dm thin: fix deadlock when swapping to thin device

This is an already known issue that dm-thin volume cannot be used as
swap, otherwise a deadlock may happen when dm-thin internal memory
demand triggers swap I/O on the dm-thin volume itself.

But thanks to commit a666e5c ("dm: fix deadlock when swapping to
encrypted device"), the limit_swap_bios target flag can also be used
for dm-thin to avoid the recursive I/O when it is used as swap.

Fix is to simply set ti->limit_swap_bios to true in both pool_ctr()
and thin_ctr().

In my test, I create a dm-thin volume /dev/vg/swap and use it as swap
device. Then I run fio on another dm-thin volume /dev/vg/main and use
large --blocksize to trigger swap I/O onto /dev/vg/swap.

The following fio command line is used in my test,
  fio --name recursive-swap-io --lockmem 1 --iodepth 128 \
     --ioengine libaio --filename /dev/vg/main --rw randrw \
    --blocksize 1M --numjobs 32 --time_based --runtime=12h

Without this fix, the whole system can be locked up within 15 seconds.

With this fix, there is no any deadlock or hung task observed after
2 hours of running fio.

Furthermore, if blocksize is changed from 1M to 128M, after around 30
seconds fio has no visible I/O, and the out-of-memory killer message
shows up in kernel message. After around 20 minutes all fio processes
are killed and the whole system is back to being alive.

This is exactly what is expected when recursive I/O happens on dm-thin
volume when it is used as swap.

Depends-on: a666e5c ("dm: fix deadlock when swapping to encrypted device")
Cc: [email protected]
Signed-off-by: Coly Li <[email protected]>
Acked-by: Mikulas Patocka <[email protected]>
Signed-off-by: Mike Snitzer <[email protected]>

Author: Coly Li

drivers/md/dm-thin.c

@@ -3369,6 +3369,7 @@ static int pool_ctr(struct dm_target *ti, unsigned int argc, char **argv)
 	pt->low_water_blocks = low_water_blocks;
 	pt->adjusted_pf = pt->requested_pf = pf;
 	ti->num_flush_bios = 1;
+	ti->limit_swap_bios = true;
 
 	/*
 	 * Only need to enable discards if the pool should pass
@@ -4249,6 +4250,7 @@ static int thin_ctr(struct dm_target *ti, unsigned int argc, char **argv)
 		goto bad;
 
 	ti->num_flush_bios = 1;
+	ti->limit_swap_bios = true;
 	ti->flush_supported = true;
 	ti->accounts_remapped_io = true;
 	ti->per_io_data_size = sizeof(struct dm_thin_endio_hook);