Re: [RFC PATCH 04/11] mm/hmm: Simplify hmm_get_or_create and make it reliable

[Jason Gunthorpe <jgg@ziepe.ca>]

On Thu, May 23, 2019 at 04:38:28PM -0700, Ralph Campbell wrote:
> 
> On 5/23/19 8:34 AM, Jason Gunthorpe wrote:
> > From: Jason Gunthorpe <jgg@mellanox.com>
> > 
> > As coded this function can false-fail in various racy situations. Make it
> > reliable by running only under the write side of the mmap_sem and avoiding
> > the false-failing compare/exchange pattern.
> > 
> > Also make the locking very easy to understand by only ever reading or
> > writing mm->hmm while holding the write side of the mmap_sem.
> > 
> > Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
> >   mm/hmm.c | 75 ++++++++++++++++++++------------------------------------
> >   1 file changed, 27 insertions(+), 48 deletions(-)
> > 
> > diff --git a/mm/hmm.c b/mm/hmm.c
> > index e27058e92508b9..ec54be54d81135 100644
> > +++ b/mm/hmm.c
> > @@ -40,16 +40,6 @@
> >   #if IS_ENABLED(CONFIG_HMM_MIRROR)
> >   static const struct mmu_notifier_ops hmm_mmu_notifier_ops;
> > -static inline struct hmm *mm_get_hmm(struct mm_struct *mm)
> > -{
> > -	struct hmm *hmm = READ_ONCE(mm->hmm);
> > -
> > -	if (hmm && kref_get_unless_zero(&hmm->kref))
> > -		return hmm;
> > -
> > -	return NULL;
> > -}
> > -
> >   /**
> >    * hmm_get_or_create - register HMM against an mm (HMM internal)
> >    *
> > @@ -64,11 +54,20 @@ static inline struct hmm *mm_get_hmm(struct mm_struct *mm)
> >    */
> >   static struct hmm *hmm_get_or_create(struct mm_struct *mm)
> >   {
> > -	struct hmm *hmm = mm_get_hmm(mm);
> > -	bool cleanup = false;
> > +	struct hmm *hmm;
> > -	if (hmm)
> > -		return hmm;
> > +	lockdep_assert_held_exclusive(mm->mmap_sem);
> > +
> > +	if (mm->hmm) {
> > +		if (kref_get_unless_zero(&mm->hmm->kref))
> > +			return mm->hmm;
> > +		/*
> > +		 * The hmm is being freed by some other CPU and is pending a
> > +		 * RCU grace period, but this CPU can NULL now it since we
> > +		 * have the mmap_sem.
> > +		 */
> > +		mm->hmm = NULL;
> 
> Shouldn't there be a "return NULL;" here so it doesn't fall through and
> allocate a struct hmm below?

No, this function should only return NULL on memory allocation
failure.

In this case another thread is busy freeing the hmm but wasn't able to
update mm->hmm to null due to a locking constraint. So we make it null
on behalf of the other thread and allocate a fresh new hmm that is
valid. The freeing thread will complete the free and do nothing with
mm->hmm.

> >   static void hmm_fee_rcu(struct rcu_head *rcu)
> 
> I see Jerome already saw and named this hmm_free_rcu()
> which I agree with.

I do love my typos :)

> >   {
> > +	struct hmm *hmm = container_of(rcu, struct hmm, rcu);
> > +
> > +	down_write(&hmm->mm->mmap_sem);
> > +	if (hmm->mm->hmm == hmm)
> > +		hmm->mm->hmm = NULL;
> > +	up_write(&hmm->mm->mmap_sem);
> > +	mmdrop(hmm->mm);
> > +
> >   	kfree(container_of(rcu, struct hmm, rcu));
> >   }
> >   static void hmm_free(struct kref *kref)
> >   {
> >   	struct hmm *hmm = container_of(kref, struct hmm, kref);
> > -	struct mm_struct *mm = hmm->mm;
> > -
> > -	mmu_notifier_unregister_no_release(&hmm->mmu_notifier, mm);
> > -	spin_lock(&mm->page_table_lock);
> > -	if (mm->hmm == hmm)
> > -		mm->hmm = NULL;
> > -	spin_unlock(&mm->page_table_lock);
> > -
> > -	mmdrop(hmm->mm);
> > +	mmu_notifier_unregister_no_release(&hmm->mmu_notifier, hmm->mm);
> >   	mmu_notifier_call_srcu(&hmm->rcu, hmm_fee_rcu);
> >   }
> > 
> 
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Ah, you should not send this trailer to the public mailing lists.

Thanks,
Jason