scoutfs/kmod/src/omap.c

/*
 * Copyright (C) 2021 Versity Software, Inc.  All rights reserved.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public
 * License v2 as published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * General Public License for more details.
 */
#include <linux/kernel.h>
#include <linux/fs.h>
#include <linux/slab.h>
#include <linux/sched.h>
#include <linux/rhashtable.h>
#include <linux/rcupdate.h>

#include "format.h"
#include "counters.h"
#include "cmp.h"
#include "inode.h"
#include "client.h"
#include "server.h"
#include "omap.h"
#include "recov.h"
#include "scoutfs_trace.h"

/*
 * As a client removes an inode from its cache with an nlink of 0 it
 * needs to decide if it is the last client using the inode and should
 * fully delete all the inode's items.  It needs to know if other mounts
 * still have the inode in use.
 *
 * We need a way to communicate between mounts that an inode is in use.
 * We don't want to pay the synchronous per-file locking round trip
 * costs associated with per-inode open locks that you'd typically see
 * in systems to solve this problem.  The first prototypes of this
 * tracked open file handles so this was coined the open map, though it
 * now tracks cached inodes.
 *
 * Clients maintain bitmaps that cover groups of inodes.  As inodes
 * enter the cache their bit is set and as the inode is evicted the bit
 * is cleared.  As deletion is attempted, either by scanning orphans or
 * evicting an inode with an nlink of 0, messages are sent around the
 * cluster to get the current bitmaps for that inode's group from all
 * active mounts.  If the inode's bit is clear then it can be deleted.
 *
 * This layer maintains a list of client rids to send messages to.  The
 * server calls us as clients enter and leave the cluster.    We can't
 * process requests until all clients are present as a server starts up
 * so we hook into recovery and delay processing until all previously
 * existing clients are recovered or fenced.
 */

struct omap_rid_list {
	int nr_rids;
	struct list_head head;
};

struct omap_rid_entry {
	struct list_head head;
	u64 rid;
};

struct omap_info {
	/* client */
	struct rhashtable group_ht;

	/* server */
	struct rhashtable req_ht;
	struct llist_head requests;
	spinlock_t lock;
	struct omap_rid_list rids;
	atomic64_t next_req_id;
};

#define DECLARE_OMAP_INFO(sb, name) \
	struct omap_info *name = SCOUTFS_SB(sb)->omap_info

/*
 * The presence of an inode in the inode sets its bit in the lock
 * group's bitmap.
 *
 * We don't want to add additional global synchronization of inode cache
 * maintenance so these are tracked in an rcu hash table.  Once their
 * total reaches zero they're removed from the hash and queued for
 * freeing and readers should ignore them.
 */
struct omap_group {
	struct super_block *sb;
	struct rhash_head ht_head;
	struct rcu_head rcu;
	u64 nr;
	spinlock_t lock;
	unsigned int total;
	__le64 bits[SCOUTFS_OPEN_INO_MAP_LE64S];
};

#define trace_group(sb, which, group, bit_nr)						\
do {											\
	__typeof__(group) _grp = (group);						\
	__typeof__(bit_nr) _nr = (bit_nr);						\
											\
	trace_scoutfs_omap_group_##which(sb, _grp, _grp->nr, _grp->total, _nr);		\
} while (0)

/*
 * Each request is initialized with the rids of currently mounted
 * clients.  As each responds we remove their rid and send the response
 * once everyone has contributed.
 *
 * The request frequency will typically be low, but in a mass rm -rf
 * load we will see O(groups * clients) messages flying around.
 */
struct omap_request {
	struct llist_node llnode;
	struct rhash_head ht_head;
	struct rcu_head rcu;
	spinlock_t lock;
	u64 client_rid;
	u64 client_id;
	struct omap_rid_list rids;
	struct scoutfs_open_ino_map map;
};

static inline void init_rid_list(struct omap_rid_list *list)
{
	INIT_LIST_HEAD(&list->head);
	list->nr_rids = 0;
}

