[PATCH -next 0/4] sysvipc: ipc-key management improvements

[PATCH -next 0/4] sysvipc: ipc-key management improvements

[Davidlohr Bueso]

Hi,

Here are a few improvements I spotted while eyeballing Guillaume's
rhashtable implementation for ipc keys. The first and fourth patches
are the interesting ones, the middle two are trivial.

Applies against today's -next.
Passes ltp ipc related testcases.

Thanks!

Davidlohr Bueso (4):
  sysvipc: unteach ids->next_id for !CHECKPOINT_RESTORE
  sysvipc: duplicate lock comments wrt ipc_addid()
  sysvipc: properly name ipc_addid() limit parameter
  sysvipc: make get_maxid O(1) again

 arch/x86/Kconfig              |  2 +-
 include/linux/ipc_namespace.h |  3 ++
 ipc/sem.c                     |  1 +
 ipc/shm.c                     |  1 +
 ipc/util.c                    | 97 ++++++++++++++++++++++++-------------------
 ipc/util.h                    | 26 ++++++++----
 6 files changed, 79 insertions(+), 51 deletions(-)

-- 
2.12.0

[PATCH 1/4] sysvipc: unteach ids->next_id for !CHECKPOINT_RESTORE

[Davidlohr Bueso]

The next_id object-allocation functionality was introduced in
03f595668017f (ipc: add sysctl to specify desired next object id).
Given that these new entries are _only_ exported under the
CONFIG_CHECKPOINT_RESTORE option, there is no point for the
common case to even know about ->next_id. As such rewrite
ipc_buildid() such that it can do away with the field as well as
unnecessary branches when adding a new identifier. The end result
also better differentiates both cases, so the code ends up being
cleaner; albeit the small duplications regarding the default case.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
---
 include/linux/ipc_namespace.h |  2 ++
 ipc/util.c                    | 60 ++++++++++++++++++++++++++++++++-----------
 ipc/util.h                    |  5 ----
 3 files changed, 47 insertions(+), 20 deletions(-)

diff --git a/include/linux/ipc_namespace.h b/include/linux/ipc_namespace.h
index 83f0bf7a587d..058051566ced 100644
--- a/include/linux/ipc_namespace.h
+++ b/include/linux/ipc_namespace.h
@@ -18,7 +18,9 @@ struct ipc_ids {
 	bool tables_initialized;
 	struct rw_semaphore rwsem;
 	struct idr ipcs_idr;
+#ifdef CONFIG_CHECKPOINT_RESTORE
 	int next_id;
+#endif
 	struct rhashtable key_ht;
 };
 
diff --git a/ipc/util.c b/ipc/util.c
index 78755873cc5b..fb69152705e6 100644
--- a/ipc/util.c
+++ b/ipc/util.c
@@ -115,13 +115,15 @@ int ipc_init_ids(struct ipc_ids *ids)
 	int err;
 	ids->in_use = 0;
 	ids->seq = 0;
-	ids->next_id = -1;
 	init_rwsem(&ids->rwsem);
 	err = rhashtable_init(&ids->key_ht, &ipc_kht_params);
 	if (err)
 		return err;
 	idr_init(&ids->ipcs_idr);
 	ids->tables_initialized = true;
+#ifdef CONFIG_CHECKPOINT_RESTORE
+	ids->next_id = -1;
+#endif
 	return 0;
 }
 
