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

[Liam R. Howlett]

* Michal Hocko <mhocko@xxxxxxxx> [230123 15:00]:
> 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.

exit_mmap() is already a special case for locking (and statistics).
This exists today to optimize the special exit scenario.  I don't think
it's a question of sub-optimal code but what we can get away without
doing in the case of the process exit.