diff --git a/fs/cifs/cifsacl.c b/fs/cifs/cifsacl.c
index 42b3fe9..f4508ee 100644
--- a/fs/cifs/cifsacl.c
+++ b/fs/cifs/cifsacl.c
@@ -44,128 +44,6 @@
 
 static const struct cred *root_cred;
 
-static void
-shrink_idmap_tree(struct rb_root *root, int nr_to_scan, int *nr_rem,
-			int *nr_del)
-{
-	struct rb_node *node;
-	struct rb_node *tmp;
-	struct cifs_sid_id *psidid;
-
-	node = rb_first(root);
-	while (node) {
-		tmp = node;
-		node = rb_next(tmp);
-		psidid = rb_entry(tmp, struct cifs_sid_id, rbnode);
-		if (nr_to_scan == 0 || *nr_del == nr_to_scan)
-			++(*nr_rem);
-		else {
-			if (time_after(jiffies, psidid->time + SID_MAP_EXPIRE)
-						&& psidid->refcount == 0) {
-				rb_erase(tmp, root);
-				++(*nr_del);
-			} else
-				++(*nr_rem);
-		}
-	}
-}
-
-/*
- * Run idmap cache shrinker.
- */
-static int
-cifs_idmap_shrinker(struct shrinker *shrink, struct shrink_control *sc)
-{
-	int nr_to_scan = sc->nr_to_scan;
-	int nr_del = 0;
-	int nr_rem = 0;
-	struct rb_root *root;
-
-	root = &uidtree;
-	spin_lock(&siduidlock);
-	shrink_idmap_tree(root, nr_to_scan, &nr_rem, &nr_del);
-	spin_unlock(&siduidlock);
-
-	root = &gidtree;
-	spin_lock(&sidgidlock);
-	shrink_idmap_tree(root, nr_to_scan, &nr_rem, &nr_del);
-	spin_unlock(&sidgidlock);
-
-	root = &siduidtree;
-	spin_lock(&uidsidlock);
-	shrink_idmap_tree(root, nr_to_scan, &nr_rem, &nr_del);
-	spin_unlock(&uidsidlock);
-
-	root = &sidgidtree;
-	spin_lock(&gidsidlock);
-	shrink_idmap_tree(root, nr_to_scan, &nr_rem, &nr_del);
-	spin_unlock(&gidsidlock);
-
-	return nr_rem;
-}
-
-static void
-sid_rb_insert(struct rb_root *root, unsigned long cid,
-		struct cifs_sid_id **psidid, char *typestr)
-{
-	char *strptr;
-	struct rb_node *node = root->rb_node;
-	struct rb_node *parent = NULL;
-	struct rb_node **linkto = &(root->rb_node);
-	struct cifs_sid_id *lsidid;
-
-	while (node) {
-		lsidid = rb_entry(node, struct cifs_sid_id, rbnode);
-		parent = node;
-		if (cid > lsidid->id) {
-			linkto = &(node->rb_left);
-			node = node->rb_left;
-		}
-		if (cid < lsidid->id) {
-			linkto = &(node->rb_right);
-			node = node->rb_right;
-		}
-	}
-
-	(*psidid)->id = cid;
-	(*psidid)->time = jiffies - (SID_MAP_RETRY + 1);
-	(*psidid)->refcount = 0;
-
-	sprintf((*psidid)->sidstr, "%s", typestr);
-	strptr = (*psidid)->sidstr + strlen((*psidid)->sidstr);
-	sprintf(strptr, "%ld", cid);
-
-	clear_bit(SID_ID_PENDING, &(*psidid)->state);
-	clear_bit(SID_ID_MAPPED, &(*psidid)->state);
-
-	rb_link_node(&(*psidid)->rbnode, parent, linkto);
-	rb_insert_color(&(*psidid)->rbnode, root);
-}
-
-static struct cifs_sid_id *
-sid_rb_search(struct rb_root *root, unsigned long cid)
-{
-	struct rb_node *node = root->rb_node;
-	struct cifs_sid_id *lsidid;
-
-	while (node) {
-		lsidid = rb_entry(node, struct cifs_sid_id, rbnode);
-		if (cid > lsidid->id)
-			node = node->rb_left;
-		else if (cid < lsidid->id)
-			node = node->rb_right;
-		else /* node found */
-			return lsidid;
-	}
-
-	return NULL;
-}
-
-static struct shrinker cifs_shrinker = {
-	.shrink = cifs_idmap_shrinker,
-	.seeks = DEFAULT_SEEKS,
-};
-
 static int
 cifs_idmap_key_instantiate(struct key *key, struct key_preparsed_payload *prep)
 {
@@ -195,30 +73,39 @@
 	.match       = user_match,
 };
 
