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

Andi Kleen
2011-03-10T16:00:32Z

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.

> This is the first of a two part series that implements safe writes to
> /proc/pid/mem. I will be posting the second series to lkml shortly. These

Making every syscall slower for /proc/pid/mem doesn't seem like a good
tradeoff to me. Please solve this in some other way.

-Andi