shmem: set shmem_writepage() variables early

shmem_writepage() sets up variables typically used *after* a possible huge
page split.  However even if that does happen the address space mapping
should not change, and the inode does not change either.  So it should be
safe to set that from the very beginning.

This commit makes no functional changes.

Link: https://lkml.kernel.org/r/20230309230545.2930737-3-mcgrof@kernel.orgSigned-off-by: Luis Chamberlain <mcgrof@kernel.org>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Tested-by: Xin Hao <xhao@linux.alibaba.com>
Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>
Cc: Adam Manzanares <a.manzanares@samsung.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Hugh Dickins <hughd@google.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Pankaj Raghav <p.raghav@samsung.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/shmem.c

@@ -1331,9 +1331,9 @@ int shmem_unuse(unsigned int type)
 static int shmem_writepage(struct page *page, struct writeback_control *wbc)
 {
 	struct folio *folio = page_folio(page);
-	struct shmem_inode_info *info;
-	struct address_space *mapping;
-	struct inode *inode;
+	struct address_space *mapping = folio->mapping;
+	struct inode *inode = mapping->host;
+	struct shmem_inode_info *info = SHMEM_I(inode);
 	swp_entry_t swap;
 	pgoff_t index;
 
@@ -1351,10 +1351,7 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
 		folio_clear_dirty(folio);
 	}
 
-	mapping = folio->mapping;
 	index = folio->index;
-	inode = mapping->host;
-	info = SHMEM_I(inode);
 	if (info->flags & VM_LOCKED)
 		goto redirty;
 	if (!total_swap_pages)