Re: [PATCH v2 hmm 06/11] mm/hmm: Hold on to the mmget for the lifetime of the range

[Ralph Campbell <rcampbell@xxxxxxxxxx>]

On 6/6/19 11:44 AM, Jason Gunthorpe wrote:
> From: Jason Gunthorpe <jgg@xxxxxxxxxxxx>
>
> Range functions like hmm_range_snapshot() and hmm_range_fault() call
> find_vma, which requires hodling the mmget() and the mmap_sem for the mm.
>
> Make this simpler for the callers by holding the mmget() inside the range
> for the lifetime of the range. Other functions that accept a range should
> only be called if the range is registered.
>
> This has the side effect of directly preventing hmm_release() from
> happening while a range is registered. That means range->dead cannot be
> false during the lifetime of the range, so remove dead and
> hmm_mirror_mm_is_alive() entirely.
>
> Signed-off-by: Jason Gunthorpe <jgg@xxxxxxxxxxxx>

Looks good to me.
Reviewed-by: Ralph Campbell <rcampbell@xxxxxxxxxx>

> ---
> v2:
>   - Use Jerome's idea of just holding the mmget() for the range lifetime,
>     rework the patch to use that as as simplification to remove dead in
>     one step
> ---
>   include/linux/hmm.h | 26 --------------------------
>   mm/hmm.c            | 28 ++++++++++------------------
>   2 files changed, 10 insertions(+), 44 deletions(-)
>
> diff --git a/include/linux/hmm.h b/include/linux/hmm.h
> index 2ab35b40992b24..0e20566802967a 100644
> --- a/include/linux/hmm.h
> +++ b/include/linux/hmm.h
> @@ -91,7 +91,6 @@
>    * @mirrors_sem: read/write semaphore protecting the mirrors list
>    * @wq: wait queue for user waiting on a range invalidation
>    * @notifiers: count of active mmu notifiers
> - * @dead: is the mm dead ?
>    */
>   struct hmm {
>   	struct mm_struct	*mm;
> @@ -104,7 +103,6 @@ struct hmm {
>   	wait_queue_head_t	wq;
>   	struct rcu_head		rcu;
>   	long			notifiers;
> -	bool			dead;
>   };
>   
>   /*
> @@ -469,30 +467,6 @@ struct hmm_mirror {
>   int hmm_mirror_register(struct hmm_mirror *mirror, struct mm_struct *mm);
>   void hmm_mirror_unregister(struct hmm_mirror *mirror);
>   
> -/*
> - * hmm_mirror_mm_is_alive() - test if mm is still alive
> - * @mirror: the HMM mm mirror for which we want to lock the mmap_sem
> - * Return: false if the mm is dead, true otherwise
> - *
> - * This is an optimization, it will not always accurately return false if the
> - * mm is dead; i.e., there can be false negatives (process is being killed but
> - * HMM is not yet informed of that). It is only intended to be used to optimize
> - * out cases where the driver is about to do something time consuming and it
> - * would be better to skip it if the mm is dead.
> - */
> -static inline bool hmm_mirror_mm_is_alive(struct hmm_mirror *mirror)
> -{
> -	struct mm_struct *mm;
> -
> -	if (!mirror || !mirror->hmm)
> -		return false;
> -	mm = READ_ONCE(mirror->hmm->mm);
> -	if (mirror->hmm->dead || !mm)
> -		return false;
> -
> -	return true;
> -}
> -
>   /*
>    * Please see Documentation/vm/hmm.rst for how to use the range API.
>    */
> diff --git a/mm/hmm.c b/mm/hmm.c
> index dc30edad9a8a02..f67ba32983d9f1 100644
> --- a/mm/hmm.c
> +++ b/mm/hmm.c
> @@ -80,7 +80,6 @@ static struct hmm *hmm_get_or_create(struct mm_struct *mm)
>   	mutex_init(&hmm->lock);
>   	kref_init(&hmm->kref);
>   	hmm->notifiers = 0;
> -	hmm->dead = false;
>   	hmm->mm = mm;
>   
>   	hmm->mmu_notifier.ops = &hmm_mmu_notifier_ops;
> @@ -124,20 +123,17 @@ static void hmm_release(struct mmu_notifier *mn, struct mm_struct *mm)
>   {
>   	struct hmm *hmm = container_of(mn, struct hmm, mmu_notifier);
>   	struct hmm_mirror *mirror;
> -	struct hmm_range *range;
>   
>   	/* hmm is in progress to free */
>   	if (!kref_get_unless_zero(&hmm->kref))
>   		return;
>   
> -	/* Report this HMM as dying. */
> -	hmm->dead = true;
> -
> -	/* Wake-up everyone waiting on any range. */
>   	mutex_lock(&hmm->lock);
> -	list_for_each_entry(range, &hmm->ranges, list)
> -		range->valid = false;
> -	wake_up_all(&hmm->wq);
> +	/*
> +	 * Since hmm_range_register() holds the mmget() lock hmm_release() is
> +	 * prevented as long as a range exists.
> +	 */
> +	WARN_ON(!list_empty(&hmm->ranges));
>   	mutex_unlock(&hmm->lock);
>   
>   	down_write(&hmm->mirrors_sem);
> @@ -909,8 +905,8 @@ int hmm_range_register(struct hmm_range *range,
>   	range->start = start;
>   	range->end = end;
>   
> -	/* Check if hmm_mm_destroy() was call. */
> -	if (hmm->mm == NULL || hmm->dead)
> +	/* Prevent hmm_release() from running while the range is valid */
> +	if (!mmget_not_zero(hmm->mm))
>   		return -EFAULT;
>   
>   	range->hmm = hmm;
> @@ -955,6 +951,7 @@ void hmm_range_unregister(struct hmm_range *range)
>   
>   	/* Drop reference taken by hmm_range_register() */
>   	range->valid = false;
> +	mmput(hmm->mm);
>   	hmm_put(hmm);
>   	range->hmm = NULL;
>   }
> @@ -982,10 +979,7 @@ long hmm_range_snapshot(struct hmm_range *range)
>   	struct vm_area_struct *vma;
>   	struct mm_walk mm_walk;
>   
> -	/* Check if hmm_mm_destroy() was call. */
> -	if (hmm->mm == NULL || hmm->dead)
> -		return -EFAULT;
> -
> +	lockdep_assert_held(&hmm->mm->mmap_sem);
>   	do {
>   		/* If range is no longer valid force retry. */
>   		if (!range->valid)
> @@ -1080,9 +1074,7 @@ long hmm_range_fault(struct hmm_range *range, bool block)
>   	struct mm_walk mm_walk;
>   	int ret;
>   
> -	/* Check if hmm_mm_destroy() was call. */
> -	if (hmm->mm == NULL || hmm->dead)
> -		return -EFAULT;
> +	lockdep_assert_held(&hmm->mm->mmap_sem);
>   
>   	do {
>   		/* If range is no longer valid force retry. */