On Mon 02-09-24 21:03:27, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@xxxxxxxxxx>
>
> Consider the following merge chain:
>
> Process 1 Process 2 Process 3 Process 4
> (BIC1) (BIC2) (BIC3) (BIC4)
> Λ | | |
> \--------------\ \-------------\ \-------------\|
> V V V
> bfqq1--------->bfqq2---------->bfqq3----------->bfqq4
>
> IO from Process 1 will get bfqf2 from BIC1 first, then
> bfq_setup_cooperator() will found bfqq2 already merged to bfqq3 and then
> handle this IO from bfqq3. However, the merge chain can be much deeper
> and bfqq3 can be merged to other bfqq as well.
>
> Fix this problem by iterating to the last bfqq in
> bfq_setup_cooperator().
>
> Fixes: 36eca8948323 ("block, bfq: add Early Queue Merge (EQM)")
> Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
Good catch. Feel free to add:
Reviewed-by: Jan Kara <jack@xxxxxxx>
Honza
> ---
> block/bfq-iosched.c | 8 ++++++--
> 1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
> index 83adac3e71db..ffaa0d56328a 100644
> --- a/block/bfq-iosched.c
> +++ b/block/bfq-iosched.c
> @@ -2911,8 +2911,12 @@ bfq_setup_cooperator(struct bfq_data *bfqd, struct bfq_queue *bfqq,
> struct bfq_iocq_bfqq_data *bfqq_data = &bic->bfqq_data[a_idx];
>
> /* if a merge has already been setup, then proceed with that first */
> - if (bfqq->new_bfqq)
> - return bfqq->new_bfqq;
> + new_bfqq = bfqq->new_bfqq;
> + if (new_bfqq) {
> + while (new_bfqq->new_bfqq)
> + new_bfqq = new_bfqq->new_bfqq;
> + return new_bfqq;
> + }
>
> /*
> * Check delayed stable merge for rotational or non-queueing
> --
> 2.39.2
>
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
