[PATCH v5 5/7] mm: shmem: fix missing cache flush in shmem_mfill_atomic_pte()

[Muchun Song]

The userfaultfd calls shmem_mfill_atomic_pte() which does not do any
cache flushing for the target page.  Then the target page will be mapped
to the user space with a different address (user address), which might
have an alias issue with the kernel address used to copy the data from the
user to.  Insert flush_dcache_page() in non-zero-page case.  And replace
clear_highpage() with clear_user_highpage() which already considers
the cache maintenance.

Fixes: 8d1039634206 ("userfaultfd: shmem: add shmem_mfill_zeropage_pte for userfaultfd support")
Fixes: 4c27fe4c4c84 ("userfaultfd: shmem: add shmem_mcopy_atomic_pte for userfaultfd support")
Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
---
 mm/shmem.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index eb0fd9001130..2e17ec9231a2 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2371,8 +2371,10 @@ int shmem_mfill_atomic_pte(struct mm_struct *dst_mm,
 				/* don't free the page */
 				goto out_unacct_blocks;
 			}
+
+			flush_dcache_page(page);
 		} else {		/* ZEROPAGE */
-			clear_highpage(page);
+			clear_user_highpage(page, dst_addr);
 		}
 	} else {
 		page = *pagep;
-- 
2.11.0