[RESEND PATCH 1/3] ipc: Allow boot time extension of IPCMNI from 32k to 16M

[RESEND PATCH 1/3] ipc: Allow boot time extension of IPCMNI from 32k to 16M

[Waiman Long]

The maximum number of unique System V IPC identifiers was limited to
32k.  That limit should be big enough for most use cases.

However, there are some users out there requesting for more, especially
those that are migrating from Solaris which uses 24 bits for unique
identifiers. To satisfy the need of those users, a new boot time kernel
option "ipcmni_extend" is added to extend the IPCMNI value to 16M. This
is a 512X increase which should be big enough for users out there that
need a large number of unique IPC identifier.

The use of this new option will change the pattern of the IPC identifiers
returned by functions like shmget(2). An application that depends on
such pattern may not work properly.  So it should only be used if the
users really need more than 32k of unique IPC numbers.

This new option does have the side effect of reducing the maximum number
of unique sequence numbers from 64k down to 128. So it is a trade-off.

The computation of a new IPC id is not done in the performance critical
path.  So a little bit of additional overhead shouldn't have any real
performance impact.

Signed-off-by: Waiman Long <longman@redhat.com>
Acked-by: Manfred Spraul <manfred@colorfullife.com>
---
 .../admin-guide/kernel-parameters.txt         |  3 ++
 ipc/ipc_sysctl.c                              | 12 ++++-
 ipc/util.c                                    | 10 ++---
 ipc/util.h                                    | 44 +++++++++++++++----
 4 files changed, 54 insertions(+), 15 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 2b8ee90bb644..8d303c845c21 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -1828,6 +1828,9 @@
 	ip=		[IP_PNP]
 			See Documentation/filesystems/nfs/nfsroot.txt.
 
+	ipcmni_extend	[KNL] Extend the maximum number of unique System V
+			IPC identifiers from 32,768 to 16,777,216.
+
 	irqaffinity=	[SMP] Set the default irq affinity mask
 			The argument is a cpu list, as described above.
 
diff --git a/ipc/ipc_sysctl.c b/ipc/ipc_sysctl.c
index 49f9bf4ffc7f..73b7782eccf4 100644
--- a/ipc/ipc_sysctl.c
+++ b/ipc/ipc_sysctl.c
@@ -120,7 +120,8 @@ static int proc_ipc_sem_dointvec(struct ctl_table *table, int write,
 static int zero;
 static int one = 1;
 static int int_max = INT_MAX;
-static int ipc_mni = IPCMNI;
+int ipc_mni = IPCMNI;
+int ipc_mni_shift = IPCMNI_SHIFT;
 
 static struct ctl_table ipc_kern_table[] = {
 	{
@@ -246,3 +247,12 @@ static int __init ipc_sysctl_init(void)
 }
 
 device_initcall(ipc_sysctl_init);
+
+static int __init ipc_mni_extend(char *str)
+{
+	ipc_mni = IPCMNI_EXTEND;
+	ipc_mni_shift = IPCMNI_EXTEND_SHIFT;
+	pr_info("IPCMNI extended to %d.\n", ipc_mni);
+	return 0;
+}
+early_param("ipcmni_extend", ipc_mni_extend);
diff --git a/ipc/util.c b/ipc/util.c
index 0af05752969f..07ae117ccdc0 100644
--- a/ipc/util.c
+++ b/ipc/util.c
@@ -110,7 +110,7 @@ static const struct rhashtable_params ipc_kht_params = {
  * @ids: ipc identifier set
  *
  * Set up the sequence range to use for the ipc identifier range (limited
- * below IPCMNI) then initialise the keys hashtable and ids idr.
+ * below ipc_mni) then initialise the keys hashtable and ids idr.
  */
 void ipc_init_ids(struct ipc_ids *ids)
 {
@@ -226,7 +226,7 @@ static inline int ipc_idr_alloc(struct ipc_ids *ids, struct kern_ipc_perm *new)
 				0, GFP_NOWAIT);
 	}
 	if (idx >= 0)
-		new->id = SEQ_MULTIPLIER * new->seq + idx;
+		new->id = (new->seq << IPCMNI_SEQ_SHIFT) + idx;
 	return idx;
 }
 
