svn commit: r357091 - head/sys/vm
Konstantin Belousov
kib at FreeBSD.org
Fri Jan 24 19:42:54 UTC 2020
Author: kib
Date: Fri Jan 24 19:42:53 2020
New Revision: 357091
URL: https://svnweb.freebsd.org/changeset/base/357091
Log:
Handle a race of collapse with a retrying fault.
Both vm_object_scan_all_shadowed() and vm_object_collapse_scan() might
observe an invalid page left in the default backing object by the
fault handler that retried. Check for the condition and refuse to collapse.
Reported and tested by: pho
Reviewed by: jeff
Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D23331
Modified:
head/sys/vm/vm_object.c
Modified: head/sys/vm/vm_object.c
==============================================================================
--- head/sys/vm/vm_object.c Fri Jan 24 17:24:02 2020 (r357090)
+++ head/sys/vm/vm_object.c Fri Jan 24 19:42:53 2020 (r357091)
@@ -1700,14 +1700,25 @@ vm_object_scan_all_shadowed(vm_object_t object)
if (new_pindex >= object->size)
break;
- /*
- * If the backing object page is busy a grandparent or older
- * page may still be undergoing CoW. It is not safe to
- * collapse the backing object until it is quiesced.
- */
- if (p != NULL && vm_page_busied(p))
- return (false);
+ if (p != NULL) {
+ /*
+ * If the backing object page is busy a
+ * grandparent or older page may still be
+ * undergoing CoW. It is not safe to collapse
+ * the backing object until it is quiesced.
+ */
+ if (vm_page_tryxbusy(p) == 0)
+ return (false);
+ /*
+ * We raced with the fault handler that left
+ * newly allocated invalid page on the object
+ * queue and retried.
+ */
+ if (!vm_page_all_valid(p))
+ goto unbusy_ret;
+ }
+
/*
* See if the parent has the page or if the parent's object
* pager has the page. If the parent has the page but the page
@@ -1717,15 +1728,24 @@ vm_object_scan_all_shadowed(vm_object_t object)
* object and we might as well give up now.
*/
pp = vm_page_lookup(object, new_pindex);
+
/*
- * The valid check here is stable due to object lock being
- * required to clear valid and initiate paging.
+ * The valid check here is stable due to object lock
+ * being required to clear valid and initiate paging.
+ * Busy of p disallows fault handler to validate pp.
*/
if ((pp == NULL || vm_page_none_valid(pp)) &&
!vm_pager_has_page(object, new_pindex, NULL, NULL))
- return (false);
+ goto unbusy_ret;
+ if (p != NULL)
+ vm_page_xunbusy(p);
}
return (true);
+
+unbusy_ret:
+ if (p != NULL)
+ vm_page_xunbusy(p);
+ return (false);
}
static void
@@ -1769,6 +1789,14 @@ vm_object_collapse_scan(vm_object_t object)
swap_pager_freespace(backing_object, p->pindex,
1);
+ KASSERT(!pmap_page_is_mapped(p),
+ ("freeing mapped page %p", p));
+ if (vm_page_remove(p))
+ vm_page_free(p);
+ continue;
+ }
+
+ if (!vm_page_all_valid(p)) {
KASSERT(!pmap_page_is_mapped(p),
("freeing mapped page %p", p));
if (vm_page_remove(p))
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