/*
 * Negative searches almost never happen.
 */
static struct omap_rid_entry *find_rid(struct omap_rid_list *list, u64 rid)
{
	struct omap_rid_entry *entry;

	list_for_each_entry(entry, &list->head, head) {
		if (rid == entry->rid)
			return entry;
	}

	return NULL;
}

static int free_rid(struct omap_rid_list *list, struct omap_rid_entry *entry)
{
	int nr;

	list_del(&entry->head);
	nr = --list->nr_rids;

	kfree(entry);
	return nr;
}

static void free_rid_list(struct omap_rid_list *list)
{
	struct omap_rid_entry *entry;
	struct omap_rid_entry *tmp;

	list_for_each_entry_safe(entry, tmp, &list->head, head)
		free_rid(list, entry);
}

static int copy_rids(struct omap_rid_list *to, struct omap_rid_list *from, spinlock_t *from_lock)
{
	struct omap_rid_entry *entry;
	struct omap_rid_entry *src;
	struct omap_rid_entry *dst;
	int nr;

	spin_lock(from_lock);

	while (to->nr_rids != from->nr_rids) {
		nr = from->nr_rids;
		spin_unlock(from_lock);

		while (to->nr_rids < nr) {
			entry = kmalloc(sizeof(struct omap_rid_entry), GFP_NOFS);
			if (!entry)
				return -ENOMEM;

			list_add_tail(&entry->head, &to->head);
			to->nr_rids++;
		}

		while (to->nr_rids > nr) {
			entry = list_first_entry(&to->head, struct omap_rid_entry, head);
			list_del(&entry->head);
			kfree(entry);
			to->nr_rids--;
		}

		spin_lock(from_lock);
	}

	dst = list_first_entry(&to->head, struct omap_rid_entry, head);
	list_for_each_entry(src, &from->head, head) {
		dst->rid = src->rid;
		dst = list_next_entry(dst, head);
	}

	spin_unlock(from_lock);

	return 0;
}

static void free_rids(struct omap_rid_list *list)
{
	struct omap_rid_entry *entry;
	struct omap_rid_entry *tmp;

	list_for_each_entry_safe(entry, tmp, &list->head, head) {
		list_del(&entry->head);
		kfree(entry);
	}
}

void scoutfs_omap_calc_group_nrs(u64 ino, u64 *group_nr, int *bit_nr)
{
	*group_nr = ino >> SCOUTFS_OPEN_INO_MAP_SHIFT;
	*bit_nr = ino & SCOUTFS_OPEN_INO_MAP_MASK;
}

static struct omap_group *alloc_group(struct super_block *sb, u64 group_nr)
{
	struct omap_group *group;

	group = kzalloc(sizeof(struct omap_group), GFP_NOFS);
	if (group) {
		group->sb = sb;
		group->nr = group_nr;
		spin_lock_init(&group->lock);

		trace_group(sb, alloc, group, -1);
	}

	return group;
}

static void free_group(struct super_block *sb, struct omap_group *group)
{
	trace_group(sb, free, group, -1);
	kfree(group);
}

static void free_group_rcu(struct rcu_head *rcu)
{
	struct omap_group *group = container_of(rcu, struct omap_group, rcu);

	free_group(group->sb, group);
}

static const struct rhashtable_params group_ht_params = {
        .key_len = member_sizeof(struct omap_group, nr),
        .key_offset = offsetof(struct omap_group, nr),
        .head_offset = offsetof(struct omap_group, ht_head),
};