-static void
-sid_to_str(struct cifs_sid *sidptr, char *sidstr)
+static char *
+sid_to_key_str(struct cifs_sid *sidptr, unsigned int type)
 {
-	int i;
+	int i, len;
 	unsigned int saval;
-	char *strptr;
+	char *sidstr, *strptr;
+
+	/* 3 bytes for prefix */
+	sidstr = kmalloc(3 + SID_STRING_BASE_SIZE +
+			 (SID_STRING_SUBAUTH_SIZE * sidptr->num_subauth),
+			 GFP_KERNEL);
+	if (!sidstr)
+		return sidstr;
 
 	strptr = sidstr;
-
-	sprintf(strptr, "S-%hhu", sidptr->revision);
-	strptr = sidstr + strlen(sidstr);
+	len = sprintf(strptr, "%cs:S-%hhu", type == SIDOWNER ? 'o' : 'g',
+			sidptr->revision);
+	strptr += len;
 
 	for (i = 0; i < NUM_AUTHS; ++i) {
 		if (sidptr->authority[i]) {
-			sprintf(strptr, "-%hhu", sidptr->authority[i]);
-			strptr = sidstr + strlen(sidstr);
+			len = sprintf(strptr, "-%hhu", sidptr->authority[i]);
+			strptr += len;
 		}
 	}
 
 	for (i = 0; i < sidptr->num_subauth; ++i) {
 		saval = le32_to_cpu(sidptr->sub_auth[i]);
-		sprintf(strptr, "-%u", saval);
-		strptr = sidstr + strlen(sidstr);
+		len = sprintf(strptr, "-%u", saval);
+		strptr += len;
 	}
+
+	return sidstr;
 }
 
