linux-kernel - Re: [RFC PATCH for 4.21 03/16] mm: Replace BUG_ON() by WARN_ON() in vm_unmap

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Date:   Thu, 1 Nov 2018 18:17:19 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-api <linux-api@...r.kernel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Dave Watson <davejwatson@...com>, Paul Turner <pjt@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King <linux@....linux.org.uk>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Andi Kleen <andi@...stfloor.org>,
        Chris Lameter <cl@...ux.com>, Ben Maurer <bmaurer@...com>,
        Josh Triplett <josh@...htriplett.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Matthew Wilcox <mawilcox@...rosoft.com>
Subject: Re: [RFC PATCH for 4.21 03/16] mm: Replace BUG_ON() by WARN_ON() in
 vm_unmap_ram()

----- On Nov 1, 2018, at 11:00 PM, Linus Torvalds torvalds@...ux-foundation.org wrote:

> On Thu, Nov 1, 2018 at 12:57 PM Mathieu Desnoyers
> <mathieu.desnoyers@...icios.com> wrote:
>>
>> > I think the graceful recovery is to simply return:
>> >
>> >       if (WARN_ON(cond))
>> >               return;
>> >
>> > is better than just
>> >
>> >       BUG_ON(cond);
>> >
>> > As that's what Linus made pretty clear at the Maintainer's Summit.
>>
>> That's it. For an unmap function, this basically boils down to
>> print a warning and leak the memory on internal unmap error.
>>
>> I will update the commit message describing this behavior.
> 
> It might be even better to use WARN_ON_ONCE().
> 
> If it's a "this shouldn't happen" situation, the advantage of
> WARN_ON_ONCE() is that it will still show the backtrace of the "how
> the heck did it happen after all" situation, but if it turns ouit to
> be user-triggerable (or simply triggerable by some odd hw situation),
> it won't spam your logs forever.
> 
> Obviously, things like rate limiting etc can also be good ideas, but
> that's just overkill for "this really should never happen" cases.
> 
> (Side note: WARN_ON_ONCE() will _warn_ just once, but will always
> return the condition that it warns for, so the return value is _not_
> "I have warned", but "I have seen the condition that I should warn
> about". Just in case people are worried about it).

Allright, I'll update this patch (and the following one implementing
vm_{map,unmap}_user_ram) to use WARN_ON_ONCE().

Thanks!

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com