Re: [PATCH] bfq: Check if bfqq is NULL in bfq_insert_request

[Paolo Valente <paolo.valente@linaro.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 <linux@roeck-us.net> ha scritto:
> 
> ping ... just in case this patch got lost in Paolo's queue.
> 
> Guenter
> 
> On Mon, Jul 22, 2019 at 10:30:48AM -0700, Guenter Roeck wrote:
>> In bfq_insert_request(), bfqq is initialized with:
>> 	bfqq = 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.
>> 
>> 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.
>> 
>> This results in the following crash.
>> 
>> 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
>> 
>> The problem is relatively easy to reproduce by running an image with
>> failslab enabled, such as with:
>> 
>> cd /sys/kernel/debug/failslab
>> echo 10 > probability
>> echo 300 > times
>> 
>> 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.
>> 
>> Fixes: 18e5a57d79878 ("block, bfq: postpone rq preparation to insert or merge")
>> Reported-by: Hsin-Yi Wang  <hsinyi@google.com>
>> Cc: Hsin-Yi Wang <hsinyi@google.com>
>> Cc: Nicolas Boichat <drinkcat@chromium.org>
>> Cc: Doug Anderson <dianders@chromium.org>
>> Signed-off-by: Guenter Roeck <linux@roeck-us.net>
>> ---
>> block/bfq-iosched.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> 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,
>> 
>> 	spin_lock_irq(&bfqd->lock);
>> 	bfqq = 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
>> -- 
>> 2.7.4
>>