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, 112 insertions, 0 deletions

diff --git a/fs/ceph/cache.c b/fs/ceph/cache.c
new file mode 100644
index 000000000..f678bab18
--- /dev/null
+++ b/fs/ceph/cache.c
@@ -0,0 +1,112 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Ceph cache definitions.
+ *
+ *  Copyright (C) 2013 by Adfin Solutions, Inc. All Rights Reserved.
+ *  Written by Milosz Tanski (milosz@adfin.com)
+ */
+
+#include <linux/ceph/ceph_debug.h>
+
+#include <linux/fs_context.h>
+#include "super.h"
+#include "cache.h"
+
+void ceph_fscache_register_inode_cookie(struct inode *inode)
+{
+	struct ceph_inode_info *ci = ceph_inode(inode);
+	struct ceph_fs_client *fsc = ceph_inode_to_fs_client(inode);
+
+	/* No caching for filesystem? */
+	if (!fsc->fscache)
+		return;
+
+	/* Regular files only */
+	if (!S_ISREG(inode->i_mode))
+		return;
+
+	/* Only new inodes! */
+	if (!(inode_state_read_once(inode) & I_NEW))
+		return;
+
+	WARN_ON_ONCE(ci->netfs.cache);
+
+	ci->netfs.cache =
+		fscache_acquire_cookie(fsc->fscache, 0,
+				       &ci->i_vino, sizeof(ci->i_vino),
+				       &ci->i_version, sizeof(ci->i_version),
+				       i_size_read(inode));
+	if (ci->netfs.cache)
+		mapping_set_release_always(inode->i_mapping);
+}
+
+void ceph_fscache_unregister_inode_cookie(struct ceph_inode_info *ci)
+{
+	fscache_relinquish_cookie(ceph_fscache_cookie(ci), false);
+}
+
+void ceph_fscache_use_cookie(struct inode *inode, bool will_modify)
+{
+	struct ceph_inode_info *ci = ceph_inode(inode);
+
+	fscache_use_cookie(ceph_fscache_cookie(ci), will_modify);
+}
+
+void ceph_fscache_unuse_cookie(struct inode *inode, bool update)
+{
+	struct ceph_inode_info *ci = ceph_inode(inode);
+
+	if (update) {
+		loff_t i_size = i_size_read(inode);
+
+		fscache_unuse_cookie(ceph_fscache_cookie(ci),
+				     &ci->i_version, &i_size);
+	} else {
+		fscache_unuse_cookie(ceph_fscache_cookie(ci), NULL, NULL);
+	}
+}
+
+void ceph_fscache_update(struct inode *inode)
+{
+	struct ceph_inode_info *ci = ceph_inode(inode);
+	loff_t i_size = i_size_read(inode);
+
+	fscache_update_cookie(ceph_fscache_cookie(ci), &ci->i_version, &i_size);
+}
+
+void ceph_fscache_invalidate(struct inode *inode, bool dio_write)
+{
+	struct ceph_inode_info *ci = ceph_inode(inode);
+
+	fscache_invalidate(ceph_fscache_cookie(ci),
+			   &ci->i_version, i_size_read(inode),
+			   dio_write ? FSCACHE_INVAL_DIO_WRITE : 0);
+}
+
+int ceph_fscache_register_fs(struct ceph_fs_client* fsc, struct fs_context *fc)
+{
+	const struct ceph_fsid *fsid = &fsc->client->fsid;
+	const char *fscache_uniq = fsc->mount_options->fscache_uniq;
+	size_t uniq_len = fscache_uniq ? strlen(fscache_uniq) : 0;
+	char *name;
+	int err = 0;
+
+	name = kasprintf(GFP_KERNEL, "ceph,%pU%s%s", fsid, uniq_len ? "," : "",
+			 uniq_len ? fscache_uniq : "");
+	if (!name)
+		return -ENOMEM;
+
+	fsc->fscache = fscache_acquire_volume(name, NULL, NULL, 0);
+	if (IS_ERR_OR_NULL(fsc->fscache)) {
+		errorfc(fc, "Unable to register fscache cookie for %s", name);
+		err = fsc->fscache ? PTR_ERR(fsc->fscache) : -EOPNOTSUPP;
+		fsc->fscache = NULL;
+	}
+	kfree(name);
+	return err;
+}
+
+void ceph_fscache_unregister_fs(struct ceph_fs_client* fsc)
+{
+	fscache_relinquish_volume(fsc->fscache, NULL, false);
+}