@@ -215,6 +217,46 @@ int ipc_get_maxid(struct ipc_ids *ids)
 	return max_id;
 }
 
+#ifdef CONFIG_CHECKPOINT_RESTORE
+/*
+ * Specify desired id for next allocated IPC object.
+ */
+#define ipc_idr_alloc(ids, new)						\
+	idr_alloc(&(ids)->ipcs_idr, (new),				\
+		  (ids)->next_id < 0 ? 0: ipcid_to_idx((ids)->next_id), \
+		  0, GFP_NOWAIT);
+
+static inline int ipc_buildid(int id, struct ipc_ids *ids,
+			      struct kern_ipc_perm *new)
+{
+	if (ids->next_id < 0) { /* default, behave as !CHECKPOINT_RESTORE */
+		new->seq = ids->seq++;
+		if (ids->seq > IPCID_SEQ_MAX)
+			ids->seq = 0;
+	} else {
+		new->seq = ipcid_to_seqx(ids->next_id);
+		ids->next_id = -1;
+	}
+
+	return SEQ_MULTIPLIER * new->seq + id;
+}
+
+#else
+#define ipc_idr_alloc(ids, new)					\
+	idr_alloc(&(ids)->ipcs_idr, (new), 0, 0, GFP_NOWAIT);
+
+static inline int ipc_buildid(int id, struct ipc_ids *ids,
+			      struct kern_ipc_perm *new)
+{
+	new->seq = ids->seq++;
+	if (ids->seq > IPCID_SEQ_MAX)
+		ids->seq = 0;
+
+	return SEQ_MULTIPLIER * new->seq + id;
+}
+
+#endif /* CONFIG_CHECKPOINT_RESTORE */
+
 /**
  * ipc_addid - add an ipc identifier
  * @ids: ipc identifier set
@@ -233,7 +275,6 @@ int ipc_addid(struct ipc_ids *ids, struct kern_ipc_perm *new, int size)
 	kuid_t euid;
 	kgid_t egid;
 	int id, err;
-	int next_id = ids->next_id;
 
 	if (size > IPCMNI)
 		size = IPCMNI;
@@ -253,9 +294,7 @@ int ipc_addid(struct ipc_ids *ids, struct kern_ipc_perm *new, int size)
 	new->cuid = new->uid = euid;
 	new->gid = new->cgid = egid;
 
-	id = idr_alloc(&ids->ipcs_idr, new,
-		       (next_id < 0) ? 0 : ipcid_to_idx(next_id), 0,
-		       GFP_NOWAIT);
+	id = ipc_idr_alloc(ids, new);
 	idr_preload_end();
 
 	if (id >= 0 && new->key != IPC_PRIVATE) {
@@ -273,17 +312,8 @@ int ipc_addid(struct ipc_ids *ids, struct kern_ipc_perm *new, int size)
 	}
 
 	ids->in_use++;
+	new->id = ipc_buildid(id, ids, new);
 
-	if (next_id < 0) {
-		new->seq = ids->seq++;
-		if (ids->seq > IPCID_SEQ_MAX)
-			ids->seq = 0;
-	} else {
-		new->seq = ipcid_to_seqx(next_id);
-		ids->next_id = -1;
-	}
-
-	new->id = ipc_buildid(id, new->seq);
 	return id;
 }
 
diff --git a/ipc/util.h b/ipc/util.h
index 80c9f51c3f07..17c8d2f14990 100644
--- a/ipc/util.h
+++ b/ipc/util.h
@@ -145,11 +145,6 @@ extern struct msg_msg *load_msg(const void __user *src, size_t len);
 extern struct msg_msg *copy_msg(struct msg_msg *src, struct msg_msg *dst);
 extern int store_msg(void __user *dest, struct msg_msg *msg, size_t len);
 
-static inline int ipc_buildid(int id, int seq)
-{
-	return SEQ_MULTIPLIER * seq + id;
-}
-
 static inline int ipc_checkid(struct kern_ipc_perm *ipcp, int uid)
 {
 	return uid / SEQ_MULTIPLIER != ipcp->seq;
-- 
2.12.0

Re: [PATCH 1/4] sysvipc: unteach ids->next_id for !CHECKPOINT_RESTORE

[Andrew Morton]

On Thu, 31 Aug 2017 10:20:46 -0700 Davidlohr Bueso <dave@stgolabs.net> wrote:

