Re: [PATCH 1/2] mm: use_mm: fix for arches checking mm_users to optimize TLB flushes

[Michal Hocko]

On Tue 18-02-20 13:56:18, Andrea Arcangeli wrote:
> Hi Michal!
> 
> On Tue, Feb 18, 2020 at 12:31:03PM +0100, Michal Hocko wrote:
> > On Mon 03-02-20 15:17:44, Andrea Arcangeli wrote:
> > > alpha, ia64, mips, powerpc, sh, sparc are relying on a check on
> > > mm->mm_users to know if they can skip some remote TLB flushes for
> > > single threaded processes.
> > > 
> > > Most callers of use_mm() tend to invoke mmget_not_zero() or
> > > get_task_mm() before use_mm() to ensure the mm will remain alive in
> > > between use_mm() and unuse_mm().
> > > 
> > > Some callers however don't increase mm_users and they instead rely on
> > > serialization in __mmput() to ensure the mm will remain alive in
> > > between use_mm() and unuse_mm(). Not increasing mm_users during
> > > use_mm() is however unsafe for aforementioned arch TLB flushes
> > > optimizations. So either mmget()/mmput() should be added to the
> > > problematic callers of use_mm()/unuse_mm() or we can embed them in
> > > use_mm()/unuse_mm() which is more robust.
> > 
> > I would prefer we do not do that because then the real owner of the mm
> > cannot really tear down the address space and the life time of it is
> > bound to a kernel thread doing the use_mm. This is undesirable I would
> > really prefer if the existing few users would use mmget only when they
> > really need to access mm.
> 
> If the existing few users that don't already do the explicit mmget
> will have to start doing it too, the end result will be exactly the
> same that you described in your "cons" (lieftime of the mm will still
> be up to who did mmget;use_mm and didn't call unuse_mm;mmput yet).

Well, they should use mmget only for moments when they access the
address space.

> One reason to prefer adding the mmget to the callers to forget it,
> would be to avoid an atomic op in use_mm (for those callers that
> didn't forget it), but if anybody is doing use_mm in a fast path that
> won't be very fast anyway so I didn't think this was worth the
> risk. If that microoptimization in a slow path is the reason we should
> add mmget to the callers that forgot it that would be fine with me
> although I think it's risky because if already happened once and it
> could happen again (and when it happens it only bits a few arches if
> used with a few drivers).

The primary concern really is that use_mm is usually not bound in time
and we do not want to pin the address space for such a long time without
any binding to a killable process.
-- 
Michal Hocko
SUSE Labs