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

[Nadav Amit <nadav.amit@gmail.com>]

[cc’ing IOMMU people, which for some reason are not cc’d]

Andrea Arcangeli <aarcange@redhat.com> wrote:

> 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.

Thanks for the clarifications. It now makes much more sense.

> 
>> 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

For both CoW and KSM, the correctness is maintained by calling
ptep_clear_flush_notify(). If you defer the secondary MMU invalidation
(i.e., replacing ptep_clear_flush_notify() with ptep_clear_flush() ), you
will cause memory corruption, and page-lock would not be enough.

BTW: I see some calls to ptep_clear_flush_notify() which are followed
immediately after by set_pte_at_notify(). I do not understand why it makes
sense, as both notifications end up doing the same thing - invalidating the
secondary MMU. The set_pte_at_notify() in these cases can be changed to
set_pte(). No?

> 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..

I think that basically you are correct, and assuming that you always
notify/invalidate unconditionally any PTE range you read/write, you are
safe. Yet, I want to have another look at the code. Anyhow, just deferring
all the TLB flushes, including those of set_pte_at_notify(), is likely to
result in errors.

Regards,
Nadav
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