Recheck PG_BUSY after locking the page.

Another thread can set the bit if we sleep during rw_enter(9) in which case
the page shouldn't be touched.

ok semarie@

diff --git a/sys/uvm/uvm_pdaemon.c b/sys/uvm/uvm_pdaemon.c
index 59a5e28..e7eebee 100644 (file)
--- a/sys/uvm/uvm_pdaemon.c
+++ b/sys/uvm/uvm_pdaemon.c
@@ -1,4 +1,4 @@
-/*     $OpenBSD: uvm_pdaemon.c,v 1.96 2022/04/11 16:43:49 mpi Exp $    */
+/*     $OpenBSD: uvm_pdaemon.c,v 1.97 2022/04/30 17:58:43 mpi Exp $    */
 /*     $NetBSD: uvm_pdaemon.c,v 1.23 2000/08/20 10:24:14 bjh21 Exp $   */
 
 /*
@@ -879,6 +879,8 @@ uvmpd_scan(void)
        int free, inactive_shortage, swap_shortage, pages_freed;
        struct vm_page *p, *nextpg;
        struct uvm_object *uobj;
+       struct vm_anon *anon;
+       struct rwlock *slock;
        boolean_t got_it;
 
        MUTEX_ASSERT_LOCKED(&uvm.pageqlock);
@@ -947,20 +949,34 @@ uvmpd_scan(void)
             p != NULL && (inactive_shortage > 0 || swap_shortage > 0);
             p = nextpg) {
                nextpg = TAILQ_NEXT(p, pageq);
-
-               /* skip this page if it's busy. */
-               if (p->pg_flags & PG_BUSY)
+               if (p->pg_flags & PG_BUSY) {
                        continue;
+               }
 
-               if (p->pg_flags & PQ_ANON) {
-                       KASSERT(p->uanon != NULL);
-                       if (rw_enter(p->uanon->an_lock, RW_WRITE|RW_NOSLEEP))
+               /*
+                * lock the page's owner.
+                */
+               if (p->uobject != NULL) {
+                       uobj = p->uobject;
+                       slock = uobj->vmobjlock;
+                       if (rw_enter(slock, RW_WRITE|RW_NOSLEEP)) {
                                continue;
+                       }
                } else {
-                       KASSERT(p->uobject != NULL);
-                       if (rw_enter(p->uobject->vmobjlock,
-                           RW_WRITE|RW_NOSLEEP))
+                       anon = p->uanon;
+                       KASSERT(p->uanon != NULL);
+                       slock = anon->an_lock;
+                       if (rw_enter(slock, RW_WRITE|RW_NOSLEEP)) {
                                continue;
+                       }
+               }
+
+               /*
+                * skip this page if it's busy.
+                */
+               if ((p->pg_flags & PG_BUSY) != 0) {
+                       rw_exit(slock);
+                       continue;
                }
 
                /*
@@ -997,10 +1013,11 @@ uvmpd_scan(void)
                        uvmexp.pddeact++;
                        inactive_shortage--;
                }
-               if (p->pg_flags & PQ_ANON)
-                       rw_exit(p->uanon->an_lock);
-               else
-                       rw_exit(p->uobject->vmobjlock);
+
+               /*
+                * we're done with this page.
+                */
+               rw_exit(slock);
        }
 }