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From:   David Laight <David.Laight@ACULAB.COM>
To:     'Jason Gunthorpe' <jgg@nvidia.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
CC:     David Hildenbrand <david@redhat.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "stable@vger.kernel.org" <stable@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Axel Rasmussen <axelrasmussen@google.com>,
        Peter Xu <peterx@redhat.com>,
        "Hugh Dickins" <hughd@google.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        "Matthew Wilcox" <willy@infradead.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        John Hubbard <jhubbard@nvidia.com>
Subject: RE: [PATCH v1] mm/gup: fix FOLL_FORCE COW security issue and remove
 FOLL_COW
Thread-Topic: [PATCH v1] mm/gup: fix FOLL_FORCE COW security issue and remove
 FOLL_COW
Thread-Index: AQHYrCRe/fPaEDNbYU+8sVbKwVGyX62nEPVw
Date:   Tue, 9 Aug 2022 21:16:15 +0000
Message-ID: <09715a42c73a499f9c90cbe3e2714389@AcuMS.aculab.com>
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From: Jason Gunthorpe
> Sent: 09 August 2022 20:08
> 
> On Tue, Aug 09, 2022 at 11:59:45AM -0700, Linus Torvalds wrote:
> 
> > But as a very good approximation, the rule is "absolutely no new
> > BUG_ON() calls _ever_". Because I really cannot see a single case
> > where "proper error handling and WARN_ON_ONCE()" isn't the right
> > thing.
> 
> Parallel to this discussion I've had ones where people more or less
> say
> 
>  Since BUG_ON crashes the machine and Linus says that crashing the
>  machine is bad, WARN_ON will also crash the machine if you set the
>  panic_on_warn parameter, so it is also bad, thus we shouldn't use
>  anything.
> 
> I've generally maintained that people who set the panic_on_warn *want*
> these crashes, because that is the entire point of it. So we should
> use WARN_ON with an error recovery for "can't happen" assertions like
> these. I think it is what you are saying here.

They don't necessarily want the crashes, it is more the people who
built the distribution think they want the crashes.

I have had issues with a customer system (with our drivers) randomly
locking up.
Someone had decided that 'PANIC_ON_OOPS' was a good idea but hadn't
enabled anything to actually take the dump.

So instead of a diagnosable problem (and a 'doh' moment) you
get several weeks of head scratching and a very annoyed user.

	David

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