VM: fix pageout stop condition

When checking whether to continue paging out or not, the pageout daemon
only considers the high free page threshold of a segment. But if e.g.
the default pager had to allocate reserved pages during a previous
pageout cycle, it could have exhausted a segment (this is currently
only seen with the DMA segment). In that case, the high threshold
cannot be reached because the segment has currently no pageable page.

This change makes the pageout daemon identify this condition and
consider the segment as usable in order to make progress. The segment
will simply be ignored on the allocation path for unprivileged threads,
and if this happens with too many segments, the system will fail at
allocation time.

* vm/vm_page.c (vm_page_seg_usable): Report usable if the segment has
no pageable page.

1 files changed, 5 insertions, 0 deletions

diff --git a/vm/vm_page.c b/vm/vm_page.c
index 2a9f27b2..567f23f3 100644
--- a/vm/vm_page.c
+++ b/vm/vm_page.c
@@ -939,6 +939,11 @@ vm_page_seg_page_available(const struct vm_page_seg *seg)
 static boolean_t
 vm_page_seg_usable(const struct vm_page_seg *seg)
 {
+    if ((seg->nr_active_pages + seg->nr_inactive_pages) == 0) {
+        /* Nothing to page out, assume segment is usable */
+        return TRUE;
+    }
+
     return (seg->nr_free_pages >= seg->high_free_pages);
 }