@@ -254,8 +254,8 @@ int ipc_addid(struct ipc_ids *ids, struct kern_ipc_perm *new, int limit)
 	/* 1) Initialize the refcount so that ipc_rcu_putref works */
 	refcount_set(&new->refcount, 1);
 
-	if (limit > IPCMNI)
-		limit = IPCMNI;
+	if (limit > ipc_mni)
+		limit = ipc_mni;
 
 	if (ids->in_use >= limit)
 		return -ENOSPC;
@@ -738,7 +738,7 @@ static struct kern_ipc_perm *sysvipc_find_ipc(struct ipc_ids *ids, loff_t pos,
 	if (total >= ids->in_use)
 		return NULL;
 
-	for (; pos < IPCMNI; pos++) {
+	for (; pos < ipc_mni; pos++) {
 		ipc = idr_find(&ids->ipcs_idr, pos);
 		if (ipc != NULL) {
 			*new_pos = pos + 1;
diff --git a/ipc/util.h b/ipc/util.h
index e272be622ae7..9746886757de 100644
--- a/ipc/util.h
+++ b/ipc/util.h
@@ -15,8 +15,34 @@
 #include <linux/err.h>
 #include <linux/ipc_namespace.h>
 
-#define IPCMNI 32768  /* <= MAX_INT limit for ipc arrays (including sysctl changes) */
-#define SEQ_MULTIPLIER	(IPCMNI)
+/*
+ * The IPC ID contains 2 separate numbers - index and sequence number.
+ * By default,
+ *   bits  0-14: index (32k, 15 bits)
+ *   bits 15-30: sequence number (64k, 16 bits)
+ *
+ * When IPCMNI extension mode is turned on, the composition changes:
+ *   bits  0-23: index (16M, 24 bits)
+ *   bits 24-30: sequence number (128, 7 bits)
+ */
+#define IPCMNI_SHIFT		15
+#define IPCMNI_EXTEND_SHIFT	24
+#define IPCMNI			(1 << IPCMNI_SHIFT)
+#define IPCMNI_EXTEND		(1 << IPCMNI_EXTEND_SHIFT)
+
+#ifdef CONFIG_SYSVIPC_SYSCTL
+extern int ipc_mni;
+extern int ipc_mni_shift;
+
+#define IPCMNI_SEQ_SHIFT	ipc_mni_shift
+#define IPCMNI_IDX_MASK		((1 << ipc_mni_shift) - 1)
+
+#else /* CONFIG_SYSVIPC_SYSCTL */
+
+#define ipc_mni			IPCMNI
+#define IPCMNI_SEQ_SHIFT	IPCMNI_SHIFT
+#define IPCMNI_IDX_MASK		((1 << IPCMNI_SHIFT) - 1)
+#endif /* CONFIG_SYSVIPC_SYSCTL */
 
 void sem_init(void);
 void msg_init(void);
@@ -96,9 +122,9 @@ struct pid_namespace *ipc_seq_pid_ns(struct seq_file *);
 #define IPC_MSG_IDS	1
 #define IPC_SHM_IDS	2
 
-#define ipcid_to_idx(id) ((id) % SEQ_MULTIPLIER)
-#define ipcid_to_seqx(id) ((id) / SEQ_MULTIPLIER)
-#define IPCID_SEQ_MAX min_t(int, INT_MAX/SEQ_MULTIPLIER, USHRT_MAX)
+#define ipcid_to_idx(id)  ((id) & IPCMNI_IDX_MASK)
+#define ipcid_to_seqx(id) ((id) >> IPCMNI_SEQ_SHIFT)
+#define IPCID_SEQ_MAX	  (INT_MAX >> IPCMNI_SEQ_SHIFT)
 
 /* must be called with ids->rwsem acquired for writing */
 int ipc_addid(struct ipc_ids *, struct kern_ipc_perm *, int);
@@ -123,8 +149,8 @@ static inline int ipc_get_maxidx(struct ipc_ids *ids)
 	if (ids->in_use == 0)
 		return -1;
 
