From 4cbe4c48a7e574ec1bb919351d7a57d252b00b62 Mon Sep 17 00:00:00 2001
From: Konstantin Belousov <kib@FreeBSD.org>
Date: Sat, 18 Nov 2023 10:55:48 +0200
Subject: [PATCH] VFS: add VOP_GETLOWVNODE()

It is similar to VOP_GETWRITEMOUNT(), and for given vnode vp should
return the lower vnode which would actually handle write to vp.
Flags allow to specify FREAD or FWRITE for benefit of possible unionfs
implementation.

Reviewed by:	markj, Olivier Certner <olce.freebsd@certner.fr>
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D42603
---
 sys/fs/nullfs/null_vnops.c | 18 ++++++++++++++++++
 sys/kern/vfs_default.c     | 10 ++++++++++
 sys/kern/vnode_if.src      |  7 +++++++
 3 files changed, 35 insertions(+)

diff --git a/sys/fs/nullfs/null_vnops.c b/sys/fs/nullfs/null_vnops.c
index 7607b44e36c..c05e3394a9a 100644
--- a/sys/fs/nullfs/null_vnops.c
+++ b/sys/fs/nullfs/null_vnops.c
@@ -1127,6 +1127,23 @@ null_vput_pair(struct vop_vput_pair_args *ap)
 	return (res);
 }
 
+static int
+null_getlowvnode(struct vop_getlowvnode_args *ap)
+{
+	struct vnode *vp, *vpl;
+
+	vp = ap->a_vp;
+	if (vn_lock(vp, LK_SHARED) != 0)
+		return (EBADF);
+
+	vpl = NULLVPTOLOWERVP(vp);
+	vhold(vpl);
+	VOP_UNLOCK(vp);
+	VOP_GETLOWVNODE(vpl, ap->a_vplp, ap->a_flags);
+	vdrop(vpl);
+	return (0);
+}
+
 /*
  * Global vfs data structures
  */
@@ -1139,6 +1156,7 @@ struct vop_vector null_vnodeops = {
 	.vop_bmap =		VOP_EOPNOTSUPP,
 	.vop_stat =		null_stat,
 	.vop_getattr =		null_getattr,
+	.vop_getlowvnode =	null_getlowvnode,
 	.vop_getwritemount =	null_getwritemount,
 	.vop_inactive =		null_inactive,
 	.vop_need_inactive =	null_need_inactive,
diff --git a/sys/kern/vfs_default.c b/sys/kern/vfs_default.c
index a342cbb80c9..67c7ace5f72 100644
--- a/sys/kern/vfs_default.c
+++ b/sys/kern/vfs_default.c
@@ -82,6 +82,7 @@ static int vop_stdgetpages_async(struct vop_getpages_async_args *ap);
 static int vop_stdread_pgcache(struct vop_read_pgcache_args *ap);
 static int vop_stdstat(struct vop_stat_args *ap);
 static int vop_stdvput_pair(struct vop_vput_pair_args *ap);
+static int vop_stdgetlowvnode(struct vop_getlowvnode_args *ap);
 
 /*
  * This vnode table stores what we want to do if the filesystem doesn't
@@ -112,6 +113,7 @@ struct vop_vector default_vnodeops = {
 	.vop_fsync =		VOP_NULL,
 	.vop_stat =		vop_stdstat,
 	.vop_fdatasync =	vop_stdfdatasync,
+	.vop_getlowvnode = 	vop_stdgetlowvnode,
 	.vop_getpages =		vop_stdgetpages,
 	.vop_getpages_async =	vop_stdgetpages_async,
 	.vop_getwritemount = 	vop_stdgetwritemount,
@@ -1607,3 +1609,11 @@ vop_stdvput_pair(struct vop_vput_pair_args *ap)
 		vput(vp);
 	return (0);
 }
+
+static int
+vop_stdgetlowvnode(struct vop_getlowvnode_args *ap)
+{
+	vref(ap->a_vp);
+	*ap->a_vplp = ap->a_vp;
+	return (0);
+}
diff --git a/sys/kern/vnode_if.src b/sys/kern/vnode_if.src
index c4051fdfe32..a2b6a7c8ff9 100644
--- a/sys/kern/vnode_if.src
+++ b/sys/kern/vnode_if.src
@@ -468,6 +468,13 @@ vop_getwritemount {
 	OUT struct mount **mpp;
 };
 
+%% getwritevnode vp	= = =
+
+vop_getlowvnode {
+	IN struct vnode *vp;
+	OUT struct vnode **vplp;
+	IN int flags;
+};
 
 %% print	vp	- - -