[PATCH 1/4] block: don't unconditionally set max_discard_sectors in blk_queue_max_discard_sectors

[Ming Lei]

On Fri, Jul 07, 2023 at 11:46:13AM +0200, Christoph Hellwig wrote:
> max_discard_sectors is split into a hardware and a tunable value, but
> blk_queue_max_discard_sectors sets both unconditionally, thus dropping
> any user stored value on a rescan.  Fix blk_queue_max_discard_sectors to
> only set max_discard_sectors if it either wasn't set, or the new hardware
> limit is smaller than the previous user limit.
> 
> Fixes: 0034af036554 ("block: make /sys/block/<dev>/queue/discard_max_bytes writeable")

It is hard to say a fix, given discard_max_bytes can still be changed
by kernel. I'd suggest to document this behavior in Documentation/ABI/stable/sysfs-block.

> Signed-off-by: Christoph Hellwig <hch at lst.de>
> ---
>  block/blk-settings.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/block/blk-settings.c b/block/blk-settings.c
> index 0046b447268f91..978d2e1fd67a51 100644
> --- a/block/blk-settings.c
> +++ b/block/blk-settings.c
> @@ -179,7 +179,9 @@ void blk_queue_max_discard_sectors(struct request_queue *q,
>  		unsigned int max_discard_sectors)
>  {
>  	q->limits.max_hw_discard_sectors = max_discard_sectors;
> -	q->limits.max_discard_sectors = max_discard_sectors;
> +	if (!q->limits.max_discard_sectors ||
> +	     q->limits.max_discard_sectors > max_discard_sectors)
> +		q->limits.max_discard_sectors = max_discard_sectors;
>  }

Userspace may write 0 to discard_max_bytes, and this patch still can
override user setting.

Thanks,
Ming