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[88.147.34.208]) by smtp.gmail.com with ESMTPSA id k63sm78640645wmb.2.2019.07.30.01.55.27 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 30 Jul 2019 01:55:27 -0700 (PDT) Content-Type: text/plain; charset=us-ascii Mime-Version: 1.0 (Mac OS X Mail 12.4 \(3445.104.8\)) Subject: Re: [PATCH] bfq: Check if bfqq is NULL in bfq_insert_request From: Paolo Valente In-Reply-To: <20190728151931.GA29181@roeck-us.net> Date: Tue, 30 Jul 2019 10:55:24 +0200 Cc: Jens Axboe , linux-block , kernel list , Hsin-Yi Wang , Nicolas Boichat , Doug Anderson Content-Transfer-Encoding: quoted-printable Message-Id: <0BCD5EDA-6D08-4023-9EEA-087F0AB99D47@linaro.org> References: <1563816648-12057-1-git-send-email-linux@roeck-us.net> <20190728151931.GA29181@roeck-us.net> To: Guenter Roeck X-Mailer: Apple Mail (2.3445.104.8) Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org Hi Guenter, sorry for the delay (Dolomiti's fault). I didn't consider that rq->elv-icq might have been NULL also because of OOM. Thanks for spotting this issue. As for the other places where the return value of bfq_init_rq is used, unfortunately I think they matter too. Those other places are related to request merging, which is the alternative destiny of requests (instead of being just inserted). But, regardless of whether a request is to be merged or inserted, that request may be destined to a bfq_queue (possibly merged with a request already in a bfq_queue), and a NULL return value by bfq_init_rq leads to a crash. I guess you can reproduce your failure also for the merge case, by generating sequential, direct I/O with queue depth > 1, and of course by enabling failslab. If my considerations above are correct, do you want to propose a complete fix yourself? Thanks, Paolo > Il giorno 28 lug 2019, alle ore 17:19, Guenter Roeck = ha scritto: >=20 > ping ... just in case this patch got lost in Paolo's queue. >=20 > Guenter >=20 > On Mon, Jul 22, 2019 at 10:30:48AM -0700, Guenter Roeck wrote: >> In bfq_insert_request(), bfqq is initialized with: >> bfqq =3D bfq_init_rq(rq); >> In bfq_init_rq(), we find: >> if (unlikely(!rq->elv.icq)) >> return NULL; >> Indeed, rq->elv.icq can be NULL if the memory allocation in >> create_task_io_context() failed. >>=20 >> A comment in bfq_insert_request() suggests that bfqq is supposed to = be >> non-NULL if 'at_head || blk_rq_is_passthrough(rq)' is false. Yet, as >> debugging and practical experience shows, this is not the case in the >> above situation. >>=20 >> This results in the following crash. >>=20 >> Unable to handle kernel NULL pointer dereference >> at virtual address 00000000000001b0 >> ... >> Call trace: >> bfq_setup_cooperator+0x44/0x134 >> bfq_insert_requests+0x10c/0x630 >> blk_mq_sched_insert_requests+0x60/0xb4 >> blk_mq_flush_plug_list+0x290/0x2d4 >> blk_flush_plug_list+0xe0/0x230 >> blk_finish_plug+0x30/0x40 >> generic_writepages+0x60/0x94 >> blkdev_writepages+0x24/0x30 >> do_writepages+0x74/0xac >> __filemap_fdatawrite_range+0x94/0xc8 >> file_write_and_wait_range+0x44/0xa0 >> blkdev_fsync+0x38/0x68 >> vfs_fsync_range+0x68/0x80 >> do_fsync+0x44/0x80 >>=20 >> The problem is relatively easy to reproduce by running an image with >> failslab enabled, such as with: >>=20 >> cd /sys/kernel/debug/failslab >> echo 10 > probability >> echo 300 > times >>=20 >> Avoid the problem by checking if bfqq is NULL before using it. With = the >> NULL check in place, requests with missing io context are queued >> immediately, and the crash is no longer seen. >>=20 >> Fixes: 18e5a57d79878 ("block, bfq: postpone rq preparation to insert = or merge") >> Reported-by: Hsin-Yi Wang >> Cc: Hsin-Yi Wang >> Cc: Nicolas Boichat >> Cc: Doug Anderson >> Signed-off-by: Guenter Roeck >> --- >> block/bfq-iosched.c | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >>=20 >> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c >> index 72860325245a..56f3f4227010 100644 >> --- a/block/bfq-iosched.c >> +++ b/block/bfq-iosched.c >> @@ -5417,7 +5417,7 @@ static void bfq_insert_request(struct = blk_mq_hw_ctx *hctx, struct request *rq, >>=20 >> spin_lock_irq(&bfqd->lock); >> bfqq =3D bfq_init_rq(rq); >> - if (at_head || blk_rq_is_passthrough(rq)) { >> + if (!bfqq || at_head || blk_rq_is_passthrough(rq)) { >> if (at_head) >> list_add(&rq->queuelist, &bfqd->dispatch); >> else >> --=20 >> 2.7.4 >>=20