Sat May 4 17:58:24 2024 UTC (28d)

radixtree: allocate memory with KM_NOSLEEP to prevent pagedaemon hangs

Revert the part of rev 1.32 (reapplying "Do away with separate pool_cache
for some kernel objects") that changed the memory allocation for radixtree
nodes from PR_NOWAIT to KM_SLEEP as part of changing from a pool to kmem.
uvm_pageinsert_tree() calls into the radixtree code while holding
the object's vmobjlock, but that same lock is taken by the pagedaemon
in the process of reclaiming pages, and if the pagedaemon happens to
choose the same object to reclaim from that uvm_pageinsert_tree()
is being called on, then these two threads will deadlock.
The previous code already handled memory allocation failures
in uvm_pageinsert_tree() so we can simply change it back to nosleep.

Fixes a hang reported by simonb@, and the fix was also tested by him.


(chs)

diff -r1.33 -r1.34 src/common/lib/libc/gen/radixtree.c

--- src/common/lib/libc/gen/radixtree.c 2023/09/23 19:17:38	1.33
+++ src/common/lib/libc/gen/radixtree.c 2024/05/04 17:58:24	1.34

 @@ -1,14 +1,14 @@
-/*	$NetBSD: radixtree.c,v 1.33 2023/09/23 19:17:38 ad Exp $	*/
+/*	$NetBSD: radixtree.c,v 1.34 2024/05/04 17:58:24 chs Exp $	*/
 /*-
  * Copyright (c)2011,2012,2013 YAMAMOTO Takashi,
  * All rights reserved.
+ *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
  *    documentation and/or other materials provided with the distribution.
 @@ -102,37 +102,37 @@
  * leaf nodes with the given tag.  To reduce amount of nodes to visit for
  * these functions, this implementation keeps tagging information in internal
  * intermediate nodes and quickly skips uninterested parts of a tree.
+ *
  * A tree has RADIX_TREE_TAG_ID_MAX independent tag spaces, each of which are
  * identified by a zero-origin numbers, tagid.  For the current implementation,
  * RADIX_TREE_TAG_ID_MAX is 2.  A set of tags is described as a bitmask tagmask,
  * which is a bitwise OR of (1 << tagid).
  */
 #include <sys/cdefs.h>
 #if defined(_KERNEL) || defined(_STANDALONE)
-__KERNEL_RCSID(0, "$NetBSD: radixtree.c,v 1.33 2023/09/23 19:17:38 ad Exp $");
+__KERNEL_RCSID(0, "$NetBSD: radixtree.c,v 1.34 2024/05/04 17:58:24 chs Exp $");
 #include <sys/param.h>
 #include <sys/errno.h>
 #include <sys/kmem.h>
 #include <sys/radixtree.h>
 #include <lib/libkern/libkern.h>
 #if defined(_STANDALONE)
 #include <lib/libsa/stand.h>
 #endif /* defined(_STANDALONE) */
 #else /* defined(_KERNEL) || defined(_STANDALONE) */
-__RCSID("$NetBSD: radixtree.c,v 1.33 2023/09/23 19:17:38 ad Exp $");
+__RCSID("$NetBSD: radixtree.c,v 1.34 2024/05/04 17:58:24 chs Exp $");
 #include <assert.h>
 #include <errno.h>
 #include <stdbool.h>
 #include <stdlib.h>
 #include <string.h>
 #if 1
 #define KASSERT assert
 #else
 #define KASSERT(a)	/* nothing */
 #endif
 #endif /* defined(_KERNEL) || defined(_STANDALONE) */
 #include <sys/radixtree.h>
 @@ -400,29 +400,30 @@ radix_tree_node_count_ptrs(const struct
 	for (i = c = 0; i < RADIX_TREE_PTR_PER_NODE; i++) {
 		c += (n->n_ptrs[i] != NULL);
+	}
 	return c;
+}
 static struct radix_tree_node *
 radix_tree_alloc_node(void)
+{
 	struct radix_tree_node *n;
 #if defined(_KERNEL)
 	/*
-	 * note that kmem_alloc can block.
+	 * We must not block waiting for memory because this function
 	 * can be called in contexts where waiting for memory is illegal.
 	 */
-	n = kmem_intr_alloc(sizeof(struct radix_tree_node), KM_SLEEP);
+	n = kmem_intr_alloc(sizeof(struct radix_tree_node), KM_NOSLEEP);
 #elif defined(_STANDALONE)
 	n = alloc(sizeof(*n));
 #else /* defined(_STANDALONE) */
 	n = malloc(sizeof(*n));
 #endif /* defined(_STANDALONE) */
 	if (n != NULL) {
 		radix_tree_node_init(n);
+	}
 	KASSERT(n == NULL || radix_tree_sum_node(n) == 0);
 	return n;
+}
 static void