git: a38483fa2b3a - main - vm_fault: assert that first_m is xbusy

The branch main has been updated by kib:

URL: https://cgit.FreeBSD.org/src/commit/?id=a38483fa2b3a26414d3409b12dd35ac406c44cea

commit a38483fa2b3a26414d3409b12dd35ac406c44cea
Author:     Konstantin Belousov <kib@FreeBSD.org>
AuthorDate: 2025-08-05 15:46:56 +0000
Commit:     Konstantin Belousov <kib@FreeBSD.org>
CommitDate: 2025-09-14 20:00:36 +0000

    vm_fault: assert that first_m is xbusy
    
    There are several invariants, most important of which is prevention of
    parallel faults handling, that depend on first_m being xbusy for the
    duration of the page fault processing.
    
    Suggested by:   markj
    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 | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/sys/vm/vm_fault.c b/sys/vm/vm_fault.c
index 427c18c63eb7..3dddc7a307d4 100644
--- a/sys/vm/vm_fault.c
+++ b/sys/vm/vm_fault.c
@@ -1784,6 +1784,11 @@ found:
 	/*
 	 * A valid page has been found and busied.  The object lock
 	 * must no longer be held if the page was busied.
+	 *
+	 * Regardless of the busy state of fs.m, fs.first_m is always
+	 * exclusively busied after the first iteration of the loop
+	 * calling vm_fault_object().  This is an ordering point for
+	 * the parallel faults occuring in on the same page.
 	 */
 	vm_page_assert_busied(fs.m);
 	VM_OBJECT_ASSERT_UNLOCKED(fs.object);
@@ -1886,6 +1891,9 @@ found:
 		(*fs.m_hold) = fs.m;
 		vm_page_wire(fs.m);
 	}
+
+	KASSERT(fs.first_object == fs.object || vm_page_xbusied(fs.first_m),
+	    ("first_m must be xbusy"));
 	if (vm_page_xbusied(fs.m))
 		vm_page_xunbusy(fs.m);
 	else