[PATCH] block: ensure discard_granularity is zero when discard is not supported

[PATCH] block: ensure discard_granularity is zero when discard is not supported

[Christoph Hellwig]

Documentation/ABI/stable/sysfs-block states:

  What: /sys/block/<disk>/queue/discard_granularity
  [...]
  A discard_granularity of 0 means that the device does not support
  discard functionality.

but this got broken when sorting out the block limits updates.  Fix this
by setting the discard_granularity limit to zero when the combined
max_discard_sectors is zero.

Fixes: 3c407dc723bb ("block: default the discard granularity to sector size")
Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 block/blk-settings.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/block/blk-settings.c b/block/blk-settings.c
index a000daafbfb4..20f8b207d5e8 100644
--- a/block/blk-settings.c
+++ b/block/blk-settings.c
@@ -336,12 +336,19 @@ int blk_validate_limits(struct queue_limits *lim)
 	lim->max_discard_sectors =
 		min(lim->max_hw_discard_sectors, lim->max_user_discard_sectors);
 
+	/*
+	 * When discard is not supported, discard_granularity should be reported
+	 * as 0 to userspace.
+	 */
+	if (lim->max_discard_sectors)
+		lim->discard_granularity =
+			max(lim->discard_granularity, lim->physical_block_size);
+	else
+		lim->discard_granularity = 0;
+
 	if (!lim->max_discard_segments)
 		lim->max_discard_segments = 1;
 
-	if (lim->discard_granularity < lim->physical_block_size)
-		lim->discard_granularity = lim->physical_block_size;
-
 	/*
 	 * By default there is no limit on the segment boundary alignment,
 	 * but if there is one it can't be smaller than the page size as
-- 
2.47.2

Re: [PATCH] block: ensure discard_granularity is zero when discard is not supported

[Martin K. Petersen]

Christoph,

> +	/*
> +	 * When discard is not supported, discard_granularity should be reported
> +	 * as 0 to userspace.
> +	 */
> +	if (lim->max_discard_sectors)
> +		lim->discard_granularity =
> +			max(lim->discard_granularity, lim->physical_block_size);
> +	else
> +		lim->discard_granularity = 0;
> +

Looks good to me!

Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>

-- 
Martin K. Petersen

Re: [PATCH] block: ensure discard_granularity is zero when discard is not supported

[Jens Axboe]

On Thu, 31 Jul 2025 08:22:28 -0700, Christoph Hellwig wrote:
> Documentation/ABI/stable/sysfs-block states:
> 
>   What: /sys/block/<disk>/queue/discard_granularity
>   [...]
>   A discard_granularity of 0 means that the device does not support
>   discard functionality.
> 
> [...]

Applied, thanks!

[1/1] block: ensure discard_granularity is zero when discard is not supported
      commit: fad6551fcf537375702b9af012508156a16a1ff7

Best regards,
-- 
Jens Axboe