On Thu, Jun 06, 2019 at 03:44:31PM -0300, Jason Gunthorpe wrote:
> From: Jason Gunthorpe <jgg@xxxxxxxxxxxx>
>
> 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@xxxxxxxxxxxx>
Reviewed-by: Ira Weiny <ira.weiny@xxxxxxxxx>
> ---
> v2:
> - Fix error unwind of mmgrab (Jerome)
> - Use hmm local instead of 2nd container_of (Jerome)
> ---
> mm/hmm.c | 80 ++++++++++++++++++++------------------------------------
> 1 file changed, 29 insertions(+), 51 deletions(-)
>
> diff --git a/mm/hmm.c b/mm/hmm.c
> index cc7c26fda3300e..dc30edad9a8a02 100644
> --- a/mm/hmm.c
> +++ 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;
> + }
>
> hmm = kmalloc(sizeof(*hmm), GFP_KERNEL);
> if (!hmm)
> @@ -83,57 +82,36 @@ static struct hmm *hmm_get_or_create(struct mm_struct *mm)
> hmm->notifiers = 0;
> hmm->dead = false;
> hmm->mm = mm;
> - mmgrab(hmm->mm);
> -
> - spin_lock(&mm->page_table_lock);
> - if (!mm->hmm)
> - mm->hmm = hmm;
> - else
> - cleanup = true;
> - spin_unlock(&mm->page_table_lock);
>
> - if (cleanup)
> - goto error;
> -
> - /*
> - * We should only get here if hold the mmap_sem in write mode ie on
> - * registration of first mirror through hmm_mirror_register()
> - */
> hmm->mmu_notifier.ops = &hmm_mmu_notifier_ops;
> - if (__mmu_notifier_register(&hmm->mmu_notifier, mm))
> - goto error_mm;
> + if (__mmu_notifier_register(&hmm->mmu_notifier, mm)) {
> + kfree(hmm);
> + return NULL;
> + }
>
> + mmgrab(hmm->mm);
> + mm->hmm = hmm;
> return hmm;
> -
> -error_mm:
> - spin_lock(&mm->page_table_lock);
> - if (mm->hmm == hmm)
> - mm->hmm = NULL;
> - spin_unlock(&mm->page_table_lock);
> -error:
> - mmdrop(hmm->mm);
> - kfree(hmm);
> - return NULL;
> }
>
> static void hmm_free_rcu(struct rcu_head *rcu)
> {
> - kfree(container_of(rcu, struct hmm, rcu));
> + 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(hmm);
> }
>
> 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_free_rcu);
> }
>
> --
> 2.21.0
>
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