Re: userfaultfd: two-step UFFDIO_API always gives -EINVAL
From: stsp
Date: Mon Nov 25 2024 - 12:33:13 EST
25.11.2024 20:13, Peter Xu пишет:
Old kernels will fail with -EINVAL, new will succeed. It's already an ABI
violation, IMHO.
Not to mention I'm not sure what could happen if uffd feature flags can
change on the fly. Your proposal means it can happen when anon missing
trap is enabled at least. That's probably unwanted, and unnecessary
complexity to maintain to the kernel.
OK, thanks for considering.
By the way, as we are at it, I have
this usage question. I initially intended
to use UFFD_FEATURE_WP_ASYNC, but
it appears (and is documented so) to not
deliver any notification.
Why not?
I am currently using UFFD_FEATURE_PAGEFAULT_FLAG_WP,
but I only want to monitor the fact that
the page was written to. With
UFFD_FEATURE_WP_ASYNC it would be
much faster, as the kernel resolves the
fault for me. Yes, I've seen the mentioning
of /proc/pages in docs (I don't even have
/proc/pages - perhaps it was ment to be
/proc/<pid>/pages?), but why such a
complexity if all I need is the notification
similar to what I get from
UFFD_FEATURE_PAGEFAULT_FLAG_WP?