block, bfq: choose the last bfqq from merge chain in bfq_setup_cooperator()

[ Upstream commit 0e456dba86c7f9a19792204a044835f1ca2c8dbb ]

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@huawei.com>
Link: https://lore.kernel.org/r/20240902130329.3787024-3-yukuai1@huaweicloud.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 0b2c8e76708ff0c3f9fa7cb4552c767d2cd14f47..e68881f71ff137f1c80758b35dff9e9bfa76fea0 100644 (file)
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -2616,8 +2616,12 @@ bfq_setup_cooperator(struct bfq_data *bfqd, struct bfq_queue *bfqq,
        struct bfq_queue *in_service_bfqq, *new_bfqq;
 
        /* 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;
+       }
 
        /*
         * Do not perform queue merging if the device is non