[PATCH printk v4 7/8] printk: use printk_buffers for devkmsg

From: John Ogness
Date: Thu Jan 05 2023 - 05:39:25 EST


Replace the individual @text_buf and @buf buffers in struct
devkmsg_user with a struct printk_buffers. This reduces the number
of buffers to keep track of. Also, since @text_buf is only used for
ringbuffer reading and @buf is only used for output, it maps
perfectly to the purpose and fields of struct printk_buffers.

As a side-effect, @text_buf was 8kB large, even though it only
needed to be the max size of a ringbuffer record. By switching to
struct printk_buffers, ~7kB less memory is allocated when opening
/dev/kmsg.

And since struct printk_buffers will be used now, reduce duplicate
code by calling printk_get_next_message() to handle the record
reading and formatting.

Signed-off-by: John Ogness <john.ogness@xxxxxxxxxxxxx>
---
kernel/printk/printk.c | 32 ++++++++++++++------------------
1 file changed, 14 insertions(+), 18 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 4fb7d29fb05d..e28a3736a399 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -695,15 +695,16 @@ static ssize_t msg_print_ext_body(char *buf, size_t size,
return len;
}

+static bool printk_get_next_message(struct printk_message *pmsg, u64 seq, bool is_extended);
+
/* /dev/kmsg - userspace message inject/listen interface */
struct devkmsg_user {
atomic64_t seq;
struct ratelimit_state rs;
struct mutex lock;
- char buf[CONSOLE_EXT_LOG_MAX];

+ struct printk_buffers pbufs;
struct printk_info info;
- char text_buf[CONSOLE_EXT_LOG_MAX];
struct printk_record record;
};

@@ -787,7 +788,10 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,
{
struct devkmsg_user *user = file->private_data;
struct printk_record *r = &user->record;
- size_t len;
+ char *outbuf = &user->pbufs.outbuf[0];
+ struct printk_message pmsg = {
+ .pbufs = &user->pbufs,
+ };
ssize_t ret;

if (!user)
@@ -797,7 +801,7 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,
if (ret)
return ret;

- if (!prb_read_valid(prb, atomic64_read(&user->seq), r)) {
+ if (!printk_get_next_message(&pmsg, atomic64_read(&user->seq), true)) {
if (file->f_flags & O_NONBLOCK) {
ret = -EAGAIN;
goto out;
@@ -820,30 +824,25 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,
goto out;
}

- if (r->info->seq != atomic64_read(&user->seq)) {
+ if (pmsg.dropped) {
/* our last seen message is gone, return error and reset */
- atomic64_set(&user->seq, r->info->seq);
+ atomic64_set(&user->seq, pmsg.seq);
ret = -EPIPE;
goto out;
}

- len = info_print_ext_header(user->buf, sizeof(user->buf), r->info);
- len += msg_print_ext_body(user->buf + len, sizeof(user->buf) - len,
- &r->text_buf[0], r->info->text_len,
- &r->info->dev_info);
-
- atomic64_set(&user->seq, r->info->seq + 1);
+ atomic64_set(&user->seq, pmsg.seq + 1);

- if (len > count) {
+ if (pmsg.outbuf_len > count) {
ret = -EINVAL;
goto out;
}

- if (copy_to_user(buf, user->buf, len)) {
+ if (copy_to_user(buf, outbuf, pmsg.outbuf_len)) {
ret = -EFAULT;
goto out;
}
- ret = len;
+ ret = pmsg.outbuf_len;
out:
mutex_unlock(&user->lock);
return ret;
@@ -937,9 +936,6 @@ static int devkmsg_open(struct inode *inode, struct file *file)

mutex_init(&user->lock);

- prb_rec_init_rd(&user->record, &user->info,
- &user->text_buf[0], sizeof(user->text_buf));
-
atomic64_set(&user->seq, prb_first_valid_seq(prb));

file->private_data = user;
--
2.30.2