dm thin: fix pool_io_hints to avoid looking at max_hw_sectors

Simplify the pool_io_hints code that works to establish a max_sectors
value that is a power-of-2 factor of the thin-pool's blocksize.  The
biggest associated improvement is that the DM thin-pool is no longer
concerning itself with the data device's max_hw_sectors when adjusting
max_sectors.

This fixes the relative fragility of the original "dm thin: adjust
max_sectors_kb based on thinp blocksize" commit that only became
apparent when testing was performed using a DM thin-pool ontop of a
virtio_blk device.  One proposed upstream patch detailed the problems
inherent in virtio_blk: https://lkml.org/lkml/2014/11/20/611

So even though virtio_blk incorrectly set its max_hw_sectors it actually
helped make it clear that we need DM thinp to be tolerant of any future
Linux driver that incorrectly sets max_hw_sectors.

We only need to be concerned with modifying the thin-pool device's
max_sectors limit if it is smaller than the thin-pool's blocksize.  In
this case the value of max_sectors does become a limiting factor when
upper layers (e.g. filesystems) construct their bios.  But if the
hardware can support IOs larger than the thin-pool's blocksize the user
is encouraged to adjust the thin-pool's data device's max_sectors
accordingly -- doing so will enable the thin-pool to inherit the
established user-defined max_sectors.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>

diff --git a/drivers/md/dm-thin.c b/drivers/md/dm-thin.c
index e9e9584..8735543 100644 (file)
--- a/drivers/md/dm-thin.c
+++ b/drivers/md/dm-thin.c
@@ -3587,27 +3587,20 @@ static void pool_io_hints(struct dm_target *ti, struct queue_limits *limits)
        sector_t io_opt_sectors = limits->io_opt >> SECTOR_SHIFT;
 
        /*
-        * Adjust max_sectors_kb to highest possible power-of-2
-        * factor of pool->sectors_per_block.
+        * If max_sectors is smaller than pool->sectors_per_block adjust it
+        * to the highest possible power-of-2 factor of pool->sectors_per_block.
+        * This is especially beneficial when the pool's data device is a RAID
+        * device that has a full stripe width that matches pool->sectors_per_block
+        * -- because even though partial RAID stripe-sized IOs will be issued to a
+        *    single RAID stripe; when aggregated they will end on a full RAID stripe
+        *    boundary.. which avoids additional partial RAID stripe writes cascading
         */
-       if (limits->max_hw_sectors & (limits->max_hw_sectors - 1))
-               limits->max_sectors = rounddown_pow_of_two(limits->max_hw_sectors);
-       else
-               limits->max_sectors = limits->max_hw_sectors;
-
        if (limits->max_sectors < pool->sectors_per_block) {
                while (!is_factor(pool->sectors_per_block, limits->max_sectors)) {
                        if ((limits->max_sectors & (limits->max_sectors - 1)) == 0)
                                limits->max_sectors--;
                        limits->max_sectors = rounddown_pow_of_two(limits->max_sectors);
                }
-       } else if (block_size_is_power_of_two(pool)) {
-               /* max_sectors_kb is >= power-of-2 thinp blocksize */
-               while (!is_factor(limits->max_sectors, pool->sectors_per_block)) {
-                       if ((limits->max_sectors & (limits->max_sectors - 1)) == 0)
-                               limits->max_sectors--;
-                       limits->max_sectors = rounddown_pow_of_two(limits->max_sectors);
-               }
        }
 
        /*