/*
 * Track an cached inode in its group.  Our set can be racing with a
 * final clear that removes the group from the hash, sets total to
 * UINT_MAX, and calls rcu free.  We can retry until the dead group is
 * no longer visible in the hash table and we can insert a new allocated
 * group.
 *
 * The caller must ensure that the bit is clear, -EEXIST will be
 * returned otherwise.
 */
int scoutfs_omap_set(struct super_block *sb, u64 ino)
{
	DECLARE_OMAP_INFO(sb, ominf);
	struct omap_group *group;
	u64 group_nr;
	int bit_nr;
	bool found;
	int ret = 0;

	scoutfs_omap_calc_group_nrs(ino, &group_nr, &bit_nr);

retry:
	found = false;
	rcu_read_lock();
	group = rhashtable_lookup(&ominf->group_ht, &group_nr, group_ht_params);
	if (group) {
		spin_lock(&group->lock);
		if (group->total < UINT_MAX) {
			found = true;
			if (WARN_ON_ONCE(test_and_set_bit_le(bit_nr, group->bits)))
				ret = -EEXIST;
			else
				group->total++;
		}
		trace_group(sb, inc, group, bit_nr);
		spin_unlock(&group->lock);
	}
	rcu_read_unlock();

	if (!found) {
		group = alloc_group(sb, group_nr);
		if (group) {
			ret = rhashtable_lookup_insert_fast(&ominf->group_ht, &group->ht_head,
							    group_ht_params);
			if (ret < 0)
				free_group(sb, group);
			if (ret == -EEXIST)
				ret = 0;
			if (ret == -EBUSY) {
				/* wait for rehash to finish */
				synchronize_rcu();
				ret = 0;
			}
			if (ret == 0)
				goto retry;
		} else {
			ret = -ENOMEM;
		}
	}

	return ret;
}

bool scoutfs_omap_test(struct super_block *sb, u64 ino)
{
	DECLARE_OMAP_INFO(sb, ominf);
	struct omap_group *group;
	bool ret = false;
	u64 group_nr;
	int bit_nr;

	scoutfs_omap_calc_group_nrs(ino, &group_nr, &bit_nr);

	rcu_read_lock();
	group = rhashtable_lookup(&ominf->group_ht, &group_nr, group_ht_params);
	if (group) {
		spin_lock(&group->lock);
		ret = !!test_bit_le(bit_nr, group->bits);
		spin_unlock(&group->lock);
	}
	rcu_read_unlock();

	return ret;
}

/*
 * Clear a previously set ino bit.  Trying to clear a bit that's already
 * clear implies imbalanced set/clear or bugs freeing groups.  We only
 * free groups here as the last clear drops the group's total to 0.
 */
void scoutfs_omap_clear(struct super_block *sb, u64 ino)
{
	DECLARE_OMAP_INFO(sb, ominf);
	struct omap_group *group;
	u64 group_nr;
	int bit_nr;

	scoutfs_omap_calc_group_nrs(ino, &group_nr, &bit_nr);

	rcu_read_lock();
	group = rhashtable_lookup(&ominf->group_ht, &group_nr, group_ht_params);
	if (group) {
		spin_lock(&group->lock);
		WARN_ON_ONCE(!test_bit_le(bit_nr, group->bits));
		WARN_ON_ONCE(group->total == 0);
		WARN_ON_ONCE(group->total == UINT_MAX);
		if (test_and_clear_bit_le(bit_nr, group->bits)) {
			if (--group->total == 0) {
				group->total = UINT_MAX;
				rhashtable_remove_fast(&ominf->group_ht, &group->ht_head,
						       group_ht_params);
				call_rcu(&group->rcu, free_group_rcu);
			}
		}
		trace_group(sb, dec, group, bit_nr);
		spin_unlock(&group->lock);
	}
	rcu_read_unlock();