> The next_id object-allocation functionality was introduced in
> 03f595668017f (ipc: add sysctl to specify desired next object id).
> Given that these new entries are _only_ exported under the
> CONFIG_CHECKPOINT_RESTORE option, there is no point for the
> common case to even know about ->next_id. As such rewrite
> ipc_buildid() such that it can do away with the field as well as
> unnecessary branches when adding a new identifier. The end result
> also better differentiates both cases, so the code ends up being
> cleaner; albeit the small duplications regarding the default case.
> 
> ...
>
> +#ifdef CONFIG_CHECKPOINT_RESTORE
> +/*
> + * Specify desired id for next allocated IPC object.
> + */
> +#define ipc_idr_alloc(ids, new)						\
> +	idr_alloc(&(ids)->ipcs_idr, (new),				\
> +		  (ids)->next_id < 0 ? 0: ipcid_to_idx((ids)->next_id), \
> +		  0, GFP_NOWAIT);
> +
> 
> ...
>
> +#else
> +#define ipc_idr_alloc(ids, new)					\
> +	idr_alloc(&(ids)->ipcs_idr, (new), 0, 0, GFP_NOWAIT);
> +
> ...

Not a fan of checkpatch, I see...

--- a/ipc/util.c~sysvipc-unteach-ids-next_id-for-checkpoint_restore-checkpatch-fixes
+++ a/ipc/util.c
@@ -223,8 +223,8 @@ int ipc_get_maxid(struct ipc_ids *ids)
  */
 #define ipc_idr_alloc(ids, new)						\
 	idr_alloc(&(ids)->ipcs_idr, (new),				\
-		  (ids)->next_id < 0 ? 0: ipcid_to_idx((ids)->next_id), \
-		  0, GFP_NOWAIT);
+		  (ids)->next_id < 0 ? 0 : ipcid_to_idx((ids)->next_id),\
+		  0, GFP_NOWAIT)
 
 static inline int ipc_buildid(int id, struct ipc_ids *ids,
 			      struct kern_ipc_perm *new)
@@ -243,7 +243,7 @@ static inline int ipc_buildid(int id, st
 
 #else
 #define ipc_idr_alloc(ids, new)					\
-	idr_alloc(&(ids)->ipcs_idr, (new), 0, 0, GFP_NOWAIT);
+	idr_alloc(&(ids)->ipcs_idr, (new), 0, 0, GFP_NOWAIT)
 
 static inline int ipc_buildid(int id, struct ipc_ids *ids,
 			      struct kern_ipc_perm *new)

Did these "functions" *have* to be implemented in cpp?  Wouldn't they
be better in C?

[PATCH 2/4] sysvipc: duplicate lock comments wrt ipc_addid()

[Davidlohr Bueso]

The comment in msgqueues when using ipc_addid() is quite
useful imo. Duplicate it for shm and semaphores.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
---
 ipc/sem.c | 1 +
 ipc/shm.c | 1 +
 2 files changed, 2 insertions(+)

diff --git a/ipc/sem.c b/ipc/sem.c
index 013c7981f3c7..29c6ab6badc2 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -514,6 +514,7 @@ static int newary(struct ipc_namespace *ns, struct ipc_params *params)
 	sma->sem_nsems = nsems;
 	sma->sem_ctime = get_seconds();
 
+	/* ipc_addid() locks sma upon success. */
 	retval = ipc_addid(&sem_ids(ns), &sma->sem_perm, ns->sc_semmni);
 	if (retval < 0) {
 		call_rcu(&sma->sem_perm.rcu, sem_rcu_free);
diff --git a/ipc/shm.c b/ipc/shm.c
index 8fc97beb5234..ea64ed8782b3 100644
--- a/ipc/shm.c
+++ b/ipc/shm.c
@@ -600,6 +600,7 @@ static int newseg(struct ipc_namespace *ns, struct ipc_params *params)
 	shp->shm_file = file;
 	shp->shm_creator = current;
 
