zram writeback is a costly operation, because every target slot (unless ZRAM_HUGE) is decompressed before it gets written to a backing device. The writeback to a backing device uses submit_bio_wait() which may look like a rescheduling point. However, if the backing device has BD_HAS_SUBMIT_BIO bit set __submit_bio() calls directly disk->fops->submit_bio(bio) on the backing device and so when submit_bio_wait() calls blk_wait_io() the I/O is already done. On such systems we effective end up in a loop

for_each (target slot) { decompress(slot) __submit_bio() disk->fops->submit_bio(bio) }

Which on PREEMPT_NONE systems triggers watchdogs (since there are no explicit rescheduling points). Add cond_resched() to the zram writeback loop.

Fixes: a939888ec38b ("zram: support idle/huge page writeback") Cc: stable@vger.kernel.org Signed-off-by: Sergey Senozhatsky senozhatsky@chromium.org ---

drivers/block/zram/zram_drv.c | 2 ++ 1 file changed, 2 insertions(+)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c index 0a924fae02a4..f5fa3db6b4f8 100644 --- a/drivers/block/zram/zram_drv.c +++ b/drivers/block/zram/zram_drv.c @@ -884,6 +884,8 @@ static ssize_t writeback_store(struct device *dev, next: zram_slot_unlock(zram, index); release_pp_slot(zram, pps); + + cond_resched(); }

if (blk_idx)