	WARN_ON_ONCE(!group);
}

/*
 * The server adds rids as it discovers clients.  We add them to the
 * list of rids to send map requests to.
 */
int scoutfs_omap_add_rid(struct super_block *sb, u64 rid)
{
	DECLARE_OMAP_INFO(sb, ominf);
	struct omap_rid_entry *entry;
	struct omap_rid_entry *found;

	entry = kmalloc(sizeof(struct omap_rid_entry), GFP_NOFS);
	if (!entry)
		return -ENOMEM;

	spin_lock(&ominf->lock);
	found = find_rid(&ominf->rids, rid);
	if (!found) {
		entry->rid = rid;
		list_add_tail(&entry->head, &ominf->rids.head);
		ominf->rids.nr_rids++;
	}
	spin_unlock(&ominf->lock);

	if (found)
		kfree(entry);

	return 0;
}

static void free_req(struct omap_request *req)
{
	free_rids(&req->rids);
	kfree(req);
}

static void free_req_rcu(struct rcu_head *rcu)
{
	struct omap_request *req = container_of(rcu, struct omap_request, rcu);

	free_req(req);
}

static const struct rhashtable_params req_ht_params = {
        .key_len = member_sizeof(struct omap_request, map.args.req_id),
        .key_offset = offsetof(struct omap_request, map.args.req_id),
        .head_offset = offsetof(struct omap_request, ht_head),
};

/*
 * Remove a rid from all the pending requests.  If it's the last rid we
 * give the caller the details to send a response, they'll call back to
 * keep removing.  If their send fails they're going to shutdown the
 * server so we can queue freeing the request as we give it to them.
 */
static int remove_rid_from_reqs(struct omap_info *ominf, u64 rid, u64 *resp_rid, u64 *resp_id,
				struct scoutfs_open_ino_map *map)
{
	struct omap_rid_entry *entry;
	struct rhashtable_iter iter;
	struct omap_request *req;
	int ret = 0;

	rhashtable_walk_enter(&ominf->req_ht, &iter);
	rhashtable_walk_start(&iter);

	for (;;) {
		req = rhashtable_walk_next(&iter);
		if (req == NULL)
			break;
		if (req == ERR_PTR(-EAGAIN))
			continue;

		spin_lock(&req->lock);
		entry = find_rid(&req->rids, rid);
		if (entry && free_rid(&req->rids, entry) == 0) {
			*resp_rid = req->client_rid;
			*resp_id = req->client_id;
			memcpy(map, &req->map, sizeof(struct scoutfs_open_ino_map));
			rhashtable_remove_fast(&ominf->req_ht, &req->ht_head, req_ht_params);
			call_rcu(&req->rcu, free_req_rcu);
			ret = 1;
		}
		spin_unlock(&req->lock);
		if (ret > 0)
			break;
	}

	rhashtable_walk_stop(&iter);
	rhashtable_walk_exit(&iter);

	if (ret <= 0) {
		*resp_rid = 0;
		*resp_id = 0;
	}

	return ret;
}

/*
 * A client has been evicted.  Remove its rid from the list and walk
 * through all the pending requests and remove its rids, sending the
 * response if it was the last rid waiting for a response.
 *
 * If this returns an error then the server will shut down.
 *
 * This can be called multiple times by different servers if there are
 * errors reclaiming an evicted mount, so we allow asking to remove a
 * rid that hasn't been added.
 */
int scoutfs_omap_remove_rid(struct super_block *sb, u64 rid)
{
	DECLARE_OMAP_INFO(sb, ominf);
	struct scoutfs_open_ino_map *map = NULL;
	struct omap_rid_entry *entry;
	u64 resp_rid = 0;
	u64 resp_id = 0;
	int ret;

	spin_lock(&ominf->lock);
	entry = find_rid(&ominf->rids, rid);
	if (entry)
		free_rid(&ominf->rids, entry);
	spin_unlock(&ominf->lock);

	if (!entry) {
		ret = 0;
		goto out;
	}

	map = kmalloc(sizeof(struct scoutfs_open_ino_map), GFP_NOFS);
	if (!map) {
		ret = -ENOMEM;
		goto out;
	}

