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Date:   Tue, 10 Mar 2020 09:02:09 +0000
From:   Jonathan Cameron <Jonathan.Cameron@Huawei.com>
To:     SeongJae Park <sjpark@amazon.com>
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Subject: Re: [PATCH v6 08/14] mm/damon: Add debugfs interface
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 <20200224123047.32506-9-sjpark@amazon.com>
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On Mon, 24 Feb 2020 13:30:41 +0100
SeongJae Park <sjpark@amazon.com> wrote:

> From: SeongJae Park <sjpark@amazon.de>
> 
> This commit adds a debugfs interface for DAMON.
> 
> DAMON exports four files, ``attrs``, ``pids``, ``record``, and
> ``monitor_on`` under its debugfs directory, ``<debugfs>/damon/``.
> 
> Attributes
> ----------
> 
> Users can read and write the ``sampling interval``, ``aggregation
> interval``, ``regions update interval``, and min/max number of
> monitoring target regions by reading from and writing to the ``attrs``
> file.  For example, below commands set those values to 5 ms, 100 ms,
> 1,000 ms, 10, 1000 and check it again::
> 
>     # cd <debugfs>/damon
>     # echo 5000 100000 1000000 10 1000 > attrs
>     # cat attrs
>     5000 100000 1000000 10 1000
> 
> Target PIDs
> -----------
> 
> Users can read and write the pids of current monitoring target processes
> by reading from and writing to the ``pids`` file.  For example, below
> commands set processes having pids 42 and 4242 as the processes to be
> monitored and check it again::
> 
>     # cd <debugfs>/damon
>     # echo 42 4242 > pids
>     # cat pids
>     42 4242
> 
> Note that setting the pids doesn't starts the monitoring.
> 
> Record
> ------
> 
> DAMON support direct monitoring result record feature.  The recorded
> results are first written to a buffer and flushed to a file in batch.
> Users can set the size of the buffer and the path to the result file by
> reading from and writing to the ``record`` file.  For example, below
> commands set the buffer to be 4 KiB and the result to be saved in
> '/damon.data'.
> 
>     # cd <debugfs>/damon
>     # echo 4096 /damon.data > pids
>     # cat record
>     4096 /damon.data
> 
> Turning On/Off
> --------------
> 
> You can check current status, start and stop the monitoring by reading
> from and writing to the ``monitor_on`` file.  Writing ``on`` to the file
> starts DAMON to monitor the target processes with the attributes.
> Writing ``off`` to the file stops DAMON.  DAMON also stops if every
> target processes is be terminated.  Below example commands turn on, off,
> and check status of DAMON::
> 
>     # cd <debugfs>/damon
>     # echo on > monitor_on
>     # echo off > monitor_on
>     # cat monitor_on
>     off
> 
> Please note that you cannot write to the ``attrs`` and ``pids`` files
> while the monitoring is turned on.  If you write to the files while
> DAMON is running, ``-EINVAL`` will be returned.
> 
> Signed-off-by: SeongJae Park <sjpark@amazon.de>

Some of the code in here seems a bit fragile and convoluted.

> ---
>  mm/damon.c | 377 ++++++++++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 376 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/damon.c b/mm/damon.c
> index b3e9b9da5720..facb1d7f121b 100644
> --- a/mm/damon.c
> +++ b/mm/damon.c
> @@ -10,6 +10,7 @@
>  #define pr_fmt(fmt) "damon: " fmt
>  
>  #include <linux/damon.h>
> +#include <linux/debugfs.h>
>  #include <linux/delay.h>
>  #include <linux/kthread.h>
>  #include <linux/mm.h>
> @@ -46,6 +47,24 @@
>  /* Get a random number in [l, r) */
>  #define damon_rand(ctx, l, r) (l + prandom_u32_state(&ctx->rndseed) % (r - l))
>  
> +/*
> + * For each 'sample_interval', DAMON checks whether each region is accessed or
> + * not.  It aggregates and keeps the access information (number of accesses to
> + * each region) for 'aggr_interval' and then flushes it to the result buffer if
> + * an 'aggr_interval' surpassed.  And for each 'regions_update_interval', damon
> + * checks whether the memory mapping of the target tasks has changed (e.g., by
> + * mmap() calls from the applications) and applies the changes.
> + *
> + * All time intervals are in micro-seconds.
> + */
> +static struct damon_ctx damon_user_ctx = {
> +	.sample_interval = 5 * 1000,
> +	.aggr_interval = 100 * 1000,
> +	.regions_update_interval = 1000 * 1000,
> +	.min_nr_regions = 10,
> +	.max_nr_regions = 1000,
> +};
> +
>  /*
>   * Construct a damon_region struct
>   *
> @@ -1026,15 +1045,371 @@ int damon_set_attrs(struct damon_ctx *ctx, unsigned long sample_int,
>  	return 0;
>  }
>  
> +/*
> + * debugfs functions

Seems unnecessary when their naming makes this clear.