+	/* ipc_addid() locks shp upon success. */
 	error = ipc_addid(&shm_ids(ns), &shp->shm_perm, ns->shm_ctlmni);
 	if (error < 0)
 		goto no_id;
-- 
2.12.0

[PATCH 3/4] sysvipc: properly name ipc_addid() limit parameter

[Davidlohr Bueso]

This is better understood as a limit, instead of size;
exactly like the function comment indicates. Rename it.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
---
 ipc/util.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/ipc/util.c b/ipc/util.c
index fb69152705e6..d4091665f439 100644
--- a/ipc/util.c
+++ b/ipc/util.c
@@ -261,7 +261,7 @@ static inline int ipc_buildid(int id, struct ipc_ids *ids,
  * ipc_addid - add an ipc identifier
  * @ids: ipc identifier set
  * @new: new ipc permission set
- * @size: limit for the number of used ids
+ * @limit: limit for the number of used ids
  *
  * Add an entry 'new' to the ipc ids idr. The permissions object is
  * initialised and the first free entry is set up and the id assigned
@@ -270,16 +270,16 @@ static inline int ipc_buildid(int id, struct ipc_ids *ids,
  *
  * Called with writer ipc_ids.rwsem held.
  */
-int ipc_addid(struct ipc_ids *ids, struct kern_ipc_perm *new, int size)
+int ipc_addid(struct ipc_ids *ids, struct kern_ipc_perm *new, int limit)
 {
 	kuid_t euid;
 	kgid_t egid;
 	int id, err;
 
-	if (size > IPCMNI)
-		size = IPCMNI;
+	if (limit > IPCMNI)
+		limit = IPCMNI;
 
-	if (!ids->tables_initialized || ids->in_use >= size)
+	if (!ids->tables_initialized || ids->in_use >= limit)
 		return -ENOSPC;
 
 	idr_preload(GFP_KERNEL);
-- 
2.12.0

[PATCH 4/4] sysvipc: make get_maxid O(1) again

[Davidlohr Bueso]

For a custom microbenchmark on a 3.30GHz Xeon SandyBridge,
which calls IPC_STAT over and over, it was calculated that,
on avg the cost of ipc_get_maxid() for increasing amounts of
keys was:

10 keys: ~900 cycles
100 keys: ~15000 cycles
1000 keys: ~150000 cycles
10000 keys: ~2100000 cycles

This is unsurprising as maxid is currently O(n).

By having the max_id available in O(1) we save all those cycles
for each semctl(_STAT) command, the idr_find can be expensive --
which some real (customer) workloads actually poll on. Note that
this used to be the case, until 7ca7e564e04 (ipc: store ipcs into
IDRs). The cost is the extra idr_find when doing RMIDs, but we
simply go backwards, and should not take too many iterations to
find the new value.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
---
 arch/x86/Kconfig              |  2 +-
 include/linux/ipc_namespace.h |  1 +
 ipc/util.c                    | 43 +++++++++++++------------------------------
 ipc/util.h                    | 21 ++++++++++++++++++---
 4 files changed, 33 insertions(+), 34 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 44d139b88c32..301fe235367b 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -55,7 +55,7 @@ config X86
 	select ARCH_HAS_KCOV			if X86_64
 	select ARCH_HAS_MMIO_FLUSH
 	select ARCH_HAS_PMEM_API		if X86_64
-	select ARCH_HAS_REFCOUNT
+	select ARCH_HAS_REFCOUNT		if BROKEN
 	select ARCH_HAS_UACCESS_FLUSHCACHE	if X86_64
 	select ARCH_HAS_SET_MEMORY
 	select ARCH_HAS_SG_CHAIN
diff --git a/include/linux/ipc_namespace.h b/include/linux/ipc_namespace.h
index 058051566ced..c59b8ddee191 100644
--- a/include/linux/ipc_namespace.h
+++ b/include/linux/ipc_namespace.h
@@ -18,6 +18,7 @@ struct ipc_ids {
 	bool tables_initialized;
 	struct rw_semaphore rwsem;
 	struct idr ipcs_idr;
+	int max_id;
 #ifdef CONFIG_CHECKPOINT_RESTORE
 	int next_id;
 #endif
diff --git a/ipc/util.c b/ipc/util.c
index d4091665f439..45e84203c3fe 100644
--- a/ipc/util.c
+++ b/ipc/util.c
@@ -121,6 +121,7 @@ int ipc_init_ids(struct ipc_ids *ids)
 		return err;
 	idr_init(&ids->ipcs_idr);
 	ids->tables_initialized = true;
+	ids->max_id = -1;
 #ifdef CONFIG_CHECKPOINT_RESTORE
 	ids->next_id = -1;
 #endif
@@ -187,36 +188,6 @@ static struct kern_ipc_perm *ipc_findkey(struct ipc_ids *ids, key_t key)
 	return NULL;
 }
 