	/* remove the rid from all pending requests, sending responses if it was final */
	for (;;) {
		ret = remove_rid_from_reqs(ominf, rid, &resp_rid, &resp_id, map);
		if (ret <= 0)
			break;
		ret = scoutfs_server_send_omap_response(sb, resp_rid, resp_id, map, 0);
		if (ret < 0)
			break;
	}

out:
	kfree(map);
	return ret;
}

/*
 * Handle a single incoming request in the server.  This could have been
 * delayed by recovery.  This only returns an error if we couldn't send
 * a processing error response to the client.
 */
static int handle_request(struct super_block *sb, struct omap_request *req)
{
	DECLARE_OMAP_INFO(sb, ominf);
	struct omap_rid_list priv_rids;
	struct omap_rid_entry *entry;
	int ret;

	init_rid_list(&priv_rids);

	ret = copy_rids(&priv_rids, &ominf->rids, &ominf->lock);
	if (ret < 0)
		goto out;

	/* don't send a request to the client who originated this request */
	entry = find_rid(&priv_rids, req->client_rid);
	if (entry && free_rid(&priv_rids, entry) == 0) {
		ret = scoutfs_server_send_omap_response(sb, req->client_rid, req->client_id,
							&req->map, 0);
		kfree(req);
		req = NULL;
		goto out;
	}

	/* this lock isn't needed but sparse gave warnings with conditional locking */
	ret = copy_rids(&req->rids, &priv_rids, &ominf->lock);
	if (ret < 0)
		goto out;

	do {
		ret = rhashtable_insert_fast(&ominf->req_ht, &req->ht_head, req_ht_params);
		if (ret == -EBUSY)
			synchronize_rcu(); /* wait for rehash to finish */
	} while (ret == -EBUSY);

	if (ret < 0)
		goto out;

	/*
	 * We can start getting responses the moment we send the first response.  After
	 * we send the last request the req can be freed.
	 */
	while ((entry = list_first_entry_or_null(&priv_rids.head, struct omap_rid_entry, head))) {
		ret = scoutfs_server_send_omap_request(sb, entry->rid, &req->map.args);
		if (ret < 0) {
			rhashtable_remove_fast(&ominf->req_ht, &req->ht_head, req_ht_params);
			goto out;
		}

		free_rid(&priv_rids, entry);
	}

	ret = 0;
out:
	free_rids(&priv_rids);
	if ((ret < 0) && (req != NULL)) {
		ret = scoutfs_server_send_omap_response(sb, req->client_rid, req->client_id,
							NULL, ret);
		free_req(req);
	}

	return ret;
}

/*
 * Handle all previously received omap requests from clients.  Once
 * we've finished recovery and can send requests to all clients we can
 * handle all pending requests.  The handling function frees the request
 * and only returns an error if it couldn't send a response to the
 * client.
 */
static int handle_requests(struct super_block *sb)
{
	DECLARE_OMAP_INFO(sb, ominf);
	struct llist_node *requests;
	struct omap_request *req;
	struct omap_request *tmp;
	int ret;
	int err;

	if (scoutfs_recov_next_pending(sb, 0, SCOUTFS_RECOV_GREETING))
		return 0;

	ret = 0;
	requests = llist_del_all(&ominf->requests);

	llist_for_each_entry_safe(req, tmp, requests, llnode) {
		err = handle_request(sb, req);
		if (err < 0 && ret == 0)
			ret = err;
	}

	return ret;
}

int scoutfs_omap_finished_recovery(struct super_block *sb)
{
	return handle_requests(sb);
}