 /*
@@ -284,184 +171,38 @@
 		dst->sub_auth[i] = src->sub_auth[i];
 }
 
-static void
-id_rb_insert(struct rb_root *root, struct cifs_sid *sidptr,
-		struct cifs_sid_id **psidid, char *typestr)
+static int
+id_to_sid(unsigned int cid, uint sidtype, struct cifs_sid *ssid)
 {
 	int rc;
-	char *strptr;
-	struct rb_node *node = root->rb_node;
-	struct rb_node *parent = NULL;
-	struct rb_node **linkto = &(root->rb_node);
-	struct cifs_sid_id *lsidid;
-
-	while (node) {
-		lsidid = rb_entry(node, struct cifs_sid_id, rbnode);
-		parent = node;
-		rc = compare_sids(sidptr, &((lsidid)->sid));
-		if (rc > 0) {
-			linkto = &(node->rb_left);
-			node = node->rb_left;
-		} else if (rc < 0) {
-			linkto = &(node->rb_right);
-			node = node->rb_right;
-		}
-	}
-
-	cifs_copy_sid(&(*psidid)->sid, sidptr);
-	(*psidid)->time = jiffies - (SID_MAP_RETRY + 1);
-	(*psidid)->refcount = 0;
-
-	sprintf((*psidid)->sidstr, "%s", typestr);
-	strptr = (*psidid)->sidstr + strlen((*psidid)->sidstr);
-	sid_to_str(&(*psidid)->sid, strptr);
-
-	clear_bit(SID_ID_PENDING, &(*psidid)->state);
-	clear_bit(SID_ID_MAPPED, &(*psidid)->state);
-
-	rb_link_node(&(*psidid)->rbnode, parent, linkto);
-	rb_insert_color(&(*psidid)->rbnode, root);
-}
-
-static struct cifs_sid_id *
-id_rb_search(struct rb_root *root, struct cifs_sid *sidptr)
-{
-	int rc;
-	struct rb_node *node = root->rb_node;
-	struct cifs_sid_id *lsidid;
-
-	while (node) {
-		lsidid = rb_entry(node, struct cifs_sid_id, rbnode);
-		rc = compare_sids(sidptr, &((lsidid)->sid));
-		if (rc > 0) {
-			node = node->rb_left;
-		} else if (rc < 0) {
-			node = node->rb_right;
-		} else /* node found */
-			return lsidid;
-	}
-
-	return NULL;
-}
-
-static int
-sidid_pending_wait(void *unused)
-{
-	schedule();
-	return signal_pending(current) ? -ERESTARTSYS : 0;
-}
-
-static int
-id_to_sid(unsigned long cid, uint sidtype, struct cifs_sid *ssid)
-{
-	int rc = 0;
 	struct key *sidkey;
+	char desc[3 + 10 + 1]; /* 3 byte prefix + 10 bytes for value + NULL */
 	const struct cred *saved_cred;
-	struct cifs_sid *lsid;
-	struct cifs_sid_id *psidid, *npsidid;
-	struct rb_root *cidtree;
-	spinlock_t *cidlock;
 
-	if (sidtype == SIDOWNER) {
-		cidlock = &siduidlock;
-		cidtree = &uidtree;
-	} else if (sidtype == SIDGROUP) {
-		cidlock = &sidgidlock;
-		cidtree = &gidtree;
-	} else
+	rc = snprintf(desc, sizeof(desc), "%ci:%u",
+			sidtype == SIDOWNER ? 'o' : 'g', cid);
+	if (rc >= sizeof(desc))
 		return -EINVAL;
 
