git: 5bd4c04a4e7f - main - vm_fault: add vm_fault_might_be_cow() helper

The branch main has been updated by kib:

URL: https://cgit.FreeBSD.org/src/commit/?id=5bd4c04a4e7f7bda657e6027e64675d0caf50715

commit 5bd4c04a4e7f7bda657e6027e64675d0caf50715
Author:     Konstantin Belousov <kib@FreeBSD.org>
AuthorDate: 2025-08-14 03:39:05 +0000
Commit:     Konstantin Belousov <kib@FreeBSD.org>
CommitDate: 2025-09-14 19:12:46 +0000

    vm_fault: add vm_fault_might_be_cow() helper
    
    The helper checks that the object containing the fs->m page is not the
    top object in the shadow chain.
    
    Reviewed by:    alc, markj
    Tested by:      pho
    Sponsored by:   The FreeBSD Foundation
    MFC after:      1 week
    Differential revision:  https://reviews.freebsd.org/D51474
---
 sys/vm/vm_fault.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/sys/vm/vm_fault.c b/sys/vm/vm_fault.c
index a00134715171..226cf34b3ca4 100644
--- a/sys/vm/vm_fault.c
+++ b/sys/vm/vm_fault.c
@@ -258,6 +258,12 @@ vm_fault_unlock_vp(struct faultstate *fs)
 	}
 }
 
+static bool
+vm_fault_might_be_cow(struct faultstate *fs)
+{
+	return (fs->object != fs->first_object);
+}
+
 static void
 vm_fault_deallocate(struct faultstate *fs)
 {
@@ -265,7 +271,7 @@ vm_fault_deallocate(struct faultstate *fs)
 	vm_fault_page_release(&fs->m_cow);
 	vm_fault_page_release(&fs->m);
 	vm_object_pip_wakeup(fs->object);
-	if (fs->object != fs->first_object) {
+	if (vm_fault_might_be_cow(fs)) {
 		VM_OBJECT_WLOCK(fs->first_object);
 		vm_fault_page_free(&fs->first_m);
 		VM_OBJECT_WUNLOCK(fs->first_object);
@@ -1005,7 +1011,7 @@ vm_fault_cow(struct faultstate *fs)
 {
 	bool is_first_object_locked;
 
-	KASSERT(fs->object != fs->first_object,
+	KASSERT(vm_fault_might_be_cow(fs),
 	    ("source and target COW objects are identical"));
 
 	/*
@@ -1169,7 +1175,7 @@ vm_fault_zerofill(struct faultstate *fs)
 	 * If there's no object left, fill the page in the top
 	 * object with zeros.
 	 */
-	if (fs->object != fs->first_object) {
+	if (vm_fault_might_be_cow(fs)) {
 		vm_object_pip_wakeup(fs->object);
 		fs->object = fs->first_object;
 		fs->pindex = fs->first_pindex;
@@ -1433,7 +1439,7 @@ vm_fault_busy_sleep(struct faultstate *fs, int allocflags)
 	 * likely to reclaim it.
 	 */
 	vm_page_aflag_set(fs->m, PGA_REFERENCED);
-	if (fs->object != fs->first_object) {
+	if (vm_fault_might_be_cow(fs)) {
 		vm_fault_page_release(&fs->first_m);
 		vm_object_pip_wakeup(fs->first_object);
 	}
@@ -1710,7 +1716,7 @@ found:
 	 * top-level object, we have to copy it into a new page owned by the
 	 * top-level object.
 	 */
-	if (fs.object != fs.first_object) {
+	if (vm_fault_might_be_cow(&fs)) {
 		/*
 		 * We only really need to copy if we want to write it.
 		 */