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@linux-foundation.org wrote:

On Thu, Nov 1, 2018 at 12:57 PM Mathieu Desnoyers
[off-list ref] wrote:

quoted

quoted

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