-	spin_lock(cidlock);
-	psidid = sid_rb_search(cidtree, cid);
-
-	if (!psidid) { /* node does not exist, allocate one & attempt adding */
-		spin_unlock(cidlock);
-		npsidid = kzalloc(sizeof(struct cifs_sid_id), GFP_KERNEL);
-		if (!npsidid)
-			return -ENOMEM;
-
-		npsidid->sidstr = kmalloc(SID_STRING_MAX, GFP_KERNEL);
-		if (!npsidid->sidstr) {
-			kfree(npsidid);
-			return -ENOMEM;
-		}
-
-		spin_lock(cidlock);
-		psidid = sid_rb_search(cidtree, cid);
-		if (psidid) { /* node happened to get inserted meanwhile */
-			++psidid->refcount;
-			spin_unlock(cidlock);
-			kfree(npsidid->sidstr);
-			kfree(npsidid);
-		} else {
-			psidid = npsidid;
-			sid_rb_insert(cidtree, cid, &psidid,
-					sidtype == SIDOWNER ? "oi:" : "gi:");
-			++psidid->refcount;
-			spin_unlock(cidlock);
-		}
-	} else {
-		++psidid->refcount;
-		spin_unlock(cidlock);
-	}
-
-	/*
-	 * If we are here, it is safe to access psidid and its fields
-	 * since a reference was taken earlier while holding the spinlock.
-	 * A reference on the node is put without holding the spinlock
-	 * and it is OK to do so in this case, shrinker will not erase
-	 * this node until all references are put and we do not access
-	 * any fields of the node after a reference is put .
-	 */
-	if (test_bit(SID_ID_MAPPED, &psidid->state)) {
-		cifs_copy_sid(ssid, &psidid->sid);
-		psidid->time = jiffies; /* update ts for accessing */
-		goto id_sid_out;
-	}
-
-	if (time_after(psidid->time + SID_MAP_RETRY, jiffies)) {
+	rc = 0;
+	saved_cred = override_creds(root_cred);
+	sidkey = request_key(&cifs_idmap_key_type, desc, "");
+	if (IS_ERR(sidkey)) {
 		rc = -EINVAL;
-		goto id_sid_out;
+		cFYI(1, "%s: Can't map %cid %u to a SID", __func__,
+			sidtype == SIDOWNER ? 'u' : 'g', cid);
+		goto out_revert_creds;
+	} else if (sidkey->datalen < CIFS_SID_BASE_SIZE) {
+		rc = -EIO;
+		cFYI(1, "%s: Downcall contained malformed key "
+			"(datalen=%hu)", __func__, sidkey->datalen);
+		goto out_key_put;
 	}
-
-	if (!test_and_set_bit(SID_ID_PENDING, &psidid->state)) {
-		saved_cred = override_creds(root_cred);
-		sidkey = request_key(&cifs_idmap_key_type, psidid->sidstr, "");
-		if (IS_ERR(sidkey)) {
-			rc = -EINVAL;
-			cFYI(1, "%s: Can't map and id to a SID", __func__);
-		} else if (sidkey->datalen < CIFS_SID_BASE_SIZE) {
-			rc = -EIO;
-			cFYI(1, "%s: Downcall contained malformed key "
-				"(datalen=%hu)", __func__, sidkey->datalen);
-		} else {
-			lsid = (struct cifs_sid *)sidkey->payload.data;
-			cifs_copy_sid(&psidid->sid, lsid);
-			cifs_copy_sid(ssid, &psidid->sid);
-			set_bit(SID_ID_MAPPED, &psidid->state);
-			key_put(sidkey);
-			kfree(psidid->sidstr);
-		}
-		psidid->time = jiffies; /* update ts for accessing */
-		revert_creds(saved_cred);
-		clear_bit(SID_ID_PENDING, &psidid->state);
-		wake_up_bit(&psidid->state, SID_ID_PENDING);
-	} else {
-		rc = wait_on_bit(&psidid->state, SID_ID_PENDING,
-				sidid_pending_wait, TASK_INTERRUPTIBLE);
-		if (rc) {
-			cFYI(1, "%s: sidid_pending_wait interrupted %d",
-					__func__, rc);
-			--psidid->refcount;
-			return rc;
-		}
-		if (test_bit(SID_ID_MAPPED, &psidid->state))
-			cifs_copy_sid(ssid, &psidid->sid);
-		else
-			rc = -EINVAL;
-	}
-id_sid_out:
-	--psidid->refcount;
+	cifs_copy_sid(ssid, (struct cifs_sid *)sidkey->payload.data);
+out_key_put:
+	key_put(sidkey);
+out_revert_creds:
+	revert_creds(saved_cred);
 	return rc;
 }
 
@@ -470,111 +211,66 @@
 		struct cifs_fattr *fattr, uint sidtype)
 {
 	int rc;
-	unsigned long cid;
-	struct key *idkey;
+	struct key *sidkey;
+	char *sidstr;
 	const struct cred *saved_cred;
-	struct cifs_sid_id *psidid, *npsidid;
-	struct rb_root *cidtree;
-	spinlock_t *cidlock;
+	uid_t fuid = cifs_sb->mnt_uid;
+	gid_t fgid = cifs_sb->mnt_gid;
 
