Re: [PATCH 02/13] mm/rmap: update to new mmu_notifier semantic

[Andrea Arcangeli <aarcange@redhat.com>]

On Wed, Aug 30, 2017 at 11:00:32AM -0700, Nadav Amit wrote:
> It is not trivial to flush TLBs (primary or secondary) without holding the
> page-table lock, and as we recently encountered this resulted in several
> bugs [1]. The main problem is that even if you perform the TLB flush
> immediately after the PT-lock is released, you cause a situation in which
> other threads may make decisions based on the updated PTE value, without
> being aware that a TLB flush is needed.
> 
> For example, we recently encountered a Linux bug when two threads run
> MADV_DONTNEED concurrently on the same address range [2]. One of the threads
> may update a PTE, setting it as non-present, and then deferring the TLB
> flush (for batching). As a result, however, it would cause the second
> thread, which also changes the PTEs to assume that the PTE is already
> non-present and TLB flush is not necessary. As a result the second core may
> still hold stale PTEs in its TLB following MADV_DONTNEED.

The source of those complex races that requires taking into account
nested tlb gather to solve it, originates from skipping primary MMU
tlb flushes depending on the value of the pagetables (as an
optimization).

For mmu_notifier_invalidate_range_end we always ignore the value of
the pagetables and mmu_notifier_invalidate_range_end always runs
unconditionally invalidating the secondary MMU for the whole range
under consideration. There are no optimizations that attempts to skip
mmu_notifier_invalidate_range_end depending on the pagetable value and
there's no TLB gather for secondary MMUs either. That is to keep it
simple of course.

As long as those mmu_notifier_invalidate_range_end stay unconditional,
I don't see how those races you linked, can be possibly relevant in
evaluating if ->invalidate_range (again only for iommuv2 and
intel-svm) has to run inside the PT lock or not.

> There is a swarm of such problems, and some are not too trivial. Deferring
> TLB flushes needs to be done in a very careful manner.

I agree it's not trivial, but I don't think any complexity comes from
above.

The only complexity is about, what if the page is copied to some other
page and replaced, because the copy is the only case where coherency
could be retained by the primary MMU. What if the primary MMU starts
working on the new page in between PT lock release and
mmu_notifier_invalidate_range_end, while the secondary MMU is stuck on
the old page? That is the only problem we deal with here, the copy to
other page and replace. Any other case that doesn't involve the copy
seems non coherent by definition, and you couldn't measure it.

I can't think of a scenario that requires the explicit
mmu_notifier_invalidate_range call before releasing the PT lock, at
least for try_to_unmap_one.

Could you think of a scenario where calling ->invalidate_range inside
mmu_notifier_invalidate_range_end post PT lock breaks iommuv2 or
intel-svm? Those two are the only ones requiring
->invalidate_range calls, all other mmu notifier users are safe
without running mmu_notifier_invalidate_range_end under PT lock thanks
to mmu_notifier_invalidate_range_start blocking the secondary MMU.

Could you post a tangible scenario that invalidates my theory that
those mmu_notifier_invalidate_range calls inside PT lock would be
superfluous?

Some of the scenarios under consideration:

1) migration entry -> migration_entry_wait -> page lock, plus
   migrate_pages taking the lock so it can't race with try_to_unmap
   from other places
2) swap entry -> lookup_swap_cache -> page lock (page not really replaced)
3) CoW -> do_wp_page -> page lock on old page
4) KSM -> replace_page -> page lock on old page
5) if the pte is zapped as in MADV_DONTNEED, no coherency possible so
   it's not measurable that we let the guest run a bit longer on the
   old page past PT lock release

If you could post a multi CPU trace that shows how iommuv2 or
intel-svm are broken if ->invalidate_range is run inside
mmu_notifier_invalidate_range_end post PT lock in try_to_unmap_one it
would help.

Of course if we run mmu_notifier_invalidate_range inside PT lock and
we remove ->invalidate_range from mmu_notifier_invalidate_range_stop
all will be obviously safe, so we could just do it to avoid thinking
about the above, but the resulting code will be uglier and less
optimal (even when disarmed there will be dummy branches I wouldn't
love) and I currently can't see why it wouldn't be safe.

Replacing a page completely without any relation to the old page
content allows no coherency anyway, so even if it breaks you cannot
measure it because it's undefined.

If the page has a relation with the old contents and coherency has to
be provided for both primary MMU and secondary MMUs, this relation
between old and new page during the replacement, is enforced by some
other mean besides the PT lock, migration entry on locked old page
with migration_entry_wait and page lock in migrate_pages etc..

Thanks,
Andrea

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