Re: [PATCH v3 3/4] dm: Fix dm-zoned-reclaim zone write pointer alignment

[Mike Snitzer <snitzer@kernel.org>]

On Mon, Dec 09, 2024 at 09:23:56PM +0900, Damien Le Moal wrote:
> The zone reclaim processing of the dm-zoned device mapper uses
> blkdev_issue_zeroout() to align the write pointer of a zone being used
> for reclaiming another zone, to write the valid data blocks from the
> zone being reclaimed at the same position relative to the zone start in
> the reclaim target zone.
> 
> The first call to blkdev_issue_zeroout() will try to use hardware
> offload using a REQ_OP_WRITE_ZEROES operation if the device reports a
> non-zero max_write_zeroes_sectors queue limit. If this operation fails
> because of the lack of hardware support, blkdev_issue_zeroout() falls
> back to using a regular write operation with the zero-page as buffer.
> Currently, such REQ_OP_WRITE_ZEROES failure is automatically handled by
> the block layer zone write plugging code which will execute a report
> zones operation to ensure that the write pointer of the target zone of
> the failed operation has not changed and to "rewind" the zone write
> pointer offset of the target zone as it was advanced when the write zero
> operation was submitted. So the REQ_OP_WRITE_ZEROES failure does not
> cause any issue and blkdev_issue_zeroout() works as expected.
> 
> However, since the automatic recovery of zone write pointers by the zone
> write plugging code can potentially cause deadlocks with queue freeze
> operations, a different recovery must be implemented in preparation for
> the removal of zone write plugging report zones based recovery.
> 
> Do this by introducing the new function blk_zone_issue_zeroout(). This
> function first calls blkdev_issue_zeroout() with the flag
> BLKDEV_ZERO_NOFALLBACK to intercept failures on the first execution
> which attempt to use the device hardware offload with the
> REQ_OP_WRITE_ZEROES operation. If this attempt fails, a report zone
> operation is issued to restore the zone write pointer offset of the
> target zone to the correct position and blkdev_issue_zeroout() is called
> again without the BLKDEV_ZERO_NOFALLBACK flag. The report zones
> operation performing this recovery is implemented using the helper
> function disk_zone_sync_wp_offset() which calls the gendisk report_zones
> file operation with the callback disk_report_zones_cb(). This callback
> updates the target write pointer offset of the target zone using the new
> function disk_zone_wplug_sync_wp_offset().
> 
> dmz_reclaim_align_wp() is modified to change its call to
> blkdev_issue_zeroout() to a call to blk_zone_issue_zeroout() without any
> other change needed as the two functions are functionnally equivalent.
> 
> Fixes: dd291d77cc90 ("block: Introduce zone write plugging")
> Cc: stable@vger.kernel.org
> Signed-off-by: Damien Le Moal <dlemoal@kernel.org>

Acked-by: Mike Snitzer <snitzer@kernel.org>