dax_associate_entry(), dax_disassociate_entry(), and dax_busy_page()
call dax_to_folio(entry) before checking whether entry is a zero or
empty xarray entry.

That ordering is wrong because zero and empty entries are not folio
entries. Commit 98c183a4fccf ("fs/dax: don't disassociate zero page
entries") added guards in the associate and disassociate paths, but the
guards still come after dax_to_folio(entry), and dax_busy_page() still
has the same problem.

Move the zero/empty checks before dax_to_folio(entry) in all three helpers.

Fixes: 38607c62b34b ("fs/dax: properly refcount fs dax pages")
Signed-off-by: Seunguk Shin <seunguk.shin@arm.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Alistair Popple <apopple@nvidia.com>
---
Changes in v3:
- Rebase on current upstream
- Update the changelog for the current code state
- Link to v2: https://lore.kernel.org/all/m2jyv11mqe.fsf@arm.com/
Changes in v2:
- Add Fixes and Reviewed-by tags.
- Rebase on the latest.
- Link to v1: https://lore.kernel.org/all/18af3213-6c46-4611-ba75-da5be5a1c9b0@arm.com/
---
 fs/dax.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/fs/dax.c b/fs/dax.c
index 6d175cd47a99..4bca6e2bc342 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -480,11 +480,12 @@ static void dax_associate_entry(void *entry, struct address_space *mapping,
 				unsigned long address, bool shared)
 {
 	unsigned long size = dax_entry_size(entry), index;
-	struct folio *folio = dax_to_folio(entry);
+	struct folio *folio;
 
 	if (dax_is_zero_entry(entry) || dax_is_empty_entry(entry))
 		return;
 
+	folio = dax_to_folio(entry);
 	index = linear_page_index(vma, address & ~(size - 1));
 	if (shared && (folio->mapping || dax_folio_is_shared(folio))) {
 		if (folio->mapping)
@@ -505,21 +506,23 @@ static void dax_associate_entry(void *entry, struct address_space *mapping,
 static void dax_disassociate_entry(void *entry, struct address_space *mapping,
 				bool trunc)
 {
-	struct folio *folio = dax_to_folio(entry);
+	struct folio *folio;
 
 	if (dax_is_zero_entry(entry) || dax_is_empty_entry(entry))
 		return;
 
+	folio = dax_to_folio(entry);
 	dax_folio_put(folio);
 }
 
 static struct page *dax_busy_page(void *entry)
 {
-	struct folio *folio = dax_to_folio(entry);
+	struct folio *folio;
 
 	if (dax_is_zero_entry(entry) || dax_is_empty_entry(entry))
 		return NULL;
 
+	folio = dax_to_folio(entry);
 	if (folio_ref_count(folio) - folio_mapcount(folio))
 		return &folio->page;
 	else
-- 
2.34.1