/*
 * The server is receiving a request from a client for the bitmap of all
 * open inodes around their ino.  Queue it for processing which is
 * typically immediate and inline but which can be deferred by recovery
 * as the server first starts up.
 */
int scoutfs_omap_server_handle_request(struct super_block *sb, u64 rid, u64 id,
				       struct scoutfs_open_ino_map_args *args)
{
	DECLARE_OMAP_INFO(sb, ominf);
	struct omap_request *req;

	req = kzalloc(sizeof(struct omap_request), GFP_NOFS);
	if (req == NULL)
		return -ENOMEM;

	spin_lock_init(&req->lock);
	req->client_rid = rid;
	req->client_id = id;
	init_rid_list(&req->rids);
	req->map.args.group_nr = args->group_nr;
	req->map.args.req_id = cpu_to_le64(atomic64_inc_return(&ominf->next_req_id));

	llist_add(&req->llnode, &ominf->requests);

	return handle_requests(sb);
}

/*
 * The client is receiving a request from the server for its map for the
 * given group.  Look up the group and copy the bits to the map.
 *
 * The mount originating the request for this bitmap has the inode group
 * write locked.  We can't be adding links to any inodes in the group
 * because that requires the lock.  Inodes bits can be set and cleared
 * while we're sampling the bitmap.  These races are fine, they can't be
 * adding cached inodes if nlink is 0 and we don't have the lock.  If
 * the caller is removing a set bit then they're about to try and delete
 * the inode themselves and will first have to acquire the cluster lock
 * themselves.
 */
int scoutfs_omap_client_handle_request(struct super_block *sb, u64 id,
				       struct scoutfs_open_ino_map_args *args)
{
	DECLARE_OMAP_INFO(sb, ominf);
	u64 group_nr = le64_to_cpu(args->group_nr);
	struct scoutfs_open_ino_map *map;
	struct omap_group *group;
	bool copied = false;
	int ret;

	map = kmalloc(sizeof(struct scoutfs_open_ino_map), GFP_NOFS);
	if (!map)
		return -ENOMEM;

	map->args = *args;

	rcu_read_lock();
	group = rhashtable_lookup(&ominf->group_ht, &group_nr, group_ht_params);
	if (group) {
		spin_lock(&group->lock);
		trace_group(sb, request, group, -1);
		if (group->total > 0 && group->total < UINT_MAX) {
			memcpy(map->bits, group->bits, sizeof(map->bits));
			copied = true;
		}
		spin_unlock(&group->lock);
	}
	rcu_read_unlock();

	if (!copied)
		memset(map->bits, 0, sizeof(map->bits));

	ret = scoutfs_client_send_omap_response(sb, id, map);
	kfree(map);
	return ret;
}

/*
 * The server has received an open ino map response from a client.  Find
 * the original request that it's serving, or in the response's map, and
 * send a reply if this was the last response from a client we were
 * waiting for.
 *
 * We can get responses for requests we're no longer tracking if, for
 * example, sending to a client gets an error.  We'll have already sent
 * the response to the requesting client so we drop these responses on
 * the floor.
 */
int scoutfs_omap_server_handle_response(struct super_block *sb, u64 rid,
					struct scoutfs_open_ino_map *resp_map)
{
	DECLARE_OMAP_INFO(sb, ominf);
	struct scoutfs_open_ino_map *map;
	struct omap_rid_entry *entry;
	bool send_response = false;
	struct omap_request *req;
	u64 resp_rid;
	u64 resp_id;
	int ret;

	map = kmalloc(sizeof(struct scoutfs_open_ino_map), GFP_NOFS);
	if (!map) {
		ret = -ENOMEM;
		goto out;
	}

	rcu_read_lock();
	req = rhashtable_lookup(&ominf->req_ht, &resp_map->args.req_id, req_ht_params);
	if (req) {
		spin_lock(&req->lock);
		entry = find_rid(&req->rids, rid);
		if (entry) {
			bitmap_or((unsigned long *)req->map.bits, (unsigned long *)req->map.bits,
				  (unsigned long *)resp_map->bits, SCOUTFS_OPEN_INO_MAP_BITS);
			if (free_rid(&req->rids, entry) == 0)
				send_response = true;
		}
		spin_unlock(&req->lock);

		if (send_response) {
			resp_rid = req->client_rid;
			resp_id = req->client_id;
			memcpy(map, &req->map, sizeof(struct scoutfs_open_ino_map));
			rhashtable_remove_fast(&ominf->req_ht, &req->ht_head, req_ht_params);
			call_rcu(&req->rcu, free_req_rcu);
		}
	}
	rcu_read_unlock();

	if (send_response)
		ret = scoutfs_server_send_omap_response(sb, resp_rid, resp_id, map, 0);
	else
		ret = 0;
	kfree(map);
out:
	return ret;
}

