dm thin: fix pool_io_hints to avoid looking at max_hw_sectors (d200c30e) · Commits · 戴 / test

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>

drivers/md/dm-thin.c

+7 −14

Original line number	Diff line number	Diff line
		@@ -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);
		}
		}

		/*

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