On Thu, Nov 22, 2018 at 6:03 AM Christoph Hellwig [off-list ref] wrote:

The 0 return doesn't work for direct mappings that have ram at address
zero and a lot of IOMMUs that start allocating bus space from address
zero, so we can't consolidate on that, but I think we can move everyone
to all-Fs, which the patch here does.

Hmm. Maybe not limit it to just all ones, but actually use the
(standard, for the kernel) IS_ERR_VALUE()?

That basically reserves the last 4095 values in an unsigned long for
error values.

Then those functions could actually return *what* error they
encountered, using just plain

        return -ENOMEM;

or whatever?

               Linus