[PATCH v19 10/15] damon/debugfs: Support pidfd target id

[SeongJae Park <sjpark@amazon.com>]

From: SeongJae Park <sjpark@amazon.de>

DAMON programming interface users are safe from pid recycling problem
since it uses 'struct pid *' as target id.  However, debugfs interface
users might still get the problem since they should describe the target
ids with pid numbers.

This commit makes the debugfs interface to support pidfd target ids.
Using it, debugfs interface users can also easily be safe from the
races.  That said, the pid numbers are stil supported for the simple
command line uses.

The pid numbers target ids usages are same to the old one.  Therefore,
old users don't need to change anything.  To use pidfd target ids, users
should add a prefix, 'pidfd ' in front of the numbers.  Because pidfd is
process private, reading 'target_ids' file again will show pid numbers
of the target processes.

Signed-off-by: SeongJae Park <sjpark@amazon.de>
---
 mm/damon.c | 37 ++++++++++++++++++++++++++++++++++---
 1 file changed, 34 insertions(+), 3 deletions(-)

diff --git a/mm/damon.c b/mm/damon.c
index 01003a1ff551..61d93f1ea974 100644
--- a/mm/damon.c
+++ b/mm/damon.c
@@ -1450,11 +1450,32 @@ static unsigned long *str_to_target_ids(const char *str, ssize_t len,
 	return ids;
 }
 
+/* Returns pid for the given pidfd if it's valid, or NULL otherwise. */
+static struct pid *damon_get_pidfd_pid(unsigned int pidfd)
+{
+	struct fd f;
+	struct pid *pid;
+
+	f = fdget(pidfd);
+	if (!f.file)
+		return NULL;
+
+	pid = pidfd_pid(f.file);
+	if (!IS_ERR(pid))
+		get_pid(pid);
+	else
+		pid = NULL;
+
+	fdput(f);
+	return pid;
+}
+
 static ssize_t debugfs_target_ids_write(struct file *file,
 		const char __user *buf, size_t count, loff_t *ppos)
 {
 	struct damon_ctx *ctx = &damon_user_ctx;
-	char *kbuf;
+	char *kbuf, *nrs;
+	bool received_pidfds = false;
 	unsigned long *targets;
 	ssize_t nr_targets;
 	ssize_t ret = count;
@@ -1466,13 +1487,23 @@ static ssize_t debugfs_target_ids_write(struct file *file,
 	if (IS_ERR(kbuf))
 		return PTR_ERR(kbuf);
 
-	targets = str_to_target_ids(kbuf, ret, &nr_targets);
+	nrs = kbuf;
+	if (!strncmp(kbuf, "pidfd ", 6)) {
+		received_pidfds = true;
+		nrs = &kbuf[6];
+	}
+
+	targets = str_to_target_ids(nrs, ret, &nr_targets);
 	if (!targets) {
 		ret = -ENOMEM;
 		goto out;
 	}
 
-	if (targetid_is_pid(ctx)) {
+	if (received_pidfds) {
+		for (i = 0; i < nr_targets; i++)
+			targets[i] = (unsigned long)damon_get_pidfd_pid(
+					(unsigned int)targets[i]);
+	} else if (targetid_is_pid(ctx)) {
 		for (i = 0; i < nr_targets; i++)
 			targets[i] = (unsigned long)find_get_pid(
 					(int)targets[i]);
-- 
2.17.1