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

Kasireddy, Vivek
2023-07-20T07:43:26Z

Hi Alistair,

>
> > 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?
Yeah, that might be possible; I believe the right thing to do would be:
- return hugetlb_no_page(mm, vma, mapping, idx, address, ptep,
+ ret = hugetlb_no_page(mm, vma, mapping, idx, address, ptep,
entry, flags);
+ if (!ret) {
+ ptl = huge_pte_lock(h, mm, ptep);
+ mmu_notifier_update_mapping(vma->vm_mm, address,
+ pte_pfn(*ptep));
+ spin_unlock(ptl);
+ }

In which case I'd need to make a similar change in the shmem path as well.
And, also redo (or eliminate) the locking in udmabuf (patch) which seems a
bit excessive on a second look given our use-case (where reads and writes do
not happen simultaneously due to fence synchronization in the guest driver).

Thanks,
Vivek

>
> > + 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;
> > }