/*
 * The server is shutting down.  Free all the server state associated
 * with ongoing request processing.  Clients who still have requests
 * pending will resend them to the next server.
 */
void scoutfs_omap_server_shutdown(struct super_block *sb)
{
	DECLARE_OMAP_INFO(sb, ominf);
	struct rhashtable_iter iter;
	struct llist_node *requests;
	struct omap_request *req;
	struct omap_request *tmp;

	rhashtable_walk_enter(&ominf->req_ht, &iter);
	rhashtable_walk_start(&iter);

	for (;;) {
		req = rhashtable_walk_next(&iter);
		if (req == NULL)
			break;
		if (req == ERR_PTR(-EAGAIN))
			continue;

		if (req->rids.nr_rids != 0) {
			free_rids(&req->rids);
			rhashtable_remove_fast(&ominf->req_ht, &req->ht_head, req_ht_params);
			call_rcu(&req->rcu, free_req_rcu);
		}
	}

	rhashtable_walk_stop(&iter);
	rhashtable_walk_exit(&iter);

	requests = llist_del_all(&ominf->requests);
	llist_for_each_entry_safe(req, tmp, requests, llnode)
		kfree(req);

	spin_lock(&ominf->lock);
	free_rid_list(&ominf->rids);
	spin_unlock(&ominf->lock);

	synchronize_rcu();
}

int scoutfs_omap_setup(struct super_block *sb)
{
	struct scoutfs_sb_info *sbi = SCOUTFS_SB(sb);
	struct omap_info *ominf;
	int ret;

	ominf = kzalloc(sizeof(struct omap_info), GFP_KERNEL);
	if (!ominf) {
		ret = -ENOMEM;
		goto out;
	}

	ret = rhashtable_init(&ominf->group_ht, &group_ht_params);
	if (ret < 0) {
		kfree(ominf);
		goto out;
	}

	ret = rhashtable_init(&ominf->req_ht, &req_ht_params);
	if (ret < 0) {
		rhashtable_destroy(&ominf->group_ht);
		kfree(ominf);
		goto out;
	}

	init_llist_head(&ominf->requests);
	spin_lock_init(&ominf->lock);
	init_rid_list(&ominf->rids);
	atomic64_set(&ominf->next_req_id, 0);

	sbi->omap_info = ominf;
	ret = 0;
out:
	return ret;
}

/*
 * To get here the server must have shut down, freeing requests, and
 * evict must have been called on all cached inodes so we can just
 * synchronize all the pending group frees.
 */
void scoutfs_omap_destroy(struct super_block *sb)
{
	DECLARE_OMAP_INFO(sb, ominf);
	struct scoutfs_sb_info *sbi = SCOUTFS_SB(sb);
	struct rhashtable_iter iter;

	if (ominf) {
		synchronize_rcu();

		/* double check that all the groups deced to 0 and were freed */
		rhashtable_walk_enter(&ominf->group_ht, &iter);
		rhashtable_walk_start(&iter);
		WARN_ON_ONCE(rhashtable_walk_peek(&iter) != NULL);
		rhashtable_walk_stop(&iter);
		rhashtable_walk_exit(&iter);

		spin_lock(&ominf->lock);
		free_rid_list(&ominf->rids);
		spin_unlock(&ominf->lock);

		rhashtable_destroy(&ominf->group_ht);
		rhashtable_destroy(&ominf->req_ht);
		kfree(ominf);
		sbi->omap_info = NULL;
	}
}