-/**
- * ipc_get_maxid - get the last assigned id
- * @ids: ipc identifier set
- *
- * Called with ipc_ids.rwsem held.
- */
-int ipc_get_maxid(struct ipc_ids *ids)
-{
-	struct kern_ipc_perm *ipc;
-	int max_id = -1;
-	int total, id;
-
-	if (ids->in_use == 0)
-		return -1;
-
-	if (ids->in_use == IPCMNI)
-		return IPCMNI - 1;
-
-	/* Look for the last assigned id */
-	total = 0;
-	for (id = 0; id < IPCMNI && total < ids->in_use; id++) {
-		ipc = idr_find(&ids->ipcs_idr, id);
-		if (ipc != NULL) {
-			max_id = id;
-			total++;
-		}
-	}
-	return max_id;
-}
-
 #ifdef CONFIG_CHECKPOINT_RESTORE
 /*
  * Specify desired id for next allocated IPC object.
@@ -312,6 +283,9 @@ int ipc_addid(struct ipc_ids *ids, struct kern_ipc_perm *new, int limit)
 	}
 
 	ids->in_use++;
+	if (id > ids->max_id)
+		ids->max_id = id;
+
 	new->id = ipc_buildid(id, ids, new);
 
 	return id;
@@ -458,6 +432,15 @@ void ipc_rmid(struct ipc_ids *ids, struct kern_ipc_perm *ipcp)
 	ipc_kht_remove(ids, ipcp);
 	ids->in_use--;
 	ipcp->deleted = true;
+
+	if (unlikely(lid == ids->max_id)) {
+		do {
+			lid--;
+			if(lid == -1)
+				break;
+		} while (!idr_find(&ids->ipcs_idr, lid));
+		ids->max_id = lid;
+	}
 }
 
 /**
diff --git a/ipc/util.h b/ipc/util.h
index 17c8d2f14990..eabca0e37ce1 100644
--- a/ipc/util.h
+++ b/ipc/util.h
@@ -12,6 +12,7 @@
 
 #include <linux/unistd.h>
 #include <linux/err.h>
+#include <linux/ipc_namespace.h>
 
 #define SEQ_MULTIPLIER	(IPCMNI)
 
@@ -98,9 +99,6 @@ void __init ipc_init_proc_interface(const char *path, const char *header,
 /* must be called with ids->rwsem acquired for writing */
 int ipc_addid(struct ipc_ids *, struct kern_ipc_perm *, int);
 
-/* must be called with ids->rwsem acquired for reading */
-int ipc_get_maxid(struct ipc_ids *);
-
 /* must be called with both locks acquired. */
 void ipc_rmid(struct ipc_ids *, struct kern_ipc_perm *);
 
