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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 3 Aug 2018 09:37:35 -0700
Message-ID: <CA+55aFwWZX=CXmWDTkDGb36kf12XmTehmQjbiMPCqCRG2hi9kw@mail.gmail.com>
Subject: Re: LVM snapshot broke between 4.14 and 4.16
To:     Zdenek Kabelac <zkabelac@redhat.com>
Cc:     wgh@torlan.ru, Ilya Dryomov <idryomov@gmail.com>,
        Jens Axboe <axboe@kernel.dk>,
        linux-block <linux-block@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Sagi Grimberg <sagi@grimberg.me>,
        Mike Snitzer <snitzer@redhat.com>, dm-devel@redhat.com
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[ Dammit. I haven't had to shout and curse at people for a while, but
this is ABSOLUTELY THE MOST IMPORTANT THING IN THE UNIVERSE WHEN IT
COMES TO SOFTWARE DEVELOPMENT ]

On Fri, Aug 3, 2018 at 6:31 AM Zdenek Kabelac <zkabelac@redhat.com> wrote:
>
> IMHO (as the author of fixing lvm2 patch) user should not be upgrading kernels
> and keep running older lvm2 user-land tool (and there are very good reasons
> for this).

Yeah, HELL NO!

Guess what? You're wrong. YOU ARE MISSING THE #1 KERNEL RULE.

We do not regress, and we do not regress exactly because your are 100% wrong.

And the reason you state for your opinion is in fact exactly *WHY* you
are wrong.

Your "good reasons" are pure and utter garbage.

The whole point of "we do not regress" is so that people can upgrade
the kernel and never have to worry about it.

> Kernel had a bug which has been fixed

That is *ENTIRELY* immaterial.

Guys, whether something was buggy or not DOES NOT MATTER.

Why?

Bugs happen. That's a fact of life. Arguing that "we had to break
something because we were fixing a bug" is completely insane. We fix
tens of bugs every single day, thinking that "fixing a bug" means that
we can break something is simply NOT TRUE.

So bugs simply aren't even relevant to the discussion. They happen,
they get found, they get fixed, and it has nothing to do with "we
break users".

Because the only thing that matters IS THE USER.

How hard is that to understand?

Anybody who uses "but it was buggy" as an argument is entirely missing
the point. As far as the USER was concerned, it wasn't buggy - it
worked for him/her.

Maybe it worked *because* the user had taken the bug into account,
maybe it worked because the user didn't notice - again, it doesn't
matter. It worked for the user.

Breaking a user workflow for a "bug" is absolutely the WORST reason
for breakage you can imagine.

It's basically saying "I took something that worked, and I broke it,
but now it's better". Do you not see how f*cking insane that statement
is?

And without users, your program is not a program, it's a pointless
piece of code that you might as well throw away.

Seriously. This is *why* the #1 rule for kernel development is "we
don't break users". Because "I fixed a bug" is absolutely NOT AN
ARGUMENT if that bug fix broke a user setup. You actually introduced a
MUCH BIGGER bug by "fixing" something that the user clearly didn't
even care about.

And dammit, we upgrade the kernel ALL THE TIME without upgrading any
other programs at all. It is absolutely required, because flag-days
and dependencies are horribly bad.

And it is also required simply because I as a kernel developer do not
upgrade random other tools that I don't even care about as I develop
the kernel, and I want any of my users to feel safe doing the same
time.

So no. Your rule is COMPLETELY wrong. If you cannot upgrade a kernel
without upgrading some other random binary, then we have a problem.

                     Linus