In a few places, like the implementation of ptrace(), a thread may call

upon pmap_enter() to create a mapping within a different address space,
i.e., not the thread's own address space.  On i386, this entails the
creation of a temporary mapping to the affected page table page (PTP).  In
general, pmap_enter() will read from this PTP, allocate a PV entry, and
write to this PTP.  The trouble comes when the system is short of memory.
In order to allocate a new PV entry, an older PV entry has to be
reclaimed.  Reclaiming a PV entry involves destroying a mapping, which
requires access to the affected PTP.  Thus, the PTP mapped at the
beginning of pmap_enter() is no longer mapped at the end of pmap_enter(),
which leads to pmap_enter() modifying the wrong PTP.  To address this
problem, pmap_pv_reclaim() is changed to use an alternate method of
mapping PTPs.

Update a related comment.

Reported by:	pho
Diagnosed by:	kib
MFC after:	5 days

1 files changed, 8 insertions, 6 deletions

diff --git a/sys/i386/xen/pmap.c b/sys/i386/xen/pmap.c
index 46cb43c..2eea8c6 100644
--- a/sys/i386/xen/pmap.c
+++ b/sys/i386/xen/pmap.c
@@ -429,7 +429,8 @@ pmap_bootstrap(vm_paddr_t firstaddr)
 	SYSMAP(struct msgbuf *, unused, msgbufp, atop(round_page(msgbufsize)))
 
 	/*
-	 * ptemap is used for pmap_pte_quick
+	 * PADDR1 and PADDR2 are used by pmap_pte_quick() and pmap_pte(),
+	 * respectively.
 	 */
 	SYSMAP(pt_entry_t *, PMAP1, PADDR1, 1)
 	SYSMAP(pt_entry_t *, PMAP2, PADDR2, 1)
@@ -1976,7 +1977,6 @@ pmap_pv_reclaim(pmap_t locked_pmap)
 	pmap = NULL;
 	free = m_pc = NULL;
 	TAILQ_INIT(&newtail);
-	sched_pin();
 	while ((pc = TAILQ_FIRST(&pv_chunks)) != NULL && (pv_vafree == 0 ||
 	    free == NULL)) {
 		TAILQ_REMOVE(&pv_chunks, pc, pc_lru);
@@ -2007,10 +2007,13 @@ pmap_pv_reclaim(pmap_t locked_pmap)
 				bit = bsfl(inuse);
 				pv = &pc->pc_pventry[field * 32 + bit];
 				va = pv->pv_va;
-				pte = pmap_pte_quick(pmap, va);
-				if ((*pte & PG_W) != 0)
+				pte = pmap_pte(pmap, va);
+				tpte = *pte;
+				if ((tpte & PG_W) == 0)
+					tpte = pte_load_clear(pte);
+				pmap_pte_release(pte);
+				if ((tpte & PG_W) != 0)
 					continue;
-				tpte = pte_load_clear(pte);
 				if ((tpte & PG_G) != 0)
 					pmap_invalidate_page(pmap, va);
 				m = PHYS_TO_VM_PAGE(tpte & PG_FRAME);
@@ -2062,7 +2065,6 @@ pmap_pv_reclaim(pmap_t locked_pmap)
 		}
 	}
 out:
-	sched_unpin();
 	TAILQ_CONCAT(&pv_chunks, &newtail, pc_lru);
 	if (pmap != NULL) {
 		pmap_invalidate_all(pmap);