Re: userfaultfd: two-step UFFDIO_API always gives -EINVAL

[stsp <stsp2@yandex.ru>]

25.11.2024 19:58, Peter Xu пишет:
> On Mon, Nov 25, 2024 at 07:15:10PM +0300, stsp wrote:
>> Man page clearly talks about
>> "the userfaultfd object" (one object)
>> when mandating the "two-step handshake".
>> I spent hours of head-scratching
>> before went looking into the sources,
>> and even then I was confident the man
>> page is right: people should always assume
>> documentation is correct, code is buggy.
> Hmm yes.  I didn't pay much attention initially, but then after I read the
> latest man-pages/, especially "UFFDIO_API(2const)" I found it looks indeed
> wrong in the doc.
>
> In this case we can't change the code because we need to keep it working
> like before to not break ABI.  We may still update the doc.
I wonder if some non-ABI-breaker
is possible, like eg keep the current
behavior of "features=0", but allow
to (optionally) override that by a
non-0 request? Yes, I've seen kselftests
are trying to double-register after 0,
but IIRC they tried to register wrong
options, which would fail anyway.