@@ -110,6 +108,23 @@ void ipc_set_key_private(struct ipc_ids *, struct kern_ipc_perm *);
 /* must be called with ipcp locked */
 int ipcperms(struct ipc_namespace *ns, struct kern_ipc_perm *ipcp, short flg);
 
+/**
+ * ipc_get_maxid - get the last assigned id
+ * @ids: ipc identifier set
+ *
+ * Called with ipc_ids.rwsem held for reading.
+ */
+static inline int ipc_get_maxid(struct ipc_ids *ids)
+{
+	if (ids->in_use == 0)
+		return -1;
+
+	if (ids->in_use == IPCMNI)
+		return IPCMNI - 1;
+
+	return ids->max_id;
+}
+
 /*
  * For allocation that need to be freed by RCU.
  * Objects are reference counted, they start with reference count 1.
-- 
2.12.0

Re: [PATCH 4/4] sysvipc: make get_maxid O(1) again

[Davidlohr Bueso]

On Thu, 31 Aug 2017, Davidlohr Bueso wrote:
>diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
>index 44d139b88c32..301fe235367b 100644
>--- a/arch/x86/Kconfig
>+++ b/arch/x86/Kconfig
>@@ -55,7 +55,7 @@ config X86
> 	select ARCH_HAS_KCOV			if X86_64
> 	select ARCH_HAS_MMIO_FLUSH
> 	select ARCH_HAS_PMEM_API		if X86_64
>-	select ARCH_HAS_REFCOUNT
>+	select ARCH_HAS_REFCOUNT		if BROKEN

*sigh* sorry, I forgot this was there, otherwise nothing works :/

Here's v2. Thanks.

-----8<------------------------------------------------------
>From 4f31964d4ae75d752c913a03e9c751b13381b561 Mon Sep 17 00:00:00 2001
From: Davidlohr Bueso <dave@stgolabs.net>
Date: Thu, 31 Aug 2017 10:34:35 -0700
Subject: [PATCH v2 4/4] sysvipc: make get_maxid O(1) again

For a custom microbenchmark on a 3.30GHz Xeon SandyBridge,
which calls IPC_STAT over and over, it was calculated that,
on avg the cost of ipc_get_maxid() for increasing amounts of
keys was:

10 keys: ~900 cycles
100 keys: ~15000 cycles
1000 keys: ~150000 cycles
10000 keys: ~2100000 cycles

This is unsurprising as maxid is currently O(n).

By having the max_id available in O(1) we save all those cycles
for each semctl(_STAT) command, the idr_find can be expensive --
which some real (customer) workloads actually poll on. Note that
this used to be the case, until 7ca7e564e04 (ipc: store ipcs into
IDRs). The cost is the extra idr_find when doing RMIDs, but we
simply go backwards, and should not take too many iterations to
find the new value.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
---
 include/linux/ipc_namespace.h |  1 +
 ipc/util.c                    | 43 +++++++++++++------------------------------
 ipc/util.h                    | 21 ++++++++++++++++++---
 3 files changed, 32 insertions(+), 33 deletions(-)

diff --git a/include/linux/ipc_namespace.h b/include/linux/ipc_namespace.h
index 058051566ced..c59b8ddee191 100644
--- a/include/linux/ipc_namespace.h
+++ b/include/linux/ipc_namespace.h
@@ -18,6 +18,7 @@ struct ipc_ids {
 	bool tables_initialized;
 	struct rw_semaphore rwsem;
 	struct idr ipcs_idr;
+	int max_id;
 #ifdef CONFIG_CHECKPOINT_RESTORE
 	int next_id;
 #endif
diff --git a/ipc/util.c b/ipc/util.c
index d4091665f439..45e84203c3fe 100644
--- a/ipc/util.c
+++ b/ipc/util.c
@@ -121,6 +121,7 @@ int ipc_init_ids(struct ipc_ids *ids)
 		return err;
 	idr_init(&ids->ipcs_idr);
 	ids->tables_initialized = true;
+	ids->max_id = -1;
 #ifdef CONFIG_CHECKPOINT_RESTORE
 	ids->next_id = -1;
 #endif
@@ -187,36 +188,6 @@ static struct kern_ipc_perm *ipc_findkey(struct ipc_ids *ids, key_t key)
 	return NULL;
 }
 
