From 8d34a3bf7d64f61bc6d1519cf75c76e976572469 Mon Sep 17 00:00:00 2001
From: Konstantin Belousov <kib@FreeBSD.org>
Date: Tue, 4 Feb 2020 19:03:37 +0000
Subject: [PATCH] Enable vm_object_mightbedirty() and vm_object_page_clean()
 for swap objects backing tmpfs vnodes data.

The clean scan is limited to only remove write permissions from the
mapped pages of the objects.  This fixes the issue that tmpfs vnode
mtime is not updated from writes to the mmaped area after the initial
page-in.

Noted by:	mjg
Reviewed by:	markj
Discussed with:	jeff
Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D23432
---
 sys/vm/vm_object.c | 66 +++++++++++++++++++++++++++-------------------
 sys/vm/vm_object.h | 11 ++++++--
 2 files changed, 48 insertions(+), 29 deletions(-)

diff --git a/sys/vm/vm_object.c b/sys/vm/vm_object.c
index dc793b60456..2b2a6355e52 100644
--- a/sys/vm/vm_object.c
+++ b/sys/vm/vm_object.c
@@ -1017,6 +1017,10 @@ vm_object_page_remove_write(vm_page_t p, int flags, boolean_t *allclean)
  *	write out pages with PGA_NOSYNC set (originally comes from MAP_NOSYNC),
  *	leaving the object dirty.
  *
+ *	For swap objects backing tmpfs regular files, do not flush anything,
+ *	but remove write protection on the mapped pages to update mtime through
+ *	mmaped writes.
+ *
  *	When stuffing pages asynchronously, allow clustering.  XXX we need a
  *	synchronous clustering mode implementation.
  *
@@ -1038,8 +1042,7 @@ vm_object_page_clean(vm_object_t object, vm_ooffset_t start, vm_ooffset_t end,
 
 	VM_OBJECT_ASSERT_WLOCKED(object);
 
-	if (object->type != OBJT_VNODE || !vm_object_mightbedirty(object) ||
-	    object->resident_page_count == 0)
+	if (!vm_object_mightbedirty(object) || object->resident_page_count == 0)
 		return (TRUE);
 
 	pagerflags = (flags & (OBJPC_SYNC | OBJPC_INVAL)) != 0 ?
@@ -1072,32 +1075,36 @@ rescan:
 			vm_page_xunbusy(p);
 			continue;
 		}
+		if (object->type == OBJT_VNODE) {
+			n = vm_object_page_collect_flush(object, p, pagerflags,
+			    flags, &allclean, &eio);
+			if (eio) {
+				res = FALSE;
+				allclean = FALSE;
+			}
+			if (object->generation != curgeneration &&
+			    (flags & OBJPC_SYNC) != 0)
+				goto rescan;
 
-		n = vm_object_page_collect_flush(object, p, pagerflags,
-		    flags, &allclean, &eio);
-		if (eio) {
-			res = FALSE;
-			allclean = FALSE;
-		}
-		if (object->generation != curgeneration &&
-		    (flags & OBJPC_SYNC) != 0)
-			goto rescan;
-
-		/*
-		 * If the VOP_PUTPAGES() did a truncated write, so
-		 * that even the first page of the run is not fully
-		 * written, vm_pageout_flush() returns 0 as the run
-		 * length.  Since the condition that caused truncated
-		 * write may be permanent, e.g. exhausted free space,
-		 * accepting n == 0 would cause an infinite loop.
-		 *
-		 * Forwarding the iterator leaves the unwritten page
-		 * behind, but there is not much we can do there if
-		 * filesystem refuses to write it.
-		 */
-		if (n == 0) {
+			/*
+			 * If the VOP_PUTPAGES() did a truncated write, so
+			 * that even the first page of the run is not fully
+			 * written, vm_pageout_flush() returns 0 as the run
+			 * length.  Since the condition that caused truncated
+			 * write may be permanent, e.g. exhausted free space,
+			 * accepting n == 0 would cause an infinite loop.
+			 *
+			 * Forwarding the iterator leaves the unwritten page
+			 * behind, but there is not much we can do there if
+			 * filesystem refuses to write it.
+			 */
+			if (n == 0) {
+				n = 1;
+				allclean = FALSE;
+			}
+		} else {
 			n = 1;
-			allclean = FALSE;
+			vm_page_xunbusy(p);
 		}
 		np = vm_page_find_least(object, pi + n);
 	}
@@ -1105,7 +1112,12 @@ rescan:
 	VOP_FSYNC(vp, (pagerflags & VM_PAGER_PUT_SYNC) ? MNT_WAIT : 0);
 #endif
 
-	if (allclean)
+	/*
+	 * Leave updating cleangeneration for tmpfs objects to tmpfs
+	 * scan.  It needs to update mtime, which happens for other
+	 * filesystems during page writeouts.
+	 */
+	if (allclean && object->type == OBJT_VNODE)
 		object->cleangeneration = curgeneration;
 	return (res);
 }
diff --git a/sys/vm/vm_object.h b/sys/vm/vm_object.h
index dd0314bdb05..777b3c84aed 100644
--- a/sys/vm/vm_object.h
+++ b/sys/vm/vm_object.h
@@ -322,8 +322,15 @@ static __inline bool
 vm_object_mightbedirty(vm_object_t object)
 {
 
-	return (object->type == OBJT_VNODE &&
-	    object->generation != object->cleangeneration);
+	if (object->type != OBJT_VNODE) {
+		if ((object->flags & OBJ_TMPFS_NODE) == 0)
+			return (false);
+#ifdef KASSERT
+		KASSERT(object->type == OBJT_SWAP,
+		    ("TMPFS_NODE obj %p is not swap", object));
+#endif
+	}
+	return (object->generation != object->cleangeneration);
 }
 
 void vm_object_clear_flag(vm_object_t object, u_short bits);