-	if (sidtype == SIDOWNER) {
-		cid = cifs_sb->mnt_uid; /* default uid, in case upcall fails */
-		cidlock = &siduidlock;
-		cidtree = &uidtree;
-	} else if (sidtype == SIDGROUP) {
-		cid = cifs_sb->mnt_gid; /* default gid, in case upcall fails */
-		cidlock = &sidgidlock;
-		cidtree = &gidtree;
-	} else
-		return -ENOENT;
+	/*
+	 * If we have too many subauthorities, then something is really wrong.
+	 * Just return an error.
+	 */
+	if (unlikely(psid->num_subauth > SID_MAX_SUB_AUTHORITIES)) {
+		cFYI(1, "%s: %u subauthorities is too many!", __func__,
+			psid->num_subauth);
+		return -EIO;
+	}
 
-	spin_lock(cidlock);
-	psidid = id_rb_search(cidtree, psid);
+	sidstr = sid_to_key_str(psid, sidtype);
+	if (!sidstr)
+		return -ENOMEM;
 
-	if (!psidid) { /* node does not exist, allocate one & attempt adding */
-		spin_unlock(cidlock);
-		npsidid = kzalloc(sizeof(struct cifs_sid_id), GFP_KERNEL);
-		if (!npsidid)
-			return -ENOMEM;
-
-		npsidid->sidstr = kmalloc(SID_STRING_MAX, GFP_KERNEL);
-		if (!npsidid->sidstr) {
-			kfree(npsidid);
-			return -ENOMEM;
-		}
-
-		spin_lock(cidlock);
-		psidid = id_rb_search(cidtree, psid);
-		if (psidid) { /* node happened to get inserted meanwhile */
-			++psidid->refcount;
-			spin_unlock(cidlock);
-			kfree(npsidid->sidstr);
-			kfree(npsidid);
-		} else {
-			psidid = npsidid;
-			id_rb_insert(cidtree, psid, &psidid,
-					sidtype == SIDOWNER ? "os:" : "gs:");
-			++psidid->refcount;
-			spin_unlock(cidlock);
-		}
-	} else {
-		++psidid->refcount;
-		spin_unlock(cidlock);
+	saved_cred = override_creds(root_cred);
+	sidkey = request_key(&cifs_idmap_key_type, sidstr, "");
+	if (IS_ERR(sidkey)) {
+		rc = -EINVAL;
+		cFYI(1, "%s: Can't map SID %s to a %cid", __func__, sidstr,
+			sidtype == SIDOWNER ? 'u' : 'g');
+		goto out_revert_creds;
 	}
 
 	/*
-	 * If we are here, it is safe to access psidid and its fields
-	 * since a reference was taken earlier while holding the spinlock.
-	 * A reference on the node is put without holding the spinlock
-	 * and it is OK to do so in this case, shrinker will not erase
-	 * this node until all references are put and we do not access
-	 * any fields of the node after a reference is put .
+	 * FIXME: Here we assume that uid_t and gid_t are same size. It's
+	 * probably a safe assumption but might be better to check based on
+	 * sidtype.
 	 */
-	if (test_bit(SID_ID_MAPPED, &psidid->state)) {
-		cid = psidid->id;
-		psidid->time = jiffies; /* update ts for accessing */
-		goto sid_to_id_out;
+	if (sidkey->datalen < sizeof(uid_t)) {
+		rc = -EIO;
+		cFYI(1, "%s: Downcall contained malformed key "
+			"(datalen=%hu)", __func__, sidkey->datalen);
+		goto out_key_put;
 	}
 
