Re: [PATCH] Initialise mmu_notifier_range correctly

Matthew Wilcox
2019-01-03T04:18:38Z

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 ...

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);
}