[dm-devel] [PATCH] dm-raid: set discard_granularity non-zero if possible

[Stephan Bärwolf]

Hi

I hope this address is the right place for this patch.
It is supposed to fix the triggering of block/blklib.c:51 WARN_ON_ONCE(..) when using LVM2 raid1 with SSD-PVs.
Since commit b35fd7422c2f8e04496f5a770bd4e1a205414b3f and without this patchthere are tons of printks logging "Error: discard_granularity is 0." to kmsg.
Also there is no discard/TRIM happening anymore...

This is a rough patch for WARNING-issue

"block/blk-lib.c:51 __blkdev_issue_discard+0x1f6/0x250"
[...] "Error: discard_granularity is 0." [...]
introduced in commit b35fd7422c2f8e04496f5a770bd4e1a205414b3f
("block: check queue's limits.discard_granularity in __blkdev_issue_discard()")

in conjunction with LVM2 raid1 volumes on discardable (SSD) backing.
It seems until now, LVM-raid1 reported "discard_granularity" as 0,
as well as "max_discard_sectors" as 0. (see "lsblk --discard").

The idea here is to fix the issue by calculating "max_discard_sectors"
as the minimum over all involved block devices. (We use the meta-data
for this to work here.)
For calculating the "discard_granularity" we would have to calculate the
lcm (least common multiple) of all discard_granularities of all involved
block devices and finally round up to next power of 2.

However, since all "discard_granularity" are powers of 2, this algorithm
will simplify to just determining the maximum and filtering for "0"-cases.

Signed-off-by: Stephan Baerwolf <stephan at matrixstorm.com>
---
drivers/md/dm-raid.c | 32 ++++++++++++++++++++++++++++++--
1 file changed, 30 insertions(+), 2 deletions(-)



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