[RFC PATCH v2 1/2] ovl: Define D_REAL_FILEDATA for d_real to return dentry with data

[Stefan Berger]

Define D_REAL_FILEDATA which is to be used as a parameter for d_real()
to return the dentry that is holding the file data, which is either the
upper or the lower denry. The caller is expected to call d_real() again
on the returned dentry to get to lower layers of a stacked filesystem,
if available. This allows a caller to get all dentries involved in hold
a file's data and iterate through the layers.

Signed-off-by: Stefan Berger <stefanb@xxxxxxxxxxxxx>
---
 fs/overlayfs/super.c   | 6 ++++++
 include/linux/dcache.h | 1 +
 2 files changed, 7 insertions(+)

diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index 06a231970cb5..f466ad89b005 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -36,6 +36,7 @@ static struct dentry *ovl_d_real(struct dentry *dentry, enum d_real_type type)
 	switch (type) {
 	case D_REAL_DATA:
 	case D_REAL_METADATA:
+	case D_REAL_FILEDATA:
 		break;
 	default:
 		goto bug;
@@ -47,6 +48,11 @@ static struct dentry *ovl_d_real(struct dentry *dentry, enum d_real_type type)
 	}
 
 	upper = ovl_dentry_upper(dentry);
+	if (type == D_REAL_FILEDATA) {
+		if (ovl_has_upperdata(d_inode(dentry)))
+			return upper;
+		return ovl_dentry_lower(dentry);
+	}
 	if (upper && (type == D_REAL_METADATA ||
 		      ovl_has_upperdata(d_inode(dentry))))
 		return upper;
diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index bf53e3894aae..e4e54fb2cf4e 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -128,6 +128,7 @@ enum dentry_d_lock_class
 enum d_real_type {
 	D_REAL_DATA,
 	D_REAL_METADATA,
+	D_REAL_FILEDATA,
 };
 
 struct dentry_operations {
-- 
2.43.0