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SubjectRe: Yes, people use FOLL_FORCE ;)
Hi Linus,

> But it sounds like your JIT case actually uses it for writing -
> but if you can write a small blurb about it, that would be nice.

yes, we use it for writing. Happy to describe the scheme in more detail.

> (b) it would probably be nice to limit FOLL_FORCE in general as much
> as possible, so if your case is about writing to your very _own_
> memory mapping, as opposed to writing to another process' memory,
> maybe we can do something like
>
> if (mm == current->mm)
> flags |= FOLL_FORCE;
>
> which at least avoids the whole "let's change the VM in odd ways for a
> process that isn't even me".

While this would fix our current use case, we do have a use case for modifying
non-local address space as well (putting the JIT into a different
process). Similarly,
the rr use case precisely uses the remote mm case. I think in general
this feature
is very useful for anybody who needs to precisely control the execution of some
other process. Various debuggers (gdb/lldb/rr) certainly fall into
that category, but
there's another class of such processes (wine, various emulators) which may want
to do that kind of thing. Now, I suspect most of these will have the
other process
under ptrace control, so maybe allowing (same_mm || ptraced) would be ok, but
at least for the sandbox/remote-jit use case, it would be perfectly
reasonable to not
have the jit server be a ptracer.

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