⬢ DragonFlyBSD Kernel Audit
DF-2866 / fix.diff
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--- a/sys/vm/vm_pager.c
+++ b/sys/vm/vm_pager.c
@@ -156,7 +156,8 @@
 	&devicepagerops,	/* OBJT_DEVICE */
 	&devicepagerops,	/* OBJT_MGTDEVICE */
 	&physpagerops,		/* OBJT_PHYS */
-	&deadpagerops		/* OBJT_DEAD */
+	&deadpagerops,		/* OBJT_DEAD */
+	&deadpagerops		/* OBJT_MARKER (list-scan marker; never paged) */
 };
 
 /*
@@ -441,8 +442,34 @@
 			}
 			TAILQ_REMOVE(&bswlist_raw[iter], bp, b_freelist);
 			atomic_add_int(&pbuf_raw_count, -1);
-			if (pfreecnt)
-				atomic_add_int(pfreecnt, -1);
+			if (pfreecnt) {
+				int c;
+
+				/*
+				 * Atomically reserve against pfreecnt.
+				 * The gate at the top of the loop is
+				 * unlocked and the bucket spinlock is
+				 * per-bucket, so a plain decrement here
+				 * can race multiple CPUs past a count of 1
+				 * and drive the counter negative.
+				 */
+				for (;;) {
+					c = *(volatile int *)pfreecnt;
+					if (c <= 0)
+						break;
+					if (atomic_cmpset_int(pfreecnt,
+							      c, c - 1))
+						break;
+				}
+				if (c <= 0) {
+					/* lost the race, put it back */
+					TAILQ_INSERT_HEAD(&bswlist_raw[iter],
+							  bp, b_freelist);
+					atomic_add_int(&pbuf_raw_count, 1);
+					spin_unlock(&bswspin_raw[iter]);
+					continue;
+				}
+			}
 			spin_unlock(&bswspin_raw[iter]);
 			initpbuf(bp);
 
@@ -486,8 +513,25 @@
 			}
 			TAILQ_REMOVE(&bswlist_kva[iter], bp, b_freelist);
 			atomic_add_int(&pbuf_kva_count, -1);
-			if (pfreecnt)
-				atomic_add_int(pfreecnt, -1);
+			if (pfreecnt) {
+				int c;
+
+				for (;;) {
+					c = *(volatile int *)pfreecnt;
+					if (c <= 0)
+						break;
+					if (atomic_cmpset_int(pfreecnt,
+							      c, c - 1))
+						break;
+				}
+				if (c <= 0) {
+					TAILQ_INSERT_HEAD(&bswlist_kva[iter],
+							  bp, b_freelist);
+					atomic_add_int(&pbuf_kva_count, 1);
+					spin_unlock(&bswspin_kva[iter]);
+					continue;
+				}
+			}
 			spin_unlock(&bswspin_kva[iter]);
 			initpbuf(bp);
 
@@ -535,8 +579,25 @@
 			}
 			TAILQ_REMOVE(&bswlist_mem[iter], bp, b_freelist);
 			atomic_add_int(&pbuf_mem_count, -1);
-			if (pfreecnt)
-				atomic_add_int(pfreecnt, -1);
+			if (pfreecnt) {
+				int c;
+
+				for (;;) {
+					c = *(volatile int *)pfreecnt;
+					if (c <= 0)
+						break;
+					if (atomic_cmpset_int(pfreecnt,
+							      c, c - 1))
+						break;
+				}
+				if (c <= 0) {
+					TAILQ_INSERT_HEAD(&bswlist_mem[iter],
+							  bp, b_freelist);
+					atomic_add_int(&pbuf_mem_count, 1);
+					spin_unlock(&bswspin_mem[iter]);
+					continue;
+				}
+			}
 			spin_unlock(&bswspin_mem[iter]);
 			initpbuf(bp);
 
@@ -575,7 +636,24 @@
 		}
 		TAILQ_REMOVE(&bswlist_raw[iter], bp, b_freelist);
 		atomic_add_int(&pbuf_raw_count, -1);
-		atomic_add_int(pfreecnt, -1);
+		{
+			int c;
+
+			for (;;) {
+				c = *(volatile int *)pfreecnt;
+				if (c <= 0)
+					break;
+				if (atomic_cmpset_int(pfreecnt, c, c - 1))
+					break;
+			}
+			if (c <= 0) {
+				TAILQ_INSERT_HEAD(&bswlist_raw[iter], bp,
+						  b_freelist);
+				atomic_add_int(&pbuf_raw_count, 1);
+				spin_unlock(&bswspin_raw[iter]);
+				continue;
+			}
+		}
 
 		spin_unlock(&bswspin_raw[iter]);
 
@@ -607,7 +685,24 @@
 		}
 		TAILQ_REMOVE(&bswlist_kva[iter], bp, b_freelist);
 		atomic_add_int(&pbuf_kva_count, -1);
-		atomic_add_int(pfreecnt, -1);
+		{
+			int c;
+
+			for (;;) {
+				c = *(volatile int *)pfreecnt;
+				if (c <= 0)
+					break;
+				if (atomic_cmpset_int(pfreecnt, c, c - 1))
+					break;
+			}
+			if (c <= 0) {
+				TAILQ_INSERT_HEAD(&bswlist_kva[iter], bp,
+						  b_freelist);
+				atomic_add_int(&pbuf_kva_count, 1);
+				spin_unlock(&bswspin_kva[iter]);
+				continue;
+			}
+		}
 
 		spin_unlock(&bswspin_kva[iter]);