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

[Suren Baghdasaryan]

On Mon, Jan 23, 2023 at 10:23 AM Matthew Wilcox <willy at infradead.org> wrote:
>
> 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);

More precisely, it's:
         mmget(mm);
         vma = lock_vma_under_rcu(mm, addr); -> calls vma_read_trylock(vma)
         mmput(mm);
         vma->a = b;
         vma_read_unlock(vma);

To be fair, vma_read_unlock() does not take mm as a parameter, so one
could have an impression that mm doesn't need to be pinned at the time
of its call.


>
> 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.
>