sys/uvm: avoid waiting with the object lock held in uvn_get()
Kirill A. Korinsky <[email protected]>
| Newsgroups | gmane.os.openbsd.tech |
|---|---|
| Message-ID | <[email protected]> |
tech@,
uvm_wait() requires all locks to be released, but uvn_get() calls it with
uobj->vmobjlock held when uvm_pagealloc() fails. The page daemon may need
this lock to reclaim the object's pages; waiting for free memory while
retaining it can ends with deadlock.
Dropping and reacquiring the lock around uvm_wait() is not safe either: the
fault maps are already unlocked, so the mapping and vnode object state may
change while the thread sleeps.
So, eelease vmobjlock and return VM_PAGER_AGAIN instead; the fault handler
backs off, restarts the fault, and revalidates its state before retrying
again.
Ok?
Index: sys/uvm/uvm_vnode.c
===================================================================
RCS file: /home/cvs/src/sys/uvm/uvm_vnode.c,v
diff -u -p -r1.151 uvm_vnode.c
--- sys/uvm/uvm_vnode.c 29 Dec 2025 16:07:14 -0000 1.151
+++ sys/uvm/uvm_vnode.c 8 Aug 2026 17:52:53 -0000
@@ -1029,10 +1029,8 @@ uvn_get(struct uvm_object *uobj, voff_t
/* out of RAM? */
if (ptmp == NULL) {
- uvm_wait("uvn_getpage");
-
- /* goto top of pps while loop */
- continue;
+ rw_exit(uobj->vmobjlock);
+ return VM_PAGER_AGAIN;
}
/*
--
wbr, Kirill