linux-kernel - Re: userfaultfd: two-step UFFDIO

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Message-ID: <b0818813-5a4c-4621-9810-dc7443a23dd1@yandex.ru>
Date: Mon, 25 Nov 2024 19:15:10 +0300
From: stsp <stsp2@...dex.ru>
To: Peter Xu <peterx@...hat.com>
Cc: Linux kernel <linux-kernel@...r.kernel.org>,
 Muhammad Usama Anjum <usama.anjum@...labora.com>
Subject: Re: userfaultfd: two-step UFFDIO_API always gives -EINVAL

25.11.2024 18:59, Peter Xu пишет:
> I agree it's slightly confusing but it's intended.  It's like that since
> the start, so I think we should still keep the behavior.
>
> The userapp needs to create one extra userfaultfd to detect supported
> features in the kernel.  To use it after a probe request, you'll need to
> close() the fd, redo the userfaultfd syscall to create another fd.
Hi Peter, thanks for info.
Unfortunately man page doesn't
say that. In fact if it did, I won't be
using the second userfaultfd just
for that, anyway. :)

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.

Would it be possible to re-document
this part? As all test-cases in kernel
do not use 2-steps - how about just
removing that part from man page?
Suggesting another fd would be strange. :)