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

[Michal Hocko]

On Mon 23-01-23 19:30:43, Matthew Wilcox wrote:
> 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().

This requires special casing remove_vma for those two different paths
(exit_mmap and remove_mt).  If you ask me that sounds like a suboptimal
code to even not handle potential large munmap which might very well be
a rare thing as you say. But haven't we learned that sooner or later we
will find out there is somebody that cares afterall? Anyway, this is not
something I care about all that much. It is just weird to special case
exit_mmap, if you ask me. Up to Suren to decide which way he wants to
go. I just really didn't like the initial implementation of batching
based on a completely arbitrary batch limit and lazy freeing.
-- 
Michal Hocko
SUSE Labs