[PATCH] ipc: Modify message queue accounting to reflect both total user data and auxiliary kernel data

[Marcus Gelderie]

A while back, the message queue implementation in the kernel was
improved to use btrees to speed up retrieval of messages (commit
d6629859b36). The patch introducing the improved kernel handling of
message queues has, as a by-product, changed the meaning of the 
QSIZE field in the pseudo-file created for the queue. Before, this 
field reflected the size of the user-data in the queue. Since, it 
now also takes kernel data structures into account, this is no no longer
true. For example, if 13 bytes of user data are in the queue, on my 
machine the file reports a size of 61 bytes.

There was some discussion on this topic before (for example
https://lkml.org/lkml/2014/10/1/115). Reporting the size of the
message queue in kernel has its merits, but doing so in the QSIZE
field of the pseudo file corresponding to the queue is a breaking
change. This patch therefore returns the QSIZE field to its original
meaning. At the same time, it introduces a new field QKERSIZE that
reflects the size of the queue in kernel (user data + kernel data).

If the accounting should be improved, I'd be happy to take a look at 
that, too. I would propose to do that in terms of the newly introduced
field.

Signed-off-by: Marcus Gelderie <redmnic@xxxxxxxxx>
---
 ipc/mqueue.c | 20 ++++++++++++++------
 1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/ipc/mqueue.c b/ipc/mqueue.c
index 3aaea7f..7d4c464 100644
--- a/ipc/mqueue.c
+++ b/ipc/mqueue.c
@@ -41,7 +41,7 @@
 
 #define MQUEUE_MAGIC	0x19800202
 #define DIRENT_SIZE	20
-#define FILENT_SIZE	80
+#define FILENT_SIZE	90
 
 #define SEND		0
 #define RECV		1
@@ -82,8 +82,12 @@ struct mqueue_inode_info {
 	/* for tasks waiting for free space and messages, respectively */
 	struct ext_wait_queue e_wait_q[2];
 
-	unsigned long qsize; /* size of queue in memory (sum of all msgs) */
-};
+	/* size of queue in memory (sum of all msgs plus kernel
+	 * data structures) */
+	unsigned long qsize;
+
+	/* size of user data in the queue (sum of all msgs) */
+	unsigned long q_usersize; };
 
 static const struct inode_operations mqueue_dir_inode_operations;
 static const struct file_operations mqueue_file_operations;
@@ -151,6 +155,7 @@ static int msg_insert(struct msg_msg *msg, struct mqueue_inode_info *info)
 insert_msg:
 	info->attr.mq_curmsgs++;
 	info->qsize += msg->m_ts;
+	info->q_usersize += msg->m_ts;
 	list_add_tail(&msg->m_list, &leaf->msg_list);
 	return 0;
 }
@@ -210,6 +215,7 @@ try_again:
 	}
 	info->attr.mq_curmsgs--;
 	info->qsize -= msg->m_ts;
+	info->q_usersize -= msg->m_ts;
 	return msg;
 }
 
@@ -246,6 +252,7 @@ static struct inode *mqueue_get_inode(struct super_block *sb,
 		info->notify_owner = NULL;
 		info->notify_user_ns = NULL;
 		info->qsize = 0;
+		info->q_usersize = 0;
 		info->user = NULL;	/* set when all is ok */
 		info->msg_tree = RB_ROOT;
 		info->node_cache = NULL;
@@ -491,13 +498,14 @@ static ssize_t mqueue_read_file(struct file *filp, char __user *u_data,
 
 	spin_lock(&info->lock);
 	snprintf(buffer, sizeof(buffer),
-			"QSIZE:%-10lu NOTIFY:%-5d SIGNO:%-5d NOTIFY_PID:%-6d\n",
-			info->qsize,
+			"QSIZE:%-10lu NOTIFY:%-5d SIGNO:%-5d NOTIFY_PID:%-6d QKERSIZE:%-10lu\n",
+			info->q_usersize,
 			info->notify_owner ? info->notify.sigev_notify : 0,
 			(info->notify_owner &&
 			 info->notify.sigev_notify == SIGEV_SIGNAL) ?
 				info->notify.sigev_signo : 0,
-			pid_vnr(info->notify_owner));
+			pid_vnr(info->notify_owner),
+			info->qsize);
 	spin_unlock(&info->lock);
 	buffer[sizeof(buffer)-1] = '\0';
 
-- 
2.4.3

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