-	if (ids->in_use == IPCMNI)
-		return IPCMNI - 1;
+	if (ids->in_use == ipc_mni)
+		return ipc_mni - 1;
 
 	return ids->max_idx;
 }
@@ -216,10 +242,10 @@ void free_ipcs(struct ipc_namespace *ns, struct ipc_ids *ids,
 
 static inline int sem_check_semmni(struct ipc_namespace *ns) {
 	/*
-	 * Check semmni range [0, IPCMNI]
+	 * Check semmni range [0, ipc_mni]
 	 * semmni is the last element of sem_ctls[4] array
 	 */
-	return ((ns->sem_ctls[3] < 0) || (ns->sem_ctls[3] > IPCMNI))
+	return ((ns->sem_ctls[3] < 0) || (ns->sem_ctls[3] > ipc_mni))
 		? -ERANGE : 0;
 }
 
-- 
2.18.1

[RESEND PATCH 2/3] ipc: Conserve sequence numbers in ipcmni_extend mode

[Waiman Long]

From: Manfred Spraul <manfred@colorfullife.com>

Rewrite, based on the patch from Waiman Long:

The mixing in of a sequence number into the IPC IDs is probably to
avoid ID reuse in userspace as much as possible. With ipcmni_extend
mode, the number of usable sequence numbers is greatly reduced leading
to higher chance of ID reuse.

To address this issue, we need to conserve the sequence number space
as much as possible. Right now, the sequence number is incremented for
every new ID created. In reality, we only need to increment the sequence
number when new allocated ID is not greater than the last one allocated.
It is in such case that the new ID may collide with an existing one.
This is being done irrespective of the ipcmni mode.

In order to avoid any races, the index is first allocated and
then the pointer is replaced.

Changes compared to the initial patch:
- Handle failures from idr_alloc().
- Avoid that concurrent operations can see the wrong
  sequence number.
(This is achieved by using idr_replace()).
- IPCMNI_SEQ_SHIFT is not a constant, thus renamed to
	ipcmni_seq_shift().
- IPCMNI_SEQ_MAX is not a constant, thus renamed to
	ipcmni_seq_max().

Suggested-by: Matthew Wilcox <willy@infradead.org>
Original-patch-from: Waiman Long <longman@redhat.com>
Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Acked-by: Waiman Long <longman@redhat.com>
---
 include/linux/ipc_namespace.h |  1 +
 ipc/util.c                    | 35 ++++++++++++++++++++++++++++++-----
 ipc/util.h                    |  8 ++++----
 3 files changed, 35 insertions(+), 9 deletions(-)

diff --git a/include/linux/ipc_namespace.h b/include/linux/ipc_namespace.h
index 6ab8c1bada3f..c309f43bde45 100644
--- a/include/linux/ipc_namespace.h
+++ b/include/linux/ipc_namespace.h
@@ -19,6 +19,7 @@ struct ipc_ids {
 	struct rw_semaphore rwsem;
 	struct idr ipcs_idr;
 	int max_idx;
+	int last_idx;	/* For wrap around detection */
 #ifdef CONFIG_CHECKPOINT_RESTORE
 	int next_id;
 #endif
diff --git a/ipc/util.c b/ipc/util.c
index 07ae117ccdc0..6e0fe3410423 100644
--- a/ipc/util.c
+++ b/ipc/util.c
@@ -120,6 +120,7 @@ void ipc_init_ids(struct ipc_ids *ids)
 	rhashtable_init(&ids->key_ht, &ipc_kht_params);
 	idr_init(&ids->ipcs_idr);
 	ids->max_idx = -1;
+	ids->last_idx = -1;
 #ifdef CONFIG_CHECKPOINT_RESTORE
 	ids->next_id = -1;
 #endif
@@ -193,6 +194,10 @@ static struct kern_ipc_perm *ipc_findkey(struct ipc_ids *ids, key_t key)
  *
  * The caller must own kern_ipc_perm.lock.of the new object.
  * On error, the function returns a (negative) error code.
+ *
+ * To conserve sequence number space, especially with extended ipc_mni,
+ * the sequence number is incremented only when the returned ID is less than
+ * the last one.
  */
 static inline int ipc_idr_alloc(struct ipc_ids *ids, struct kern_ipc_perm *new)
 {
@@ -216,17 +221,37 @@ static inline int ipc_idr_alloc(struct ipc_ids *ids, struct kern_ipc_perm *new)
 	 */
 
