fs/dax: check zero or empty entry before converting xarray entry

Trying to convert zero or empty xarray entry causes kernel panic.

[    0.737679] EXT4-fs (pmem0p1): mounted filesystem 79676804-7c8b-491a-b2a6-9bae3c72af70 ro with ordered data mode. Quota mode: disabled.
[    0.737891] VFS: Mounted root (ext4 filesystem) readonly on device 259:1.
[    0.739119] devtmpfs: mounted
[    0.739476] Freeing unused kernel memory: 1920K
[    0.740156] Run /sbin/init as init process
[    0.740229]   with arguments:
[    0.740286]     /sbin/init
[    0.740321]   with environment:
[    0.740369]     HOME=/
[    0.740400]     TERM=linux
[    0.743162] Unable to handle kernel paging request at virtual address fffffdffbf000008
[    0.743285] Mem abort info:
[    0.743316]   ESR = 0x0000000096000006
[    0.743371]   EC = 0x25: DABT (current EL), IL = 32 bits
[    0.743444]   SET = 0, FnV = 0
[    0.743489]   EA = 0, S1PTW = 0
[    0.743545]   FSC = 0x06: level 2 translation fault
[    0.743610] Data abort info:
[    0.743656]   ISV = 0, ISS = 0x00000006, ISS2 = 0x00000000
[    0.743720]   CM = 0, WnR = 0, TnD = 0, TagAccess = 0
[    0.743785]   GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0
[    0.743848] swapper pgtable: 4k pages, 48-bit VAs, pgdp=00000000b9d17000
[    0.743931] [fffffdffbf000008] pgd=10000000bfa3d403, p4d=10000000bfa3d403, pud=1000000040bfe403, pmd=0000000000000000
[    0.744070] Internal error: Oops: 0000000096000006 [#1]  SMP
[    0.748888] CPU: 0 UID: 0 PID: 1 Comm: init Not tainted 6.18.4 #1 NONE
[    0.749421] pstate: 004000c5 (nzcv daIF +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[    0.749969] pc : dax_disassociate_entry.constprop.0+0x20/0x50
[    0.750444] lr : dax_insert_entry+0xcc/0x408
[    0.750802] sp : ffff80008000b9e0
[    0.751083] x29: ffff80008000b9e0 x28: 0000000000000000 x27: 0000000000000000
[    0.751682] x26: 0000000001963d01 x25: ffff0000004f7d90 x24: 0000000000000000
[    0.752264] x23: 0000000000000000 x22: ffff80008000bcc8 x21: 0000000000000011
[    0.752836] x20: ffff80008000ba90 x19: 0000000001963d01 x18: 0000000000000000
[    0.753407] x17: 0000000000000000 x16: 0000000000000000 x15: 0000000000000000
[    0.753970] x14: ffffbf3154b9ae70 x13: 0000000000000000 x12: ffffbf3154b9ae70
[    0.754548] x11: ffffffffffffffff x10: 0000000000000000 x9 : 0000000000000000
[    0.755122] x8 : 000000000000000d x7 : 000000000000001f x6 : 0000000000000000
[    0.755707] x5 : 0000000000000000 x4 : 0000000000000000 x3 : fffffdffc0000000
[    0.756287] x2 : 0000000000000008 x1 : 0000000040000000 x0 : fffffdffbf000000
[    0.756871] Call trace:
[    0.757107]  dax_disassociate_entry.constprop.0+0x20/0x50 (P)
[    0.757592]  dax_iomap_pte_fault+0x4fc/0x808
[    0.757951]  dax_iomap_fault+0x28/0x30
[    0.758258]  ext4_dax_huge_fault+0x80/0x2dc
[    0.758594]  ext4_dax_fault+0x10/0x3c
[    0.758892]  __do_fault+0x38/0x12c
[    0.759175]  __handle_mm_fault+0x530/0xcf0
[    0.759518]  handle_mm_fault+0xe4/0x230
[    0.759833]  do_page_fault+0x17c/0x4dc
[    0.760144]  do_translation_fault+0x30/0x38
[    0.760483]  do_mem_abort+0x40/0x8c
[    0.760771]  el0_ia+0x4c/0x170
[    0.761032]  el0t_64_sync_handler+0xd8/0xdc
[    0.761371]  el0t_64_sync+0x168/0x16c
[    0.761677] Code: f9453021 f2dfbfe3 cb813080 8b001860 (f9400401)
[    0.762168] ---[ end trace 0000000000000000 ]---
[    0.762550] note: init[1] exited with irqs disabled
[    0.762631] Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b

This patch just reorders checking and converting.

---
Changes in v2:
- Add Fixes and Reviewed-by tags.
- Rebase on the latest.
- Link to v1: https://lore.kernel.org/lkml/[email protected]/T/#r7160ab8ce8b04db157ea73a7c203b2c69626bfc6

---
 fs/dax.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

index 289e6254aa30..de316be2cc4e 100644
@@ -443,11 +443,12 @@ 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)

@@ -468,21 +469,23 @@ 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