Subject: Re: [RFC PATCH V2 0/5] vhost: accelerate metadata access through vmap()

From: Andrea Arcangeli <[email-masked]>
Date: Thu, 14 Mar 2019 15:33:33 -0400

Hello Jason,

On Thu, Mar 14, 2019 at 09:49:03PM +0800, Jason Wang wrote:
> Yes since we don't want to slow down 32bit.

If you've a lot of ram there's no justification to stick to a 32bit
kernel, so I don't think there's need to maintain a separate model
just for 32bit. I really wouldn't care about the performance of 32bit
with >700MB of RAM if that would cause any maintenance burden. Let's
focus on the best 64bit implementation that will work equally
optimally on 32bit with <= 700M of RAM.

Talking to Jerome about the set_page_dirty issue, he raised the point
of what happens if two thread calls a mmu notifier invalidate
simultaneously. The first mmu notifier could call set_page_dirty and
then proceed in try_to_free_buffers or page_mkclean and then the
concurrent mmu notifier that arrives second, then must not call
set_page_dirty a second time.

With KVM sptes mappings and vhost mappings you would call
set_page_dirty (if you invoked gup with FOLL_WRITE) only when
effectively tearing down any secondary mapping (you've got pointers in
both cases for the mapping). So there's no way to risk a double
set_page_dirty from concurrent mmu notifier invalidate because the
invalidate takes a lock when it has to teardown the mapping and so
set_page_dirty is only run in the first invalidate method and not in
the second. In the spte case even better, as you wouldn't need to call
it even at teardown time unless the spte is dirty (if shadow mask
allows dirty sptes with EPT2 or NPT or shadow MMU).

If you instead had to invalidate a secondary MMU mapping that isn't
tracked by the driver (again: not vhost nor KVM case), you could have
used the dirty bit of the kernel pagetable to call set_page_dirty and
disambiguate but that's really messy, and it would prevent the use of
gigapages in the direct mapping too and it'd require vmap for 4k
tracking.

To make sure set_page_dirty is run a single time no matter if the
invalidate known when a mapping is tear down, I suggested the below
model:

  access = FOLL_WRITE

repeat:
  page = gup_fast(access)
  put_page(page) /* need a way to drop FOLL_GET from gup_fast instead! */

  spin_lock(mmu_notifier_lock);
  if (race with invalidate) {
    spin_unlock..
    goto repeat;
  }
  if (access == FOLL_WRITE)
    set_page_dirty(page)
  establish writable mapping in secondary MMU on page
  spin_unlock

(replace spin_lock with mutex_lock for vhost of course if you stick to
a mutex and _start/_end instead of non-sleepable ->invalidate_range)

"race with invalidate" is the usual "mmu_notifier_retry" in kvm_host.h
to be implemented for vhost.

We could add a FOLL_DIRTY flag to add to FOLL_TOUCH to move the
set_page_dirty inside GUP forced (currently it's not forced if the
linux pte is already dirty). And we could remove FOLL_GET.

Then if you have the ability to disambiguate which is the first
invalidate that tears down a mapping to any given page (vhost can do
that trivially, it's just a pointer to a page struct to kmap), in the
mmu notifier invalidate just before dropping the spinlock you would
do this check:

def vhost_invalidate_range_start():
   [..]
   spin_lock(mmu_notifier_lock);
   [..]
   if (vhost->page_pointer) {
      if (access == FOLL_WRITE)
	VM_WARN_ON(!PageDirty(vhost->page_pointer));
      vhost->page_pointer = NULL;
      /* no put_page, already done at gup time */
   }
   spin_unlock(..

Generally speaking set_page_dirty is safer done after the last
modification to the data of the page. However the way stable page
works, as long as the mmu notifier invalidate didn't run, the PG_dirty
cannot go away.

So this model solves the issue with guaranteeing a single
set_page_dirty is run before page_mkclean or try_to_free_buffers can
run, even for drivers that implement the invalidate as a generic "TLB
flush" across the whole secondary MMU and that cannot disambiguate the
first invalidate from a second invalidate if they're issued
concurrently on the same address by two different CPUs.

