* [PATCH] ublk: remove segment count and size limits
@ 2024-04-30 0:53 Uday Shankar
2024-04-30 9:15 ` Ming Lei
2024-04-30 12:59 ` Christoph Hellwig
0 siblings, 2 replies; 3+ messages in thread
From: Uday Shankar @ 2024-04-30 0:53 UTC (permalink / raw
To: Ming Lei, Jens Axboe; +Cc: linux-block, Uday Shankar, Riley Thomasson
ublk_drv currently creates block devices with the default max_segments
and max_segment_size limits of BLK_MAX_SEGMENTS (128) and
BLK_MAX_SEGMENT_SIZE (65536) respectively. These defaults can
artificially constrain the I/O size seen by the ublk server - for
example, suppose that the ublk server has configured itself to accept
I/Os up to 1M and the application is also issuing 1M sized I/Os. If the
I/O buffer used by the application is backed by 4K pages, the buffer
could consist of up to 1M / 4K = 256 physically discontiguous segments
(even if the buffer is virtually contiguous). As such, the I/O could
exceed the default max_segments limit and get split. This can cause
unnecessary performance issues if the ublk server is optimized to handle
1M I/Os. The block layer's segment count/size limits exist to model
hardware constraints which don't exist in ublk_drv's case, so just
remove those limits for the block devices created by ublk_drv.
Signed-off-by: Uday Shankar <ushankar@purestorage.com>
Reviewed-by: Riley Thomasson <riley@purestorage.com>
---
drivers/block/ublk_drv.c | 3 +++
1 file changed, 3 insertions(+)
diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
index bea3d5cf8a83..835b0cc7c032 100644
--- a/drivers/block/ublk_drv.c
+++ b/drivers/block/ublk_drv.c
@@ -2209,6 +2209,9 @@ static int ublk_ctrl_start_dev(struct ublk_device *ub, struct io_uring_cmd *cmd)
lim.max_zone_append_sectors = p->max_zone_append_sectors;
}
+ lim.max_segments = USHRT_MAX;
+ lim.max_segment_size = UINT_MAX;
+
if (wait_for_completion_interruptible(&ub->completion) != 0)
return -EINTR;
--
2.34.1
^ permalink raw reply related [flat|nested] 3+ messages in thread
* Re: [PATCH] ublk: remove segment count and size limits
2024-04-30 0:53 [PATCH] ublk: remove segment count and size limits Uday Shankar
@ 2024-04-30 9:15 ` Ming Lei
2024-04-30 12:59 ` Christoph Hellwig
1 sibling, 0 replies; 3+ messages in thread
From: Ming Lei @ 2024-04-30 9:15 UTC (permalink / raw
To: Uday Shankar; +Cc: Jens Axboe, linux-block, Riley Thomasson
On Mon, Apr 29, 2024 at 06:53:31PM -0600, Uday Shankar wrote:
> ublk_drv currently creates block devices with the default max_segments
> and max_segment_size limits of BLK_MAX_SEGMENTS (128) and
> BLK_MAX_SEGMENT_SIZE (65536) respectively. These defaults can
> artificially constrain the I/O size seen by the ublk server - for
> example, suppose that the ublk server has configured itself to accept
> I/Os up to 1M and the application is also issuing 1M sized I/Os. If the
> I/O buffer used by the application is backed by 4K pages, the buffer
> could consist of up to 1M / 4K = 256 physically discontiguous segments
> (even if the buffer is virtually contiguous). As such, the I/O could
> exceed the default max_segments limit and get split. This can cause
> unnecessary performance issues if the ublk server is optimized to handle
> 1M I/Os. The block layer's segment count/size limits exist to model
> hardware constraints which don't exist in ublk_drv's case, so just
> remove those limits for the block devices created by ublk_drv.
>
> Signed-off-by: Uday Shankar <ushankar@purestorage.com>
> Reviewed-by: Riley Thomasson <riley@purestorage.com>
> ---
> drivers/block/ublk_drv.c | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
> index bea3d5cf8a83..835b0cc7c032 100644
> --- a/drivers/block/ublk_drv.c
> +++ b/drivers/block/ublk_drv.c
> @@ -2209,6 +2209,9 @@ static int ublk_ctrl_start_dev(struct ublk_device *ub, struct io_uring_cmd *cmd)
> lim.max_zone_append_sectors = p->max_zone_append_sectors;
> }
>
> + lim.max_segments = USHRT_MAX;
> + lim.max_segment_size = UINT_MAX;
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Thanks,
Ming
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH] ublk: remove segment count and size limits
2024-04-30 0:53 [PATCH] ublk: remove segment count and size limits Uday Shankar
2024-04-30 9:15 ` Ming Lei
@ 2024-04-30 12:59 ` Christoph Hellwig
1 sibling, 0 replies; 3+ messages in thread
From: Christoph Hellwig @ 2024-04-30 12:59 UTC (permalink / raw
To: Uday Shankar; +Cc: Ming Lei, Jens Axboe, linux-block, Riley Thomasson
On Mon, Apr 29, 2024 at 06:53:31PM -0600, Uday Shankar wrote:
> + lim.max_segments = USHRT_MAX;
> + lim.max_segment_size = UINT_MAX;
Please initialize thes at the top of the function as part of the
declaration for lim, like all the other unconditional limits.
^ permalink raw reply [flat|nested] 3+ messages in thread
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2024-04-30 0:53 [PATCH] ublk: remove segment count and size limits Uday Shankar
2024-04-30 9:15 ` Ming Lei
2024-04-30 12:59 ` Christoph Hellwig
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