linux-kernel - Re: [PATCH] bfq: Check if bfqq is NULL in bfq_insert

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Date:   Tue, 30 Jul 2019 10:06:25 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Paolo Valente <paolo.valente@...aro.org>
Cc:     Jens Axboe <axboe@...nel.dk>,
        linux-block <linux-block@...r.kernel.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Hsin-Yi Wang <hsinyi@...gle.com>,
        Nicolas Boichat <drinkcat@...omium.org>,
        Doug Anderson <dianders@...omium.org>
Subject: Re: [PATCH] bfq: Check if bfqq is NULL in bfq_insert_request

Paolo,

On Tue, Jul 30, 2019 at 10:55:24AM +0200, Paolo Valente wrote:
> 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?
> 

I had another look into the code. Unfortunately, both bfq_request_merged()
and bfq_requests_merged() simply assume that bfq_init_rq() never returns
NULL, and don't give me an idea for a path of action if it returns NULL
after all. I'll have to pass the problem off to you for a fix.

Thanks,
Guenter