 	if (next_id < 0) { /* !CHECKPOINT_RESTORE or next_id is unset */
-		new->seq = ids->seq++;
-		if (ids->seq > IPCID_SEQ_MAX)
-			ids->seq = 0;
-		idx = idr_alloc(&ids->ipcs_idr, new, 0, 0, GFP_NOWAIT);
+
+		/* allocate the idx, with a NULL struct kern_ipc_perm */
+		idx = idr_alloc(&ids->ipcs_idr, NULL, 0, 0, GFP_NOWAIT);
+
+		if (idx >= 0) {
+			/*
+			 * idx got allocated successfully.
+			 * Now calculate the sequence number and set the
+			 * pointer for real.
+			 */
+			if (idx <= ids->last_idx) {
+				ids->seq++;
+				if (ids->seq >= ipcid_seq_max())
+					ids->seq = 0;
+			}
+			ids->last_idx = idx;
+
+			new->seq = ids->seq;
+			/* no need for smp_wmb(), this is done
+			 * inside idr_replace, as part of
+			 * rcu_assign_pointer
+			 */
+			idr_replace(&ids->ipcs_idr, new, idx);
+		}
 	} else {
 		new->seq = ipcid_to_seqx(next_id);
 		idx = idr_alloc(&ids->ipcs_idr, new, ipcid_to_idx(next_id),
 				0, GFP_NOWAIT);
 	}
 	if (idx >= 0)
-		new->id = (new->seq << IPCMNI_SEQ_SHIFT) + idx;
+		new->id = (new->seq << ipcmni_seq_shift()) + idx;
 	return idx;
 }
 
diff --git a/ipc/util.h b/ipc/util.h
index 9746886757de..8c834ed39012 100644
--- a/ipc/util.h
+++ b/ipc/util.h
@@ -34,13 +34,13 @@
 extern int ipc_mni;
 extern int ipc_mni_shift;
 
-#define IPCMNI_SEQ_SHIFT	ipc_mni_shift
+#define ipcmni_seq_shift()	ipc_mni_shift
 #define IPCMNI_IDX_MASK		((1 << ipc_mni_shift) - 1)
 
 #else /* CONFIG_SYSVIPC_SYSCTL */
 
 #define ipc_mni			IPCMNI
-#define IPCMNI_SEQ_SHIFT	IPCMNI_SHIFT
+#define ipcmni_seq_shift()	IPCMNI_SHIFT
 #define IPCMNI_IDX_MASK		((1 << IPCMNI_SHIFT) - 1)
 #endif /* CONFIG_SYSVIPC_SYSCTL */
 
@@ -123,8 +123,8 @@ struct pid_namespace *ipc_seq_pid_ns(struct seq_file *);
 #define IPC_SHM_IDS	2
 
 #define ipcid_to_idx(id)  ((id) & IPCMNI_IDX_MASK)
-#define ipcid_to_seqx(id) ((id) >> IPCMNI_SEQ_SHIFT)
-#define IPCID_SEQ_MAX	  (INT_MAX >> IPCMNI_SEQ_SHIFT)
+#define ipcid_to_seqx(id) ((id) >> ipcmni_seq_shift())
+#define ipcid_seq_max()	  (INT_MAX >> ipcmni_seq_shift())
 
 /* must be called with ids->rwsem acquired for writing */
 int ipc_addid(struct ipc_ids *, struct kern_ipc_perm *, int);
-- 
2.18.1

[RESEND PATCH 3/3] ipc: Do cyclic id allocation for the ipc object.

[Waiman Long]

From: Manfred Spraul <manfred@colorfullife.com>

For ipcmni_extend mode, the sequence number space is only 7 bits. So
the chance of id reuse is relatively high compared with the non-extended
mode.

