Re: [PATCH] mm/mmn: prevent unpaired invalidate_start and invalidate_end with non-blocking

[Michal Hocko <mhocko@suse.com>]

On Wed 07-08-19 19:16:32, Jason Gunthorpe wrote:
> Many users of the mmu_notifier invalidate_range callbacks maintain
> locking/counters/etc on a paired basis and have long expected that
> invalidate_range start/end are always paired.
> 
> The recent change to add non-blocking notifiers breaks this assumption
> when multiple notifiers are present in the list as an EAGAIN return from a
> later notifier causes all earlier notifiers to get their
> invalidate_range_end() skipped.
> 
> During the development of non-blocking each user was audited to be sure
> they can skip their invalidate_range_end() if their start returns -EAGAIN,
> so the only place that has a problem is when there are multiple
> subscriptions.
> 
> Due to the RCU locking we can't reliably generate a subset of the linked
> list representing the notifiers already called, and generate an
> invalidate_range_end() pairing.
> 
> Rather than design an elaborate fix, for now, just block non-blocking
> requests early on if there are multiple subscriptions.

Which means that the oom path cannot really release any memory for
ranges covered by these notifiers which is really unfortunate because
that might cover a lot of memory. Especially when the particular range
might not be tracked at all, right?

So I cannot really say I am happy with this much. People will simply
start complaining that the OOM killer has killed more victims because
the first one hasn't really released its memory during the tear down.

If a different fix is indeed too elaborate then make sure to let users
known that there is a restriction in place and dump something useful
into the kernel log.

> Fixes: 93065ac753e4 ("mm, oom: distinguish blockable mode for mmu notifiers")
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: "Jérôme Glisse" <jglisse@redhat.com>
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
> ---
>  include/linux/mmu_notifier.h |  1 +
>  mm/mmu_notifier.c            | 15 +++++++++++++++
>  2 files changed, 16 insertions(+)
> 
> HCH suggested to make the locking common so we don't need to have an
> invalidate_range_end, but that is a longer journey.
> 
> Here is a simpler stop-gap for this bug. What do you think Michal?
> I don't have a good way to test this flow ..

Testing is quite simple if you have any blocking notifiers at hand.
Simple trigger an OOM condition, mark a process which uses notifier to
be a prime oom candidate by
echo 1000 > /proc/<pid>/oom_score_adj

and see how it behaves.

> This lightly clashes with the other mmu notififer series I just sent,
> so it should go to either -rc or hmm.git
> 
> Thanks,
> Jason
> 
> diff --git a/include/linux/mmu_notifier.h b/include/linux/mmu_notifier.h
> index b6c004bd9f6ad9..170fa2c65d659c 100644
> --- a/include/linux/mmu_notifier.h
> +++ b/include/linux/mmu_notifier.h
> @@ -53,6 +53,7 @@ struct mmu_notifier_mm {
>  	struct hlist_head list;
>  	/* to serialize the list modifications and hlist_unhashed */
>  	spinlock_t lock;
> +	bool multiple_subscriptions;
>  };
>  
>  #define MMU_NOTIFIER_RANGE_BLOCKABLE (1 << 0)
> diff --git a/mm/mmu_notifier.c b/mm/mmu_notifier.c
> index b5670620aea0fc..4e56f75c560242 100644
> --- a/mm/mmu_notifier.c
> +++ b/mm/mmu_notifier.c
> @@ -171,6 +171,19 @@ int __mmu_notifier_invalidate_range_start(struct mmu_notifier_range *range)
>  	int ret = 0;
>  	int id;
>  
> +	/*
> +	 * If there is more than one notififer subscribed to this mm then we
> +	 * cannot support the EAGAIN return. invalidate_range_start/end() must
> +	 * always be paired unless start returns -EAGAIN. When we return
> +	 * -EAGAIN from here the caller will skip all invalidate_range_end()
> +	 * calls. However, if there is more than one notififer then some
> +	 * notifiers may have had a successful invalidate_range_start() -
> +	 * causing imbalance when the end is skipped.
> +	 */
> +	if (!mmu_notifier_range_blockable(range) &&
> +	    range->mm->mmu_notifier_mm->multiple_subscriptions)
> +		return -EAGAIN;
> +
>  	id = srcu_read_lock(&srcu);
>  	hlist_for_each_entry_rcu(mn, &range->mm->mmu_notifier_mm->list, hlist) {
>  		if (mn->ops->invalidate_range_start) {
> @@ -274,6 +287,8 @@ static int do_mmu_notifier_register(struct mmu_notifier *mn,
>  	 * thanks to mm_take_all_locks().
>  	 */
>  	spin_lock(&mm->mmu_notifier_mm->lock);
> +	mm->mmu_notifier_mm->multiple_subscriptions =
> +		!hlist_empty(&mm->mmu_notifier_mm->list);
>  	hlist_add_head_rcu(&mn->hlist, &mm->mmu_notifier_mm->list);
>  	spin_unlock(&mm->mmu_notifier_mm->lock);
>  
> -- 
> 2.22.0
> 

-- 
Michal Hocko
SUSE Labs