From: Tang Junhui <tang.junhui@xxxxxxxxxx> 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. >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. > >Reported-by: Jack Douglas <jack@xxxxxxxxxxxxxxxxxxxxxxx> >Signed-off-by: Michael Lyle <mlyle@xxxxxxxx> >--- > drivers/md/bcache/writeback.c | 17 +++++++++++++++-- > 1 file changed, 15 insertions(+), 2 deletions(-) > >diff --git a/drivers/md/bcache/writeback.c b/drivers/md/bcache/writeback.c >index 56a37884ca8b..ddbbeec1f0ee 100644 >--- a/drivers/md/bcache/writeback.c >+++ b/drivers/md/bcache/writeback.c >@@ -24,10 +24,23 @@ static void __update_writeback_rate(struct cached_dev *dc) > struct cache_set *c = dc->disk.c; > uint64_t cache_sectors = c->nbuckets * c->sb.bucket_size - > bcache_flash_devs_sectors_dirty(c); >+ /* >+ * Unfortunately we don't know the exact share of dirty data for >+ * each backing device. Therefore, we need to infer the writeback >+ * for each disk based on its assumed proportion of dirty data. >+ * >+ * 16384 is chosen here as something that each backing device should >+ * be a reasonable fraction of the share, and not to blow up until >+ * individual backing devices are a petabyte. >+ */ >+ uint32_t bdev_share_per16k = >+ div64_u64(bdev_sectors(dc->bdev) << 14, >+ c->cached_dev_sectors); >+ > 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 = (cache_dirty_target * bdev_share_per16k) >> 14; > > /* > * PI controller: >-- >2.14.1 > Thanks, Tang Junhui