> + */
> +
> +static ssize_t debugfs_monitor_on_read(struct file *file,
> +		char __user *buf, size_t count, loff_t *ppos)
> +{
> +	struct damon_ctx *ctx = &damon_user_ctx;
> +	char monitor_on_buf[5];
> +	bool monitor_on;
> +	int ret;
> +
> +	spin_lock(&ctx->kdamond_lock);
> +	monitor_on = ctx->kdamond != NULL;
> +	spin_unlock(&ctx->kdamond_lock);
> +
> +	ret = snprintf(monitor_on_buf, 5, monitor_on ? "on\n" : "off\n");
> +
> +	return simple_read_from_buffer(buf, count, ppos, monitor_on_buf, ret);
> +}
> +
> +static ssize_t debugfs_monitor_on_write(struct file *file,
> +		const char __user *buf, size_t count, loff_t *ppos)
> +{
> +	struct damon_ctx *ctx = &damon_user_ctx;
> +	ssize_t ret;
> +	bool on = false;
> +	char cmdbuf[5];
> +
> +	ret = simple_write_to_buffer(cmdbuf, 5, ppos, buf, count);
> +	if (ret < 0)
> +		return ret;
> +
> +	if (sscanf(cmdbuf, "%s", cmdbuf) != 1)
> +		return -EINVAL;
> +	if (!strncmp(cmdbuf, "on", 5))
> +		on = true;
> +	else if (!strncmp(cmdbuf, "off", 5))
> +		on = false;
> +	else
> +		return -EINVAL;
> +
> +	if (damon_turn_kdamond(ctx, on))
> +		return -EINVAL;
> +
> +	return ret;
> +}
> +
> +static ssize_t damon_sprint_pids(struct damon_ctx *ctx, char *buf, ssize_t len)
> +{
> +	struct damon_task *t;
> +	int written = 0;
> +	int rc;
> +
> +	damon_for_each_task(ctx, t) {
> +		rc = snprintf(&buf[written], len - written, "%lu ", t->pid);
> +		if (!rc)
> +			return -ENOMEM;
> +		written += rc;
> +	}
> +	if (written)
> +		written -= 1;
> +	written += snprintf(&buf[written], len - written, "\n");
> +	return written;
> +}
> +
> +static ssize_t debugfs_pids_read(struct file *file,
> +		char __user *buf, size_t count, loff_t *ppos)
> +{
> +	struct damon_ctx *ctx = &damon_user_ctx;
> +	ssize_t len;
> +	char pids_buf[320];
> +
> +	len = damon_sprint_pids(ctx, pids_buf, 320);
> +	if (len < 0)
> +		return len;
> +
> +	return simple_read_from_buffer(buf, count, ppos, pids_buf, len);
> +}
> +
> +/*
> + * Converts a string into an array of unsigned long integers
> + *
> + * Returns an array of unsigned long integers if the conversion success, or
> + * NULL otherwise.
> + */
> +static unsigned long *str_to_pids(const char *str, ssize_t len,
> +				ssize_t *nr_pids)
> +{
> +	unsigned long *pids;
> +	const int max_nr_pids = 32;
> +	unsigned long pid;
> +	int pos = 0, parsed, ret;
> +
> +	*nr_pids = 0;
> +	pids = kmalloc_array(max_nr_pids, sizeof(unsigned long), GFP_KERNEL);
> +	if (!pids)
> +		return NULL;
> +	while (*nr_pids < max_nr_pids && pos < len) {
> +		ret = sscanf(&str[pos], "%lu%n", &pid, &parsed);
> +		pos += parsed;
> +		if (ret != 1)
> +			break;
> +		pids[*nr_pids] = pid;
> +		*nr_pids += 1;
> +	}
> +	if (*nr_pids == 0) {
> +		kfree(pids);
> +		pids = NULL;
> +	}
> +
> +	return pids;
> +}
> +
> +static ssize_t debugfs_pids_write(struct file *file,
> +		const char __user *buf, size_t count, loff_t *ppos)
> +{
> +	struct damon_ctx *ctx = &damon_user_ctx;
> +	char *kbuf;
> +	unsigned long *targets;
> +	ssize_t nr_targets;
> +	ssize_t ret;
> +
> +	kbuf = kmalloc_array(count, sizeof(char), GFP_KERNEL);
> +	if (!kbuf)
> +		return -ENOMEM;
> +
> +	ret = simple_write_to_buffer(kbuf, 512, ppos, buf, count);

Why only 512?