So for those drivers that can disambiguate trivially (like KVM and
vhost) we'll just add a debug check in the invalidate to validate the
common code for all mmu notifier users.

This is the solution for RDMA hardware and everything that can support
mmu notifiers too and they can take infinitely long secondary MMU
mappins without interfering with stable pages at all (i.e. long term
pins but without pinning) perfectly safe and transparent to the whole
stable page code.

I think O_DIRECT for stable pages shall be solved taking the page lock
or writeback lock or a new rwsem in the inode that is taken for
writing by page_mkclean and try_to_free_buffers and for reading by
outstanding O_DIRECT in flight I/O, like I suggested probably ages ago
but then we only made GUP take the page pin, which is fine for mmu
notifier actually (except those didn't exist back then). To solve
O_DIRECT we can leverage the 100% guarantee that the pin will be
dropped ASAP and stop page_mkclean and stop or trylock in
try_to_free_buffers in such case.

mm_take_all_locks is major hurdle that prevents usage in O_DIRECT
case, even if we "cache it" if you fork(); write; exit() in a loop
it'll still cause heavy lock overhead. MMU notifier registration isn't
intended to happen in fast and frequent paths like the write()
syscall. Dropping mm_take_all_locks would bring other downsides: a
regular quiescent point can happen in between _start/_end and _start
must be always called first all the mmu notifier retry counters we
rely on would break. One way would be to add srcu_read_lock _before_
you can call mm_has_mm_has_notifiers(mm), then yes we could replace
mm_take_all_locks with synchronize_srcu. It would save a lot of CPU
and a ton of locked operations, but it'd potentially increase the
latency of the registration so the first O_DIRECT write() in a process
could still potentially stall (still better than mm_take_all_locks
which would use a lot more CPU and hurt SMP scalability in threaded
programs). The downside is all VM fast paths would get some overhead
because of srcu_read_lock even when mmu notifier is not registered,
which is what we carefully avoided by taking a larger hit in the
registration with mm_take_all_locks. This is why I don't think mmu
notifier is a good solution to solve O_DIRECT stable pages even in
theory O_DIRECT could use the exact same model as vhost to solve
stable pages.

If we solve O_DIRECT with new per-page locking or a new rwsem inode
lock leveraging the fact we're guaranteed the pin to go away ASAP,
what's left is the case of PCI devices mapping userland memory for
indefinite amount of time that cannot support MMU notifier because of
hardware limitations.

Mentioning virtualization as a case taking long term PIN is incorrect,
that didn't happen since the introduction of MMU notifier.

vfio for device assignment to KVM takes the long term pins, but that's
because the iommus may not support the mmu notifier, mmu notifier
could solve the vfio case too.

PCI devices that pretend to keep a constant mapping on userland
virtual memory and that cannot support MMU notifier because they lack
a secondary MMU, cripple the Linux VM and there's no solution to
that. Even if we solve the stable pages problem, they will still
practically disable all advanced VM features.

I think it would be ok to solve the stable pages in 3 different ways:

1) mmu notifier as above when mmu_notifier_register doesn't need to
   run often and the hardware can support it

2) O_DIRECT with new locking stopping page_mkclean/try_to_free_buffers
   until I/O completion, leveraging the fact the pin&lock are
   guaranteed to be dropped&released ASAP

3) something else for pci devices that cannot support MMU notifier
   because of hardware limitations, bounce buffers would be fine as
   well

I'm not even sure if in 3) it is worth worrying about being able to
routinely flush to disk the dirty data, but then bounce buffers could
solve that. Altering the page mapped in the pte sounds like a complex
solution when you could copy the physical page just before issuing the
I/O in the writeback code. To find if a GUP pin exists it's enough to
check what KSM does:

		/*
		 * Check that no O_DIRECT or similar I/O is in progress on the
		 * page
		 */
		if (page_mapcount(page) + 1 + swapped != page_count(page)) {

That can give false positives (even through random pins coming from
speculative cache lookups), but not false negatives.

Thanks,
Andrea