Re: [PATCH 1/7] userfaultfd: require UFFDIO_API before other ioctls

From: Linus Torvalds <torvalds@linux-foundation.org>
To: Andrea Arcangeli <aarcange@redhat.com>
Cc: "Huangpeng (Peter)" <peter.huangpeng@huawei.com>,
	qemu-devel@nongnu.org, Paolo Bonzini <pbonzini@redhat.com>,
	Pavel Emelyanov <xemul@parallels.com>,
	Hugh Dickins <hughd@google.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	"Dr. David Alan Gilbert" <dgilbert@redhat.com>,
	Andres Lagar-Cavilla <andreslc@google.com>,
	Andy Lutomirski <luto@amacapital.net>,
	linux-mm@kvack.org, "Kirill A. Shutemov" <kirill@shutemov.name>,
	Rik van Riel <riel@redhat.com>,
	Johannes Weiner <hannes@cmpxchg.org>,
	linux-kernel@vger.kernel.org,
	Sanidhya Kashyap <sanidhya.gatech@gmail.com>,
	zhang.zhanghailiang@huawei.com,
	Dave Hansen <dave.hansen@intel.com>, Mel Gorman <mgorman@suse.de>,
	Peter Feiner <pfeiner@google.com>,
	kvm@vger.kernel.org
Subject: Re: [PATCH 1/7] userfaultfd: require UFFDIO_API before other ioctls
Date: Mon, 15 Jun 2015 11:55:38 -1000	[thread overview]
Message-ID: <CA+55aFxKz5eONz1g57ccZ8b=6ivouQOapBtVjt7U3aOtrRgw8Q@mail.gmail.com> (raw)
In-Reply-To: <20150615214338.GH18909@redhat.com>

[-- Attachment #1: Type: text/plain, Size: 1036 bytes --]

On Jun 15, 2015 11:43 AM, "Andrea Arcangeli" <aarcange@redhat.com> wrote:
>
> Several times I got very hardly reproducible bugs noticed purely
> because of BUG_ON (not VM_BUG_ON)

Feel free to use them while developing. Don't send me patches with your
broken debug code, though.

For users, a dead machine means that it is less likely you will ever get a
bug report. People set "reboot on oops", and when running X is not always
something that can be seen anyway. They'll just see a rebooting or a dead
machine, and not send you any debug output.

This is not negotiable. Seriously. Get rid of the BUG_ON if you expect your
patches to be merged mainline.

Also, even for debugging, using something like

    if (WARN_ON_ONCE(...))
         return -EINVAL;

is the right thing to do. Then you can unwind locks etc, and return
cleanly, and you'll only get one warning rather than a stream of them etc.

So stop making excuses for your bad BUG_ON use. Do it in private where
nobody can see your perversions, or do it right.

        Linus

[-- Attachment #2: Type: text/html, Size: 1322 bytes --]