[RFC 1/2] shmem: set shmem_writepage() variables early

[Luis Chamberlain <mcgrof@kernel.org>]

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. So it should be safe to set that from
the beginning.

The folio should always be locked from the start as well. It however
was not clear if the folio address can / should change, as well as
the first inode.

This commit makes no functional changes other a double check on the
folio locking which might be superflous. This change should help make
the subsequent patch easier to review.

Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
---
 mm/shmem.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index 28f3c699c8ce..a2c6aa11aab8 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1332,11 +1332,13 @@ 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;
 	swp_entry_t swap;
 	pgoff_t index;
 
+	BUG_ON(!folio_test_locked(folio));
+
 	/*
 	 * If /sys/kernel/mm/transparent_hugepage/shmem_enabled is "always" or
 	 * "force", drivers/gpu/drm/i915/gem/i915_gem_shmem.c gets huge pages,
@@ -1351,8 +1353,8 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
 		folio_clear_dirty(folio);
 	}
 
+	/* Can the folio or first inode change on after a split? */
 	BUG_ON(!folio_test_locked(folio));
-	mapping = folio->mapping;
 	index = folio->index;
 	inode = mapping->host;
 	info = SHMEM_I(inode);
-- 
2.39.0