Re: [RFC v1 1/3] mm/mmu_notifier: Add a new notifier for mapping updates (new pages)

Alistair Popple
2023-07-19T02:14:50Z

Vivek Kasireddy <vivek.kasireddy@intel.com> writes:

> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 64a3239b6407..1f2f0209101a 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -6096,8 +6096,12 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
> * hugetlb_no_page will drop vma lock and hugetlb fault
> * mutex internally, which make us return immediately.
> */
> - return hugetlb_no_page(mm, vma, mapping, idx, address, ptep,
> + ret = hugetlb_no_page(mm, vma, mapping, idx, address, ptep,
> entry, flags);
> + if (!ret)
> + mmu_notifier_update_mapping(vma->vm_mm, address,
> + pte_pfn(*ptep));

The next patch ends up calling pfn_to_page() on the result of
pte_pfn(*ptep). I don't think that's safe because couldn't the PTE have
already changed and/or the new page have been freed?

> + return ret;
>
> ret = 0;
>
> @@ -6223,6 +6227,9 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
> */
> if (need_wait_lock)
> folio_wait_locked(folio);
> + if (!ret)
> + mmu_notifier_update_mapping(vma->vm_mm, address,
> + pte_pfn(*ptep));
> return ret;
> }
>
> diff --git a/mm/mmu_notifier.c b/mm/mmu_notifier.c
> index 50c0dde1354f..6421405334b9 100644
> --- a/mm/mmu_notifier.c
> +++ b/mm/mmu_notifier.c
> @@ -441,6 +441,23 @@ void __mmu_notifier_change_pte(struct mm_struct *mm, unsigned long address,
> srcu_read_unlock(&srcu, id);
> }
>
> +void __mmu_notifier_update_mapping(struct mm_struct *mm, unsigned long address,
> + unsigned long pfn)
> +{
> + struct mmu_notifier *subscription;
> + int id;
> +
> + id = srcu_read_lock(&srcu);
> + hlist_for_each_entry_rcu(subscription,
> + &mm->notifier_subscriptions->list, hlist,
> + srcu_read_lock_held(&srcu)) {
> + if (subscription->ops->update_mapping)
> + subscription->ops->update_mapping(subscription, mm,
> + address, pfn);
> + }
> + srcu_read_unlock(&srcu, id);
> +}
> +
> static int mn_itree_invalidate(struct mmu_notifier_subscriptions *subscriptions,
> const struct mmu_notifier_range *range)
> {
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 2f2e0e618072..e59eb5fafadb 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -77,6 +77,7 @@ static struct vfsmount *shm_mnt;
> #include <linux/fcntl.h>
> #include <uapi/linux/memfd.h>
> #include <linux/rmap.h>
> +#include <linux/mmu_notifier.h>
> #include <linux/uuid.h>
>
> #include <linux/uaccess.h>
> @@ -2164,8 +2165,12 @@ static vm_fault_t shmem_fault(struct vm_fault *vmf)
> gfp, vma, vmf, &ret);
> if (err)
> return vmf_error(err);
> - if (folio)
> + if (folio) {
> vmf->page = folio_file_page(folio, vmf->pgoff);
> + if (ret == VM_FAULT_LOCKED)
> + mmu_notifier_update_mapping(vma->vm_mm, vmf->address,
> + page_to_pfn(vmf->page));
> + }
> return ret;
> }