> +	if (ret < 0)
> +		goto out;
> +
> +	targets = str_to_pids(kbuf, ret, &nr_targets);
> +	if (!targets) {
> +		ret = -ENOMEM;
> +		goto out;
> +	}
> +
> +	spin_lock(&ctx->kdamond_lock);
> +	if (ctx->kdamond)
> +		goto monitor_running;
> +
> +	damon_set_pids(ctx, targets, nr_targets);
> +	spin_unlock(&ctx->kdamond_lock);
> +
> +	goto free_targets_out;
> +
> +monitor_running:
> +	spin_unlock(&ctx->kdamond_lock);
> +	pr_err("%s: kdamond is running. Turn it off first.\n", __func__);
> +	ret = -EINVAL;
> +free_targets_out:
> +	kfree(targets);
> +out:
> +	kfree(kbuf);
> +	return ret;
> +}
> +
> +static ssize_t debugfs_record_read(struct file *file,
> +		char __user *buf, size_t count, loff_t *ppos)
> +{
> +	struct damon_ctx *ctx = &damon_user_ctx;
> +	char record_buf[20 + MAX_RFILE_PATH_LEN];
> +	int ret;
> +
> +	ret = snprintf(record_buf, ARRAY_SIZE(record_buf), "%u %s\n",
> +			ctx->rbuf_len, ctx->rfile_path);
> +	return simple_read_from_buffer(buf, count, ppos, record_buf, ret);
> +}
> +
> +static ssize_t debugfs_record_write(struct file *file,
> +		const char __user *buf, size_t count, loff_t *ppos)
> +{
> +	struct damon_ctx *ctx = &damon_user_ctx;
> +	char *kbuf;
> +	unsigned int rbuf_len;
> +	char rfile_path[MAX_RFILE_PATH_LEN];
> +	ssize_t ret;
> +
> +	kbuf = kmalloc_array(count + 1, sizeof(char), GFP_KERNEL);
> +	if (!kbuf)
> +		return -ENOMEM;
> +	kbuf[count] = '\0';
> +
> +	ret = simple_write_to_buffer(kbuf, count, ppos, buf, count);
> +	if (ret < 0)
> +		goto out;
> +	if (sscanf(kbuf, "%u %s",
> +				&rbuf_len, rfile_path) != 2) {
> +		ret = -EINVAL;
> +		goto out;
> +	}
> +
> +	spin_lock(&ctx->kdamond_lock);
> +	if (ctx->kdamond)
> +		goto monitor_running;
> +
> +	damon_set_recording(ctx, rbuf_len, rfile_path);
> +	spin_unlock(&ctx->kdamond_lock);
> +
> +	goto out;
> +
> +monitor_running:
> +	spin_unlock(&ctx->kdamond_lock);
> +	pr_err("%s: kdamond is running. Turn it off first.\n", __func__);
> +	ret = -EINVAL;
> +out:
> +	kfree(kbuf);
> +	return ret;
> +}
> +
> +
> +static ssize_t debugfs_attrs_read(struct file *file,
> +		char __user *buf, size_t count, loff_t *ppos)
> +{
> +	struct damon_ctx *ctx = &damon_user_ctx;
> +	char kbuf[128];
> +	int ret;
> +
> +	ret = snprintf(kbuf, ARRAY_SIZE(kbuf), "%lu %lu %lu %lu %lu\n",
> +			ctx->sample_interval, ctx->aggr_interval,
> +			ctx->regions_update_interval, ctx->min_nr_regions,
> +			ctx->max_nr_regions);
> +
> +	return simple_read_from_buffer(buf, count, ppos, kbuf, ret);
> +}
> +
> +static ssize_t debugfs_attrs_write(struct file *file,
> +		const char __user *buf, size_t count, loff_t *ppos)
> +{
> +	struct damon_ctx *ctx = &damon_user_ctx;
> +	unsigned long s, a, r, minr, maxr;
> +	char *kbuf;
> +	ssize_t ret;
> +
> +	kbuf = kmalloc_array(count, sizeof(char), GFP_KERNEL);

malloc fine for array of characters.   The checks on overflow etc cannot be
relevant here.

