linux-kernel - [PATCH 4.19 020/247] bfq: Avoid false bfq queue merging

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Message-Id: <20210301161032.688648140@linuxfoundation.org>
Date:   Mon,  1 Mar 2021 17:10:40 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Jan Kara <jack@...e.cz>,
        Paolo Valente <paolo.valente@...aro.org>,
        Jens Axboe <axboe@...nel.dk>
Subject: [PATCH 4.19 020/247] bfq: Avoid false bfq queue merging

From: Jan Kara <jack@...e.cz>

commit 41e76c85660c022c6bf5713bfb6c21e64a487cec upstream.

bfq_setup_cooperator() uses bfqd->in_serv_last_pos so detect whether it
makes sense to merge current bfq queue with the in-service queue.
However if the in-service queue is freshly scheduled and didn't dispatch
any requests yet, bfqd->in_serv_last_pos is stale and contains value
from the previously scheduled bfq queue which can thus result in a bogus
decision that the two queues should be merged. This bug can be observed
for example with the following fio jobfile:

[global]
direct=0
ioengine=sync
invalidate=1
size=1g
rw=read

[reader]
numjobs=4
directory=/mnt

where the 4 processes will end up in the one shared bfq queue although
they do IO to physically very distant files (for some reason I was able to
observe this only with slice_idle=1ms setting).

Fix the problem by invalidating bfqd->in_serv_last_pos when switching
in-service queue.

Fixes: 058fdecc6de7 ("block, bfq: fix in-service-queue check for queue merging")
CC: stable@...r.kernel.org
Signed-off-by: Jan Kara <jack@...e.cz>
Acked-by: Paolo Valente <paolo.valente@...aro.org>
Signed-off-by: Jens Axboe <axboe@...nel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 block/bfq-iosched.c |    1 +
 1 file changed, 1 insertion(+)

--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -2478,6 +2478,7 @@ static void __bfq_set_in_service_queue(s
 	}

 	bfqd->in_service_queue = bfqq;
+	bfqd->in_serv_last_pos = 0;
 }

 /*