isn't completely stupid, so... We're running out of time to get a fix into the package for 4.16, and I've tested the current version a fair bit. Mike