-/**
- * ipc_get_maxid - get the last assigned id
- * @ids: ipc identifier set
- *
- * Called with ipc_ids.rwsem held.
- */
-int ipc_get_maxid(struct ipc_ids *ids)
-{
-	struct kern_ipc_perm *ipc;
-	int max_id = -1;
-	int total, id;
-
-	if (ids->in_use == 0)
-		return -1;
-
-	if (ids->in_use == IPCMNI)
-		return IPCMNI - 1;
-
-	/* Look for the last assigned id */
-	total = 0;
-	for (id = 0; id < IPCMNI && total < ids->in_use; id++) {
-		ipc = idr_find(&ids->ipcs_idr, id);
-		if (ipc != NULL) {
-			max_id = id;
-			total++;
-		}
-	}
-	return max_id;
-}
-
 #ifdef CONFIG_CHECKPOINT_RESTORE
 /*
  * Specify desired id for next allocated IPC object.
@@ -312,6 +283,9 @@ int ipc_addid(struct ipc_ids *ids, struct kern_ipc_perm *new, int limit)
 	}
 
 	ids->in_use++;
+	if (id > ids->max_id)
+		ids->max_id = id;
+
 	new->id = ipc_buildid(id, ids, new);
 
 	return id;
@@ -458,6 +432,15 @@ void ipc_rmid(struct ipc_ids *ids, struct kern_ipc_perm *ipcp)
 	ipc_kht_remove(ids, ipcp);
 	ids->in_use--;
 	ipcp->deleted = true;
+
+	if (unlikely(lid == ids->max_id)) {
+		do {
+			lid--;
+			if(lid == -1)
+				break;
+		} while (!idr_find(&ids->ipcs_idr, lid));
+		ids->max_id = lid;
+	}
 }
 
 /**
diff --git a/ipc/util.h b/ipc/util.h
index 17c8d2f14990..eabca0e37ce1 100644
--- a/ipc/util.h
+++ b/ipc/util.h
@@ -12,6 +12,7 @@
 
 #include <linux/unistd.h>
 #include <linux/err.h>
+#include <linux/ipc_namespace.h>
 
 #define SEQ_MULTIPLIER	(IPCMNI)
 
@@ -98,9 +99,6 @@ void __init ipc_init_proc_interface(const char *path, const char *header,
 /* must be called with ids->rwsem acquired for writing */
 int ipc_addid(struct ipc_ids *, struct kern_ipc_perm *, int);
 
-/* must be called with ids->rwsem acquired for reading */
-int ipc_get_maxid(struct ipc_ids *);
-
 /* must be called with both locks acquired. */
 void ipc_rmid(struct ipc_ids *, struct kern_ipc_perm *);
 
@@ -110,6 +108,23 @@ void ipc_set_key_private(struct ipc_ids *, struct kern_ipc_perm *);
 /* must be called with ipcp locked */
 int ipcperms(struct ipc_namespace *ns, struct kern_ipc_perm *ipcp, short flg);
 
+/**
+ * ipc_get_maxid - get the last assigned id
+ * @ids: ipc identifier set
+ *
+ * Called with ipc_ids.rwsem held for reading.
+ */
+static inline int ipc_get_maxid(struct ipc_ids *ids)
+{
+	if (ids->in_use == 0)
+		return -1;
+
+	if (ids->in_use == IPCMNI)
+		return IPCMNI - 1;
+
+	return ids->max_id;
+}
+
 /*
  * For allocation that need to be freed by RCU.
  * Objects are reference counted, they start with reference count 1.
-- 
2.12.0