linux-kernel - Re: [dm-devel] [PATCH v5] fault-injection: introduce kvmalloc fallback options

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Date:   Mon, 30 Apr 2018 14:27:16 -0400
From:   "John Stoffel" <john@...ffel.org>
To:     Mikulas Patocka <mpatocka@...hat.com>
Cc:     John Stoffel <john@...ffel.org>, Andrew@...ffel.org,
        eric.dumazet@...il.com, mst@...hat.com, edumazet@...gle.com,
        netdev@...r.kernel.org, jasowang@...hat.com,
        Randy Dunlap <rdunlap@...radead.org>,
        linux-kernel@...r.kernel.org, Matthew Wilcox <willy@...radead.org>,
        Hocko <mhocko@...nel.org>,
        James Bottomley <James.Bottomley@...senPartnership.com>,
        Michal@...ffel.org, dm-devel@...hat.com,
        David Miller <davem@...emloft.net>,
        David Rientjes <rientjes@...gle.com>,
        Morton <akpm@...ux-foundation.org>,
        virtualization@...ts.linux-foundation.org, linux-mm@...ck.org,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [dm-devel] [PATCH v5] fault-injection: introduce kvmalloc
 fallback options

>>>>> "Mikulas" == Mikulas Patocka <mpatocka@...hat.com> writes:

Mikulas> On Thu, 26 Apr 2018, John Stoffel wrote:

>> >>>>> "James" == James Bottomley <James.Bottomley@...senPartnership.com> writes:
>> 
James> I may be an atypical developer but I'd rather have a root canal
James> than browse through menuconfig options.  The way to get people
James> to learn about new debugging options is to blog about it (or
James> write an lwn.net article) which google will find the next time
James> I ask it how I debug XXX.  Google (probably as a service to
James> humanity) rarely turns up Kconfig options in response to a
James> query.
>> 
>> I agree with James here.  Looking at the SLAB vs SLUB Kconfig entries
>> tells me *nothing* about why I should pick one or the other, as an
>> example.
>> 
>> John

Mikulas> I see your point - and I think the misunderstanding is this.

Thanks.

Mikulas> This patch is not really helping people to debug existing crashes. It is 
Mikulas> not like "you get a crash" - "you google for some keywords" - "you get a 
Mikulas> page that suggests to turn this option on" - "you turn it on and solve the 
Mikulas> crash".

Mikulas> What this patch really does is that - it makes the kernel deliberately 
Mikulas> crash in a situation when the code violates the specification, but it 
Mikulas> would not crash otherwise or it would crash very rarely. It helps to 
Mikulas> detect specification violations.

Mikulas> If the kernel developer (or tester) doesn't use this option, his buggy 
Mikulas> code won't crash - and if it won't crash, he won't fix the bug or report 
Mikulas> it. How is the user or developer supposed to learn about this option, if 
Mikulas> he gets no crash at all?

So why do we make this a KConfig option at all?  Just turn it on and
let it rip.  Now I also think that Linus has the right idea to not
just sprinkle BUG_ONs into the code, just dump and oops and keep going
if you can.  If it's a filesystem or a device, turn it read only so
that people notice right away.