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 7d13420a16d6a7e5984de5fdf2725b088cf29ae1..bdb7ceb680890e431b70df870f3626e4802bea3d 100644 (file)
--- 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