Re: [RFC PATCH for 4.21 03/16] mm: Replace BUG_ON() by WARN_ON() in vm_unmap_ram()

From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: rostedt <rostedt@goodmis.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Peter Zijlstra <peterz@infradead.org>,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	Boqun Feng <boqun.feng@gmail.com>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	linux-api <linux-api@vger.kernel.org>,
	Andy Lutomirski <luto@amacapital.net>,
	Dave Watson <davejwatson@fb.com>, Paul Turner <pjt@google.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Russell King <linux@arm.linux.org.uk>,
	Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
	Andi Kleen <andi@firstfloor.org>, Chris Lameter <cl@linux.com>,
	Ben Maurer <bmaurer@fb.com>,
	Josh Triplett <josh@joshtriplett.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	Michael Kerrisk <mtk.manpages@gmail.com>,
	Joel Fernandes <joelaf@google.com>,
	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
	Matthew Wilcox <mawilcox@microsoft.com>
Subject: Re: [RFC PATCH for 4.21 03/16] mm: Replace BUG_ON() by WARN_ON() in vm_unmap_ram()
Date: Thu, 1 Nov 2018 18:17:19 -0400 (EDT)	[thread overview]
Message-ID: <1770995215.4046.1541110639818.JavaMail.zimbra@efficios.com> (raw)
In-Reply-To: <CAHk-=wiOm8x5KnBQH+FYN76sSjCeP2_9cxgzu4WArR3jP0MD8g@mail.gmail.com>

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

> On Thu, Nov 1, 2018 at 12:57 PM Mathieu Desnoyers
> <mathieu.desnoyers@efficios.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