Re: [PATCH 39/41] kernel/fork: throttle call_rcu() calls in vm_area_free

[Matthew Wilcox]

On Mon, Jan 23, 2023 at 08:18:37PM +0100, Michal Hocko wrote:
> On Mon 23-01-23 18:23:08, Matthew Wilcox wrote:
> > On Mon, Jan 23, 2023 at 09:46:20AM -0800, Suren Baghdasaryan wrote:
> [...]
> > > Yes, batching the vmas into a list and draining it in remove_mt() and
> > > exit_mmap() as you suggested makes sense to me and is quite simple.
> > > Let's do that if nobody has objections.
> > 
> > I object.  We *know* nobody has a reference to any of the VMAs because
> > you have to have a refcount on the mm before you can get a reference
> > to a VMA.  If Michal is saying that somebody could do:
> > 
> > 	mmget(mm);
> > 	vma = find_vma(mm);
> > 	lock_vma(vma);
> > 	mmput(mm);
> > 	vma->a = b;
> > 	unlock_vma(mm, vma);
> > 
> > then that's something we'd catch in review -- you obviously can't use
> > the mm after you've dropped your reference to it.
> 
> I am not claiming this is possible now. I do not think we want to have
> something like that in the future either but that is really hard to
> envision. I am claiming that it is subtle and potentially error prone to
> have two different ways of mass vma freeing wrt. locking. Also, don't we
> have a very similar situation during last munmaps?

We shouldn't have two ways of mass VMA freeing.  Nobody's suggesting that.
There are two cases; there's munmap(), which typically frees a single
VMA (yes, theoretically, you can free hundreds of VMAs with a single
call which spans multiple VMAs, but in practice that doesn't happen),
and there's exit_mmap() which happens on exec() and exit().

For the munmap() case, just RCU-free each one individually.  For the
exit_mmap() case, there's no need to use RCU because nobody should still
have a VMA pointer after calling mmdrop() [1]

[1] Sorry, the above example should have been mmgrab()/mmdrop(), not
mmget()/mmput(); you're not allowed to look at the VMA list with an
mmget(), you need to have grabbed.