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
