Re: [PATCH] Initialise mmu_notifier_range correctly

Jerome Glisse
2019-01-03T14:30:05Z

On Wed, Jan 02, 2019 at 08:18:33PM -0800, Matthew Wilcox wrote:
> On Wed, Jan 02, 2019 at 07:32:08PM -0800, John Hubbard wrote:
> > Having the range struct declared in separate places from the mmu_notifier_range_init()
> > calls is not great. But I'm not sure I see a way to make it significantly cleaner, given
> > that __follow_pte_pmd uses the range pointer as a way to decide to issue the mmn calls.
>
> Yeah, I don't think there's anything we can do. But I started reviewing
> the comments, and they don't make sense together:
>
> /*
> * Note because we provide range to follow_pte_pmd it will
> * call mmu_notifier_invalidate_range_start() on our behalf
> * before taking any lock.
> */
> if (follow_pte_pmd(vma->vm_mm, address, &range,
> &ptep, &pmdp, &ptl))
> continue;
>
> /*
> * No need to call mmu_notifier_invalidate_range() as we are
> * downgrading page table protection not changing it to point
> * to a new page.
> *
> * See Documentation/vm/mmu_notifier.rst
> */
>
> So if we don't call mmu_notifier_invalidate_range, why are we calling
> mmu_notifier_invalidate_range_start and mmu_notifier_invalidate_range_end?
> ie, why not this ...

Thus comments looks wrong to me ... we need to call
mmu_notifier_invalidate_range() those are use by
IOMMU. I might be to blame for those comments thought.

>
> diff --git a/fs/dax.c b/fs/dax.c
> index 6959837cc465..905340149924 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -777,7 +777,6 @@ static void dax_entry_mkclean(struct address_space *mapping, pgoff_t index,
>
> i_mmap_lock_read(mapping);
> vma_interval_tree_foreach(vma, &mapping->i_mmap, index, index) {
> - struct mmu_notifier_range range;
> unsigned long address;
>
> cond_resched();
> @@ -787,12 +786,7 @@ static void dax_entry_mkclean(struct address_space *mapping, pgoff_t index,
>
> address = pgoff_address(index, vma);
>
> - /*
> - * Note because we provide start/end to follow_pte_pmd it will
> - * call mmu_notifier_invalidate_range_start() on our behalf
> - * before taking any lock.
> - */
> - if (follow_pte_pmd(vma->vm_mm, address, &range,
> + if (follow_pte_pmd(vma->vm_mm, address, NULL,
> &ptep, &pmdp, &ptl))
> continue;
>
> @@ -834,8 +828,6 @@ static void dax_entry_mkclean(struct address_space *mapping, pgoff_t index,
> unlock_pte:
> pte_unmap_unlock(ptep, ptl);
> }
> -
> - mmu_notifier_invalidate_range_end(&range);
> }
> i_mmap_unlock_read(mapping);
> }