Re: [PATCH 0/5] make *_gate_vma accept mm_struct instead of task_struct II

Andi Kleen
2011-03-10T16:38:09Z

On Thu, Mar 10, 2011 at 08:00:32AM -0800, Andi Kleen wrote:
> On Tue, Mar 08, 2011 at 07:31:56PM -0500, Stephen Wilson wrote:
> >
> > Morally, the question of whether an address lies in a gate vma should be asked
> > with respect to an mm, not a particular task.
> >
> > Practically, dropping the dependency on task_struct will help make current and
> > future operations on mm's more flexible and convenient. In particular, it
> > allows some code paths to avoid the need to hold task_lock.
> >
> > The only architecture this change impacts in any significant way is x86_64.
> > The principle change on that architecture is to mirror TIF_IA32 via
> > a new flag in mm_context_t.
>
> The problem is -- you're adding a likely cache miss on mm_struct for
> every 32bit compat syscall now, even if they don't need mm_struct
> currently (and a lot of them do not) Unless there's a very good
> justification to make up for this performance issue elsewhere
> (including numbers) this seems like a bad idea.

Hmm I see you're only setting it on exec time actually on rereading
the patches. I thought you were changing TS_COMPAT which is in
the syscall path.

Never mind. I have no problems with doing such a change on exec
time.

-Andi