Author: alc
Date: Thu Jul 5 00:08:47 2012
New Revision: 238124
URL: http://svn.freebsd.org/changeset/base/238124
Log:
Correct an error in r237513. The call to reserve_pv_entries() must come
before pmap_demote_pde() updates the PDE. Otherwise, pmap_pv_demote_pde()
can crash.
Crash reported by: kib
Patch tested by: kib
Modified:
head/sys/amd64/amd64/pmap.c
Modified: head/sys/amd64/amd64/pmap.c
==============================================================================
--- head/sys/amd64/amd64/pmap.c Wed Jul 4 22:12:10 2012 (r238123)
+++ head/sys/amd64/amd64/pmap.c Thu Jul 5 00:08:47 2012 (r238124)
@@ -2491,7 +2491,6 @@ pmap_pv_demote_pde(pmap_t pmap, vm_offse
PMAP_LOCK_ASSERT(pmap, MA_OWNED);
KASSERT((pa & PDRMASK) == 0,
("pmap_pv_demote_pde: pa is not 2mpage aligned"));
- reserve_pv_entries(pmap, NPTEPG - 1, lockp);
CHANGE_PV_LIST_LOCK_TO_PHYS(lockp, pa);
/*
@@ -2751,6 +2750,17 @@ pmap_demote_pde_locked(pmap_t pmap, pd_e
pmap_fill_ptp(firstpte, newpte);
/*
+ * The spare PV entries must be reserved prior to demoting the
+ * mapping, that is, prior to changing the PDE. Otherwise, the state
+ * of the PDE and the PV lists will be inconsistent, which can result
+ * in reclaim_pv_chunk() attempting to remove a PV entry from the
+ * wrong PV list and pmap_pv_demote_pde() failing to find the expected
+ * PV entry for the 2MB page mapping that is being demoted.
+ */
+ if ((oldpde & PG_MANAGED) != 0)
+ reserve_pv_entries(pmap, NPTEPG - 1, lockp);
+
+ /*
* Demote the mapping. This pmap is locked. The old PDE has
* PG_A set. If the old PDE has PG_RW set, it also has PG_M
* set. Thus, there is no danger of a race with another
@@ -2769,13 +2779,7 @@ pmap_demote_pde_locked(pmap_t pmap, pd_e
pmap_invalidate_page(pmap, (vm_offset_t)vtopte(va));
/*
- * Demote the pv entry. This depends on the earlier demotion
- * of the mapping. Specifically, the (re)creation of a per-
- * page pv entry might trigger the execution of reclaim_pv_chunk(),
- * which might reclaim a newly (re)created per-page pv entry
- * and destroy the associated mapping. In order to destroy
- * the mapping, the PDE must have already changed from mapping
- * the 2mpage to referencing the page table page.
+ * Demote the PV entry.
*/
if ((oldpde & PG_MANAGED) != 0)
pmap_pv_demote_pde(pmap, va, oldpde & PG_PS_FRAME, lockp);
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