To alleviate this id reuse problem, this patch enables cyclic allocation
for the index to the radix tree (idx).
The disadvantage is that this can cause a slight slow-down of the fast
path, as the radix tree could be higher than necessary.

To limit the radix tree height, I have chosen the following limits:
- 1) The cycling is done over in_use*1.5.
- 2) At least, the cycling is done over
   "normal" ipcnmi mode: RADIX_TREE_MAP_SIZE elements
   "ipcmni_extended": 4096 elements

Result:
- for normal mode:
	No change for <= 42 active ipc elements. With more than 42
	active ipc elements, a 2nd level would be added to the radix
	tree.
	Without cyclic allocation, a 2nd level would be added only with
	more than 63 active elements.

- for extended mode:
	Cycling creates always at least a 2-level radix tree.
	With more than 2730 active objects, a 3rd level would be
	added, instead of > 4095 active objects until the 3rd level
	is added without cyclic allocation.

For a 2-level radix tree compared to a 1-level radix tree, I have
observed < 1% performance impact.

Notes:
1) Normal "x=semget();y=semget();" is unaffected: Then the idx
  is e.g. a and a+1, regardless if idr_alloc() or idr_alloc_cyclic()
  is used.

2) The -1% happens in a microbenchmark after this situation:
	x=semget();
	for(i=0;i<4000;i++) {t=semget();semctl(t,0,IPC_RMID);}
	y=semget();
	Now perform semget calls on x and y that do not sleep.

3) The worst-case reuse cycle time is unfortunately unaffected:
   If you have 2^24-1 ipc objects allocated, and get/remove the last
   possible element in a loop, then the id is reused after 128
   get/remove pairs.

Performance check:
A microbenchmark that performes no-op semop() randomly on two IDs,
with only these two IDs allocated.
The IDs were set using /proc/sys/kernel/sem_next_id.
The test was run 5 times, averages are shown.

1 & 2: Base (6.22 seconds for 10.000.000 semops)
1 & 40: -0.2%
1 & 3348: - 0.8%
1 & 27348: - 1.6%
1 & 15777204: - 3.2%

Or: ~12.6 cpu cycles per additional radix tree level.
The cpu is an Intel I3-5010U. ~1300 cpu cycles/syscall is slower
than what I remember (spectre impact?).

V2 of the patch:
- use "min" and "max"
- use RADIX_TREE_MAP_SIZE * RADIX_TREE_MAP_SIZE instead of
	(2<<12).

Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Acked-by: Waiman Long <longman@redhat.com>
---
 ipc/ipc_sysctl.c | 2 ++
 ipc/util.c       | 7 ++++++-
 ipc/util.h       | 3 +++
 3 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/ipc/ipc_sysctl.c b/ipc/ipc_sysctl.c
index 73b7782eccf4..bfaae457810c 100644
--- a/ipc/ipc_sysctl.c
+++ b/ipc/ipc_sysctl.c
@@ -122,6 +122,7 @@ static int one = 1;
 static int int_max = INT_MAX;
 int ipc_mni = IPCMNI;
 int ipc_mni_shift = IPCMNI_SHIFT;
+int ipc_min_cycle = RADIX_TREE_MAP_SIZE;
 
 static struct ctl_table ipc_kern_table[] = {
 	{
@@ -252,6 +253,7 @@ static int __init ipc_mni_extend(char *str)
 {
 	ipc_mni = IPCMNI_EXTEND;
 	ipc_mni_shift = IPCMNI_EXTEND_SHIFT;
+	ipc_min_cycle = IPCMNI_EXTEND_MIN_CYCLE;
 	pr_info("IPCMNI extended to %d.\n", ipc_mni);
 	return 0;
 }
diff --git a/ipc/util.c b/ipc/util.c
index 6e0fe3410423..1a492afb1d8b 100644
--- a/ipc/util.c
+++ b/ipc/util.c
@@ -221,9 +221,14 @@ static inline int ipc_idr_alloc(struct ipc_ids *ids, struct kern_ipc_perm *new)
 	 */
 
 	if (next_id < 0) { /* !CHECKPOINT_RESTORE or next_id is unset */
+		int max_idx;
+
+		max_idx = max(ids->in_use*3/2, ipc_min_cycle);
+		max_idx = min(max_idx, ipc_mni);
 
