Re: [PATCH/RFC] ummunot: Userspace support for MMU notifications

[Roland Dreier <rdreier@cisco.com>]

 > > >  > >  3. mmap() one page at offset 0 to map a kernel page that contains a
 > > >  > >     generation counter that is incremented each time an event is
 > > >  > >     generated.  This allows userspace to have a fast path that checks
 > > >  > >     that no events have occurred without a system call.

 > Looks like a vsyscall to me.

Yes, in a way, although it is quite a bit simpler in the sense that it
doesn't require any arch-specific code (or indeed any code mapped from
the kernel) and is automatically available in a portable way.
Implementing this as a vsyscall seems as if it would add a lot of
complexity to the kernel side without much simplification on the
userspace side (in fact, hooking up the vsyscall is probably more code
than just doing mmap() + dereferencing a pointer).

 > # mount -t debugfs nodev /sys/kernel/debug
 > # ls /sys/kernel/debug/tracing

The use case I have in mind is for unprivileged user applications to use
this.  So requiring debugfs to be mounted hurts (since that isn't done
by default), and using the files in tracing really hurts, since they're
currently created with mode 0644 and so tracing can't be controlled by
unprivileged users.

[ASIDE: why is trace_marker created with the strange permission of 0220
when it is owned by root:root -- is there any reason for the group write
permission, or should it just be 0200 permission?]

In fact the whole model of ftrace seems to be a single privileged user
controlling a single context; the use case for ummunotify is that a lot
of processes running unprivileged (and possibly as multiple different
users) each want to get events for parts of their own address space.

So

 > # echo "ptr > 0xffffffff81100000 && ptr < 0xffffffff8113000" > events/kmem/kmalloc/filter 

is very cool; but what I would want is for a given process to be able to
say "please give me events for ptr in the following 100 ranges A..B,
C..D, ..." and "oh and add this range X..Y" and "oh don't give me events
for C..D anymore".  And that process should only get events about its
own address range; and 10 other (unprivileged) processes should be able
to do the same thing simultaneously.

Also is there a raw format for setting the filters that lets userspace
swap them atomically (ie change from filter A to filter B with a
guarantee that filter A is in effect right up to the time filter B is in
effect with no window where eg no filter is in effect).

 > Well, if you need to add hooks, definitely at least use tracepoints. (see 
 > the TRACE_EVENT code in include/trace/events/*.h)

I don't think I'm adding hooks -- the mmu notifier infrastructure
already suits me perfectly.  The only thing I'm doing is forwarding the
events delivered by mmu notifiers up to userspace, but not really in a
way that's very close to what ftrace does (I don't think).

It seems handling multiple unprivileged contexts accessing different
streams of trace events is going to require pretty huge ftrace changes.
And ummunotify is currently about 400 lines of code total (+ 300 lines
of comments :) so we're not going to simplify that code dramatically.
The hope I guess would be that a common interface would make things
conceptually simpler, but I don't see how to slot ftrace and ummunotify
together very cleanly.

Thanks,
  Roland