[PATCH v4, RESEND] fs/dax: check zero or empty entry before converting xarray entry

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 <[email protected]>
Reviewed-by: Jan Kara <[email protected]>
Reviewed-by: Alistair Popple <[email protected]>
---
Changes in v4:
- Rebase on current upstream
- Link to v3: https://lore.kernel.org/all/[email protected]/
Changes in v3:
- Rebase on current upstream
- Update the changelog for the current code state
- Link to v2: https://lore.kernel.org/all/[email protected]/
Changes in v2:
- Add Fixes and Reviewed-by tags.
- Rebase on the latest.
- Link to v1: https://lore.kernel.org/all/[email protected]/
---
 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
--
2.43.0