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

[Mathieu Desnoyers]

----- On Nov 1, 2018, at 11:00 PM, Linus Torvalds torvalds@xxxxxxxxxxxxxxxxxxxx wrote:

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