-	if (time_after(psidid->time + SID_MAP_RETRY, jiffies))
-		goto sid_to_id_out;
-
-	if (!test_and_set_bit(SID_ID_PENDING, &psidid->state)) {
-		saved_cred = override_creds(root_cred);
-		idkey = request_key(&cifs_idmap_key_type, psidid->sidstr, "");
-		if (IS_ERR(idkey))
-			cFYI(1, "%s: Can't map SID to an id", __func__);
-		else {
-			cid = *(unsigned long *)idkey->payload.value;
-			psidid->id = cid;
-			set_bit(SID_ID_MAPPED, &psidid->state);
-			key_put(idkey);
-			kfree(psidid->sidstr);
-		}
-		revert_creds(saved_cred);
-		psidid->time = jiffies; /* update ts for accessing */
-		clear_bit(SID_ID_PENDING, &psidid->state);
-		wake_up_bit(&psidid->state, SID_ID_PENDING);
-	} else {
-		rc = wait_on_bit(&psidid->state, SID_ID_PENDING,
-				sidid_pending_wait, TASK_INTERRUPTIBLE);
-		if (rc) {
-			cFYI(1, "%s: sidid_pending_wait interrupted %d",
-					__func__, rc);
-			--psidid->refcount; /* decremented without spinlock */
-			return rc;
-		}
-		if (test_bit(SID_ID_MAPPED, &psidid->state))
-			cid = psidid->id;
-	}
-
-sid_to_id_out:
-	--psidid->refcount; /* decremented without spinlock */
 	if (sidtype == SIDOWNER)
-		fattr->cf_uid = cid;
+		fuid = *(uid_t *)sidkey->payload.value;
 	else
-		fattr->cf_gid = cid;
+		fgid = *(gid_t *)sidkey->payload.value;
 
+out_key_put:
+	key_put(sidkey);
+out_revert_creds:
+	revert_creds(saved_cred);
+	kfree(sidstr);
+
+	/*
+	 * Note that we return 0 here unconditionally. If the mapping
+	 * fails then we just fall back to using the mnt_uid/mnt_gid.
+	 */
+	if (sidtype == SIDOWNER)
+		fattr->cf_uid = fuid;
+	else
+		fattr->cf_gid = fgid;
 	return 0;
 }
 
@@ -621,17 +317,6 @@
 	cred->jit_keyring = KEY_REQKEY_DEFL_THREAD_KEYRING;
 	root_cred = cred;
 
-	spin_lock_init(&siduidlock);
-	uidtree = RB_ROOT;
-	spin_lock_init(&sidgidlock);
-	gidtree = RB_ROOT;
-
-	spin_lock_init(&uidsidlock);
-	siduidtree = RB_ROOT;
-	spin_lock_init(&gidsidlock);
-	sidgidtree = RB_ROOT;
-	register_shrinker(&cifs_shrinker);
-
 	cFYI(1, "cifs idmap keyring: %d", key_serial(keyring));
 	return 0;
 
@@ -648,41 +333,9 @@
 	key_revoke(root_cred->thread_keyring);
 	unregister_key_type(&cifs_idmap_key_type);
 	put_cred(root_cred);
-	unregister_shrinker(&cifs_shrinker);
 	cFYI(1, "Unregistered %s key type", cifs_idmap_key_type.name);
 }
 
-void
-cifs_destroy_idmaptrees(void)
-{
-	struct rb_root *root;
-	struct rb_node *node;
-
-	root = &uidtree;
-	spin_lock(&siduidlock);
-	while ((node = rb_first(root)))
-		rb_erase(node, root);
-	spin_unlock(&siduidlock);
-
-	root = &gidtree;
-	spin_lock(&sidgidlock);
-	while ((node = rb_first(root)))
-		rb_erase(node, root);
-	spin_unlock(&sidgidlock);
-
-	root = &siduidtree;
-	spin_lock(&uidsidlock);
-	while ((node = rb_first(root)))
-		rb_erase(node, root);
-	spin_unlock(&uidsidlock);
-
-	root = &sidgidtree;
-	spin_lock(&gidsidlock);
-	while ((node = rb_first(root)))
-		rb_erase(node, root);
-	spin_unlock(&gidsidlock);
-}
-
 /* copy ntsd, owner sid, and group sid from a security descriptor to another */
 static void copy_sec_desc(const struct cifs_ntsd *pntsd,
 				struct cifs_ntsd *pnntsd, __u32 sidsoffset)