[PATCH i-g-t 2/2] runner/executor: Limit reading dmesg to chunks

[Kamil Konieczny]

There was no disk limit checks in reading kernel dmesg and that
could lead to writing really huge dumps longer than 400MB,
greatly exceeding disk limits used by CI and hardly useful for
developers. Make a dmesg dumping in chunks, size depending on
number of CPUs present, with a minimum of 64KB.
This could also allow to kick in disk limits checks if a driver
starts spilling messages into dmesg.

Closes: https://gitlab.freedesktop.org/drm/igt-gpu-tools/-/issues/129
Cc: Petri Latvala <adrinael at adrinael.net>
Cc: Karol Krol <karol.krol at intel.com>
Cc: Ewelina Musial <ewelina.musial at intel.com>
Signed-off-by: Kamil Konieczny <kamil.konieczny at linux.intel.com>
---
 runner/executor.c | 36 +++++++++++++++++++++++++++++-------
 1 file changed, 29 insertions(+), 7 deletions(-)

diff --git a/runner/executor.c b/runner/executor.c
index 3939f92f1..9283aad97 100644
--- a/runner/executor.c
+++ b/runner/executor.c
@@ -585,13 +585,14 @@ void close_outputs(int *fds)
 }
 
 /* Returns the number of bytes written to disk, or a negative number on error */
-static long dump_dmesg(int kmsgfd, int outfd)
+static long dump_dmesg(int kmsgfd, int outfd, ssize_t size)
 {
 	/*
 	 * Write kernel messages to the log file until we reach
-	 * 'now'. Unfortunately, /dev/kmsg doesn't support seeking to
-	 * -1 from SEEK_END so we need to use a second fd to read a
-	 * message to match against, or stop when we reach EAGAIN.
+	 * 'now' or we read at least size bytes. Unfortunately,
+	 * /dev/kmsg doesn't support seeking to -1 from SEEK_END
+	 * so we need to use a second fd to read a message to
+	 *  match against, or stop when we reach EAGAIN.
 	 */
 
 	int comparefd;
@@ -606,6 +607,9 @@ static long dump_dmesg(int kmsgfd, int outfd)
 	if (kmsgfd < 0)
 		return 0;
 
+	if (size <= 0)
+		return 0;
+
 	comparefd = open("/dev/kmsg", O_RDONLY | O_NONBLOCK);
 	if (comparefd < 0) {
 		errf("Error opening another fd for /dev/kmsg\n");
@@ -690,6 +694,13 @@ static long dump_dmesg(int kmsgfd, int outfd)
 			if (seq >= cmpseq)
 				return written;
 		}
+
+		if (written >= size) {
+			if (comparefd >= 0)
+				close(comparefd);
+
+			return written;
+		}
 	}
 }
 
@@ -883,6 +894,14 @@ static void write_packet_with_canary(int fd, struct runnerpacket *packet, bool s
 /* TODO: Refactor this macro from here and from various tests to lib */
 #define KB(x) ((x) * 1024)
 
+static size_t calc_last_dmesg_chunk(size_t limit, size_t disk_usage)
+{
+	if (!limit)
+		return KB(128 * 1024); /* 128MB */
+
+	return limit > disk_usage ? limit - disk_usage : 0;
+}
+
 /*
  * Returns:
  *  =0 - Success
@@ -915,6 +934,7 @@ static int monitor_output(pid_t child,
 	unsigned long taints = 0;
 	bool aborting = false;
 	size_t disk_usage = 0;
+	size_t dmsg_chunk_size = 4096 * max_t(size_t, sysconf(_SC_NPROCESSORS_ONLN), 16);
 	bool socket_comms_used = false; /* whether the test actually uses comms */
 	bool results_received = false; /* whether we already have test results that might need overriding if we detect an abort condition */
 
@@ -1244,7 +1264,7 @@ static int monitor_output(pid_t child,
 
 			time_last_activity = time_now;
 
-			dmesgwritten = dump_dmesg(kmsgfd, outputs[_F_DMESG]);
+			dmesgwritten = dump_dmesg(kmsgfd, outputs[_F_DMESG], dmsg_chunk_size);
 			if (settings->sync)
 				fdatasync(outputs[_F_DMESG]);
 
@@ -1482,7 +1502,8 @@ static int monitor_output(pid_t child,
 					asprintf(abortreason, "Child refuses to die, tainted 0x%lx.", taints);
 				}
 
-				dump_dmesg(kmsgfd, outputs[_F_DMESG]);
+				dmsg_chunk_size = calc_last_dmesg_chunk(settings->disk_usage_limit, disk_usage);
+				dump_dmesg(kmsgfd, outputs[_F_DMESG], dmsg_chunk_size);
 				if (settings->sync)
 					fdatasync(outputs[_F_DMESG]);
 
@@ -1508,7 +1529,8 @@ static int monitor_output(pid_t child,
 		}
 	}
 
-	dump_dmesg(kmsgfd, outputs[_F_DMESG]);
+	dmsg_chunk_size = calc_last_dmesg_chunk(settings->disk_usage_limit, disk_usage);
+	dump_dmesg(kmsgfd, outputs[_F_DMESG], dmsg_chunk_size);
 	if (settings->sync)
 		fdatasync(outputs[_F_DMESG]);
 
-- 
2.47.0