> +	if (!kbuf)
> +		return -ENOMEM;
> +
> +	ret = simple_write_to_buffer(kbuf, count, ppos, buf, count);
> +	if (ret < 0)
> +		goto out;
> +
> +	if (sscanf(kbuf, "%lu %lu %lu %lu %lu",
> +				&s, &a, &r, &minr, &maxr) != 5) {
> +		ret = -EINVAL;
> +		goto out;
> +	}
> +
> +	spin_lock(&ctx->kdamond_lock);
> +	if (ctx->kdamond)
> +		goto monitor_running;
> +
> +	damon_set_attrs(ctx, s, a, r, minr, maxr);
> +	spin_unlock(&ctx->kdamond_lock);
> +
> +	goto out;
> +
> +monitor_running:
> +	spin_unlock(&ctx->kdamond_lock);
> +	pr_err("%s: kdamond is running. Turn it off first.\n", __func__);
> +	ret = -EINVAL;

This complex exit path is a bad idea from maintainability point of view...
Just put the pr_err and spin_unlock in the error path above.

> +out:
> +	kfree(kbuf);
> +	return ret;
> +}
> +
> +static const struct file_operations monitor_on_fops = {
> +	.owner = THIS_MODULE,
> +	.read = debugfs_monitor_on_read,
> +	.write = debugfs_monitor_on_write,
> +};
> +
> +static const struct file_operations pids_fops = {
> +	.owner = THIS_MODULE,
> +	.read = debugfs_pids_read,
> +	.write = debugfs_pids_write,
> +};
> +
> +static const struct file_operations record_fops = {
> +	.owner = THIS_MODULE,
> +	.read = debugfs_record_read,
> +	.write = debugfs_record_write,
> +};
> +
> +static const struct file_operations attrs_fops = {
> +	.owner = THIS_MODULE,
> +	.read = debugfs_attrs_read,
> +	.write = debugfs_attrs_write,
> +};
> +
> +static struct dentry *debugfs_root;
> +
> +static int __init debugfs_init(void)

Prefix this function.  Chances of sometime getting a header
that includes debugfs_init feels rather too high!

> +{
> +	const char * const file_names[] = {"attrs", "record",
> +		"pids", "monitor_on"};
> +	const struct file_operations *fops[] = {&attrs_fops, &record_fops,
> +		&pids_fops, &monitor_on_fops};
> +	int i;
> +
> +	debugfs_root = debugfs_create_dir("damon", NULL);
> +	if (!debugfs_root) {
> +		pr_err("failed to create the debugfs dir\n");
> +		return -ENOMEM;
> +	}
> +
> +	for (i = 0; i < ARRAY_SIZE(file_names); i++) {
> +		if (!debugfs_create_file(file_names[i], 0600, debugfs_root,
> +					NULL, fops[i])) {
> +			pr_err("failed to create %s file\n", file_names[i]);
> +			return -ENOMEM;
> +		}
> +	}
> +
> +	return 0;
> +}
> +
> +static int __init damon_init_user_ctx(void)
> +{
> +	int rc;
> +
> +	struct damon_ctx *ctx = &damon_user_ctx;
> +
> +	ktime_get_coarse_ts64(&ctx->last_aggregation);
> +	ctx->last_regions_update = ctx->last_aggregation;
> +
> +	ctx->rbuf_offset = 0;
> +	rc = damon_set_recording(ctx, 1024 * 1024, "/damon.data");
> +	if (rc)
> +		return rc;
> +
> +	ctx->kdamond = NULL;
> +	ctx->kdamond_stop = false;
> +	spin_lock_init(&ctx->kdamond_lock);
> +
> +	prandom_seed_state(&ctx->rndseed, 42);

:)

> +	INIT_LIST_HEAD(&ctx->tasks_list);
> +
> +	ctx->sample_cb = NULL;
> +	ctx->aggregate_cb = NULL;

Should already be set to 0.

> +
> +	return 0;
> +}
> +
>  static int __init damon_init(void)
>  {
> +	int rc;
> +
>  	pr_info("init\n");
>  
> -	return 0;
> +	rc = damon_init_user_ctx();
> +	if (rc)
> +		return rc;
> +
> +	return debugfs_init();

In theory no code should ever be dependent on debugfs succeeding..
There might be other daemon users so you should just eat the return
code.


>  }
>  
>  static void __exit damon_exit(void)
>  {
> +	damon_turn_kdamond(&damon_user_ctx, false);
> +	debugfs_remove_recursive(debugfs_root);
> +
> +	kfree(damon_user_ctx.rbuf);
> +	kfree(damon_user_ctx.rfile_path);
> +
>  	pr_info("exit\n");
>  }
>