linux-kernel - Re: [PATCH v12 10/16] mm/damon: Add debugfs interface

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Date:   Tue, 19 May 2020 16:02:21 +0200
From:   SeongJae Park <sjpark@...zon.com>
To:     SeongJae Park <sjpark@...zon.com>
CC:     <akpm@...ux-foundation.org>, SeongJae Park <sjpark@...zon.de>,
        <Jonathan.Cameron@...wei.com>, <aarcange@...hat.com>,
        <acme@...nel.org>, <alexander.shishkin@...ux.intel.com>,
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        <vdavydov.dev@...il.com>, <yang.shi@...ux.alibaba.com>,
        <ying.huang@...el.com>, <linux-damon@...zon.com>,
        <linux-mm@...ck.org>, <linux-doc@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v12 10/16] mm/damon: Add debugfs interface

On Mon, 18 May 2020 12:00:12 +0200 SeongJae Park <sjpark@...zon.com> wrote:

> From: SeongJae Park <sjpark@...zon.de>
> 
> This commit adds a debugfs interface for DAMON.
[...]
> diff --git a/mm/damon.c b/mm/damon.c
> index ddd78843f79a..f31310536c79 100644
> --- a/mm/damon.c
> +++ b/mm/damon.c
> @@ -10,6 +10,7 @@
>  #define pr_fmt(fmt) "damon: " fmt
>  
[...]
> +
> +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(t, ctx) {
> +		rc = snprintf(&buf[written], len - written, "%d ", 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);

This could race with concurrent pids debugfs file writers.  Should be
synchronized.  Same to other debugfs files except 'monitor_on', which is
already synchronized with corresponding writers.

I will enclose this function call with the context mutex in next revision.


Thanks,
SeongJae Park

> +	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 int *str_to_pids(const char *str, ssize_t len, ssize_t *nr_pids)
> +{
> +	int *pids;
> +	const int max_nr_pids = 32;
> +	int pid;
> +	int pos = 0, parsed, ret;
> +
> +	*nr_pids = 0;
> +	pids = kmalloc_array(max_nr_pids, sizeof(pid), GFP_KERNEL);
> +	if (!pids)
> +		return NULL;
> +	while (*nr_pids < max_nr_pids && pos < len) {
> +		ret = sscanf(&str[pos], "%d%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;
> +	int *targets;
> +	ssize_t nr_targets;
> +	ssize_t ret;
> +	int err;
> +
> +	kbuf = kmalloc(count, GFP_KERNEL);
> +	if (!kbuf)
> +		return -ENOMEM;
> +
> +	ret = simple_write_to_buffer(kbuf, count, ppos, buf, count);
> +	if (ret < 0)
> +		goto out;
> +
> +	targets = str_to_pids(kbuf, ret, &nr_targets);
> +	if (!targets) {
> +		ret = -ENOMEM;
> +		goto out;
> +	}
> +
> +	mutex_lock(&ctx->kdamond_lock);
> +	if (ctx->kdamond) {
> +		ret = -EINVAL;
> +		goto unlock_out;
> +	}
> +
> +	err = damon_set_pids(ctx, targets, nr_targets);
> +	if (err)
> +		ret = err;
> +unlock_out:
> +	mutex_unlock(&ctx->kdamond_lock);
> +	kfree(targets);
> +out:
> +	kfree(kbuf);
> +	return ret;
> +}
[...]