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

[Matthew Wilcox]

On Mon, Jan 23, 2023 at 09:46:20AM -0800, Suren Baghdasaryan wrote:
> On Mon, Jan 23, 2023 at 9:16 AM Michal Hocko <mhocko at suse.com> wrote:
> >
> > On Mon 23-01-23 09:07:34, Suren Baghdasaryan wrote:
> > > On Mon, Jan 23, 2023 at 8:55 AM Michal Hocko <mhocko at suse.com> wrote:
> > > >
> > > > On Mon 23-01-23 08:22:53, Suren Baghdasaryan wrote:
> > > > > On Mon, Jan 23, 2023 at 1:56 AM Michal Hocko <mhocko at suse.com> wrote:
> > > > > >
> > > > > > On Fri 20-01-23 09:50:01, Suren Baghdasaryan wrote:
> > > > > > > On Fri, Jan 20, 2023 at 9:32 AM Matthew Wilcox <willy at infradead.org> wrote:
> > > > > > [...]
> > > > > > > > The page fault handler (or whatever other reader -- ptrace, proc, etc)
> > > > > > > > should have a refcount on the mm_struct, so we can't be in this path
> > > > > > > > trying to free VMAs.  Right?
> > > > > > >
> > > > > > > Hmm. That sounds right. I checked process_mrelease() as well, which
> > > > > > > operated on mm with only mmgrab()+mmap_read_lock() but it only unmaps
> > > > > > > VMAs without freeing them, so we are still good. Michal, do you agree
> > > > > > > this is ok?
> > > > > >
> > > > > > Don't we need RCU procetions for the vma life time assurance? Jann has
> > > > > > already shown how rwsem is not safe wrt to unlock and free without RCU.
> > > > >
> > > > > Jann's case requires a thread freeing the VMA to be blocked on vma
> > > > > write lock waiting for the vma real lock to be released by a page
> > > > > fault handler. However exit_mmap() means mm->mm_users==0, which in
> > > > > turn suggests that there are no racing page fault handlers and no new
> > > > > page fault handlers will appear. Is that a correct assumption? If so,
> > > > > then races with page fault handlers can't happen while in exit_mmap().
> > > > > Any other path (other than page fault handlers), accesses vma->lock
> > > > > under protection of mmap_lock (for read or write, does not matter).
> > > > > One exception is when we operate on an isolated VMA, then we don't
> > > > > need mmap_lock protection, but exit_mmap() does not deal with isolated
> > > > > VMAs, so out of scope here. exit_mmap() frees vm_area_structs under
> > > > > protection of mmap_lock in write mode, so races with anything other
> > > > > than page fault handler should be safe as they are today.
> > > >
> > > > I do not see you talking about #PF (RCU + vma read lock protected) with
> > > > munmap. It is my understanding that the latter will synchronize over per
> > > > vma lock (along with mmap_lock exclusive locking). But then we are back
> > > > to the lifetime guarantees, or do I miss anything.
> > >
> > > munmap() or any VMA-freeing operation other than exit_mmap() will free
> > > using call_rcu(), as implemented today. The suggestion is to free VMAs
> > > directly, without RCU grace period only when done from exit_mmap().
> >
> > OK, I have clearly missed that. This makes more sense but it also adds
> > some more complexity and assumptions - a harder to maintain code in the
> > end. Whoever wants to touch this scheme in future would have to
> > re-evaluate all of them. So, I would just avoid that special casing if
> > that is feasible.
> 
> Ok, I understand your point.
> 
> >
> > Dealing with the flood of call_rcu during exit_mmap is a trivial thing
> > to deal with as proposed elsewhere (just batch all of them in a single
> > run). This will surely add some more code but at least the locking would
> > consistent.
> 
> 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.

Having all this extra code to solve two problems badly is a very poor
choice.  We have two distinct problems, each of which has a simple,
efficient solution.