 		/* allocate the idx, with a NULL struct kern_ipc_perm */
-		idx = idr_alloc(&ids->ipcs_idr, NULL, 0, 0, GFP_NOWAIT);
+		idx = idr_alloc_cyclic(&ids->ipcs_idr, NULL, 0, max_idx,
+					GFP_NOWAIT);
 
 		if (idx >= 0) {
 			/*
diff --git a/ipc/util.h b/ipc/util.h
index 8c834ed39012..d316399f0c32 100644
--- a/ipc/util.h
+++ b/ipc/util.h
@@ -27,12 +27,14 @@
  */
 #define IPCMNI_SHIFT		15
 #define IPCMNI_EXTEND_SHIFT	24
+#define IPCMNI_EXTEND_MIN_CYCLE	(RADIX_TREE_MAP_SIZE * RADIX_TREE_MAP_SIZE)
 #define IPCMNI			(1 << IPCMNI_SHIFT)
 #define IPCMNI_EXTEND		(1 << IPCMNI_EXTEND_SHIFT)
 
 #ifdef CONFIG_SYSVIPC_SYSCTL
 extern int ipc_mni;
 extern int ipc_mni_shift;
+extern int ipc_min_cycle;
 
 #define ipcmni_seq_shift()	ipc_mni_shift
 #define IPCMNI_IDX_MASK		((1 << ipc_mni_shift) - 1)
@@ -40,6 +42,7 @@ extern int ipc_mni_shift;
 #else /* CONFIG_SYSVIPC_SYSCTL */
 
 #define ipc_mni			IPCMNI
+#define ipc_min_cycle		RADIX_TREE_MAP_SIZE
 #define ipcmni_seq_shift()	IPCMNI_SHIFT
 #define IPCMNI_IDX_MASK		((1 << IPCMNI_SHIFT) - 1)
 #endif /* CONFIG_SYSVIPC_SYSCTL */
-- 
2.18.1

Re: [RESEND PATCH 2/3] ipc: Conserve sequence numbers in ipcmni_extend mode

[Andrew Morton]

On Fri, 29 Mar 2019 16:49:29 -0400 Waiman Long <longman@redhat.com> wrote:

> From: Manfred Spraul <manfred@colorfullife.com>
> 
> Rewrite, based on the patch from Waiman Long:
> 
> ...
>
> Suggested-by: Matthew Wilcox <willy@infradead.org>
> Original-patch-from: Waiman Long <longman@redhat.com>
> Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
> Acked-by: Waiman Long <longman@redhat.com>

I added your Signed-off-by: to this.  Because you contributed and
because you were on the patch delivery path (either of these require a
SOB).

And I removed the Original-patch-from line.  The changelog already says
that and Original-patch-from isn't a standard changelog tag.

Re: [RESEND PATCH 2/3] ipc: Conserve sequence numbers in ipcmni_extend mode

[Waiman Long]

On 04/02/2019 05:22 PM, Andrew Morton wrote:
> On Fri, 29 Mar 2019 16:49:29 -0400 Waiman Long <longman@redhat.com> wrote:
>
>> From: Manfred Spraul <manfred@colorfullife.com>
>>
>> Rewrite, based on the patch from Waiman Long:
>>
>> ...
>>
>> Suggested-by: Matthew Wilcox <willy@infradead.org>
>> Original-patch-from: Waiman Long <longman@redhat.com>
>> Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
>> Acked-by: Waiman Long <longman@redhat.com>
> I added your Signed-off-by: to this.  Because you contributed and
> because you were on the patch delivery path (either of these require a
> SOB).
>
> And I removed the Original-patch-from line.  The changelog already says
> that and Original-patch-from isn't a standard changelog tag.

Thanks for including those patches in your tree. Hopefully they can make
5.2.

Cheers,
Longman