sched_ext: Maintain an online cid mask in the scheduler arenagrafted

Schedulers on the default cid mapping treat [0, nr_online_cids) as the
online set and restart on hotplug. Schedulers that install their own mapping
with scx_bpf_cid_override() have no way to learn which cids are online: the
count no longer identifies members and the CPU-form cpumask is unusable from
cid programs. This is an obvious hole in the cid API.

Add scx_bpf_online_cmask(), a kernel-maintained cmask in the scheduler's
arena, allocated alongside the per-CPU scratch masks and populated after the
cid mapping is finalized and before ops.init(), for child schedulers too.
The pointer stays valid through ops.exit() with no reference to take. It is
the arena offset as a void pointer, the same form struct_ops arena arguments
arrive in. The verifier types the void return as a scalar for the program's
arena cast.

The mask follows the SCX hotplug notifications: seeded from cpu_active_mask
and updated before ops.cid_online/offline() runs, so it lags cpu_online_mask
only inside a hotplug transition. Updates walk the scheduler list under the
lock that also serializes unlinking. Reads are live, not atomic snapshots.
Root initialization excludes hotplug.

v2: Reworded the getter kerneldoc (Andrea Righi).

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>

1 files changed, 137 insertions, 0 deletions

diff --git a/fs/cachefiles/key.c b/fs/cachefiles/key.c
new file mode 100644
index 000000000..aae86af48
--- /dev/null
+++ b/fs/cachefiles/key.c
@@ -0,0 +1,137 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/* Key to pathname encoder
+ *
+ * Copyright (C) 2021 Red Hat, Inc. All Rights Reserved.
+ * Written by David Howells (dhowells@redhat.com)
+ */
+
+#include <linux/slab.h>
+#include "internal.h"
+
+static const char cachefiles_charmap[64] __nonstring =
+	"0123456789"			/* 0 - 9 */
+	"abcdefghijklmnopqrstuvwxyz"	/* 10 - 35 */
+	"ABCDEFGHIJKLMNOPQRSTUVWXYZ"	/* 36 - 61 */
+	"_-"				/* 62 - 63 */
+	;
+
+static const char cachefiles_filecharmap[256] = {
+	/* we skip space and tab and control chars */
+	[33 ... 46] = 1,		/* '!' -> '.' */
+	/* we skip '/' as it's significant to pathwalk */
+	[48 ... 127] = 1,		/* '0' -> '~' */
+};
+
+static inline unsigned int how_many_hex_digits(unsigned int x)
+{
+	return x ? round_up(ilog2(x) + 1, 4) / 4 : 0;
+}
+
+/*
+ * turn the raw key into something cooked
+ * - the key may be up to NAME_MAX in length (including the length word)
+ *   - "base64" encode the strange keys, mapping 3 bytes of raw to four of
+ *     cooked
+ *   - need to cut the cooked key into 252 char lengths (189 raw bytes)
+ */
+bool cachefiles_cook_key(struct cachefiles_object *object)
+{
+	const u8 *key = fscache_get_key(object->cookie), *kend;
+	unsigned char ch;
+	unsigned int acc, i, n, nle, nbe, keylen = object->cookie->key_len;
+	unsigned int b64len, len, print, pad;
+	char *name, sep;
+
+	_enter(",%u,%*phN", keylen, keylen, key);
+
+	BUG_ON(keylen > NAME_MAX - 3);
+
+	print = 1;
+	for (i = 0; i < keylen; i++) {
+		ch = key[i];
+		print &= cachefiles_filecharmap[ch];
+	}
+
+	/* If the path is usable ASCII, then we render it directly */
+	if (print) {
+		len = 1 + keylen;
+		name = kmalloc(len + 1, GFP_KERNEL);
+		if (!name)
+			return false;
+
+		name[0] = 'D'; /* Data object type, string encoding */
+		memcpy(name + 1, key, keylen);
+		goto success;
+	}
+
+	/* See if it makes sense to encode it as "hex,hex,hex" for each 32-bit
+	 * chunk.  We rely on the key having been padded out to a whole number
+	 * of 32-bit words.
+	 */
+	n = round_up(keylen, 4);
+	nbe = nle = 0;
+	for (i = 0; i < n; i += 4) {
+		u32 be = be32_to_cpu(*(__be32 *)(key + i));
+		u32 le = le32_to_cpu(*(__le32 *)(key + i));
+
+		nbe += 1 + how_many_hex_digits(be);
+		nle += 1 + how_many_hex_digits(le);
+	}
+
+	b64len = DIV_ROUND_UP(keylen, 3);
+	pad = b64len * 3 - keylen;
+	b64len = 2 + b64len * 4; /* Length if we base64-encode it */
+	_debug("len=%u nbe=%u nle=%u b64=%u", keylen, nbe, nle, b64len);
+	if (nbe < b64len || nle < b64len) {
+		unsigned int nlen = min(nbe, nle) + 1;
+		name = kmalloc(nlen, GFP_KERNEL);
+		if (!name)
+			return false;
+		sep = (nbe <= nle) ? 'S' : 'T'; /* Encoding indicator */
+		len = 0;
+		for (i = 0; i < n; i += 4) {
+			u32 x;
+			if (nbe <= nle)
+				x = be32_to_cpu(*(__be32 *)(key + i));
+			else
+				x = le32_to_cpu(*(__le32 *)(key + i));
+			name[len++] = sep;
+			if (x != 0)
+				len += snprintf(name + len, nlen - len, "%x", x);
+			sep = ',';
+		}
+		goto success;
+	}
+
+	/* We need to base64-encode it */
+	name = kmalloc(b64len + 1, GFP_KERNEL);
+	if (!name)
+		return false;
+
+	name[0] = 'E';
+	name[1] = '0' + pad;
+	len = 2;
+	kend = key + keylen;
+	do {
+		acc  = *key++;
+		if (key < kend) {
+			acc |= *key++ << 8;
+			if (key < kend)
+				acc |= *key++ << 16;
+		}
+
+		name[len++] = cachefiles_charmap[acc & 63];
+		acc >>= 6;
+		name[len++] = cachefiles_charmap[acc & 63];
+		acc >>= 6;
+		name[len++] = cachefiles_charmap[acc & 63];
+		acc >>= 6;
+		name[len++] = cachefiles_charmap[acc & 63];
+	} while (key < kend);
+
+success:
+	name[len] = 0;
+	object->d_name = name;
+	_leave(" = %s", object->d_name);
+	return true;
+}