Re: [PATCH] blk-wbt: Always change enable_state to MANUAL when setting latency

[guzebing <guzebing1612@gmail.com>]

On 7/17/26 8:50 PM, Tang Yizhou wrote:
> From: Tang Yizhou <yizhou.tang@shopee.com>
> 
> Commit 1e56f30a73f3 ("block: Make WBT latency writes honor enable
> state") compares only the boolean enabled state in the no-op check.
> Writing the current latency value while WBT is in WBT_STATE_ON_DEFAULT
> is therefore still skipped, and enable_state is not changed to
> WBT_STATE_ON_MANUAL. A subsequent elevator switch to BFQ then disables
> WBT through wbt_disable_default(), silently discarding the explicit
> sysfs setting:
> 
>   # enable_state = WBT_STATE_ON_DEFAULT, min_lat_nsec = 2000000
>   echo 2000 > /sys/block/nullb0/queue/wbt_lat_usec  # skipped as a no-op
>   echo bfq > /sys/block/nullb0/queue/scheduler
>   cat /sys/block/nullb0/queue/wbt_lat_usec          # 0, WBT disabled
> 
> Skip the update only if the stored latency matches and enable_state
> already equals the exact target state: WBT_STATE_ON_MANUAL for a
> non-zero value, WBT_STATE_OFF_MANUAL for zero.
> 
> A write that passes the check then reaches wbt_set_min_lat(), which
> performs the actual transition to the MANUAL state.
> 
> Rename the helper to wbt_lat_changed() to improve readability.
> 
> Fixes: 1e56f30a73f3 ("block: Make WBT latency writes honor enable state")
> Signed-off-by: Tang Yizhou <yizhou.tang@shopee.com>
> ---
>  block/blk-wbt.c | 14 +++++++++++---
>  1 file changed, 11 insertions(+), 3 deletions(-)

Indeed, this edge case was missed. Thanks for catching it. The patch
looks good to me.

Reviewed-by: Guzebing <guzebing1612@gmail.com>