[PATCH 2/2] blktrace: limit allowed total trace buffer size

[Ming Lei <ming.lei@redhat.com>]

On some ARCHs, such as aarch64, page size may be 64K, meantime there may
be lots of CPU cores. relay_open() needs to allocate pages on each CPU
blktrace, so easily too many pages are taken by blktrace. For example,
on one ARM64 server: 224 CPU cores, 16G RAM, blktrace finally got
allocated 7GB in case of 'blktrace -b 8192' which is used by device-mapper
test suite[1]. This way could cause OOM easily.

Fix the issue by limiting max allowed pages to be 1/8 of totalram_pages().

[1] https://github.com/jthornber/device-mapper-test-suite.git

Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 kernel/trace/blktrace.c | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
index c221e4c3f625..8403ff19d533 100644
--- a/kernel/trace/blktrace.c
+++ b/kernel/trace/blktrace.c
@@ -466,6 +466,35 @@ static void blk_trace_setup_lba(struct blk_trace *bt,
 	}
 }
 
+/* limit total allocated buffer size is <= 1/8 of total pages */
+static void validate_and_adjust_buf(struct blk_user_trace_setup *buts)
+{
+	unsigned buf_size = buts->buf_size;
+	unsigned buf_nr = buts->buf_nr;
+	unsigned long max_allowed_pages = totalram_pages() >> 3;
+	unsigned long req_pages = PAGE_ALIGN(buf_size * buf_nr) >> PAGE_SHIFT;
+
+	if (req_pages * num_online_cpus() <= max_allowed_pages)
+		return;
+
+	req_pages = DIV_ROUND_UP(max_allowed_pages, num_online_cpus());
+
+	if (req_pages == 0) {
+		buf_size = PAGE_SIZE;
+		buf_nr = 1;
+	} else {
+		buf_size = req_pages << PAGE_SHIFT / buf_nr;
+		if (buf_size < PAGE_SIZE)
+			buf_size = PAGE_SIZE;
+		buf_nr = req_pages << PAGE_SHIFT / buf_size;
+		if (buf_nr == 0)
+			buf_nr = 1;
+	}
+
+	buts->buf_size = min_t(unsigned, buf_size, buts->buf_size);
+	buts->buf_nr = min_t(unsigned, buf_nr, buts->buf_nr);
+}
+
 /*
  * Setup everything required to start tracing
  */
@@ -482,6 +511,9 @@ static int do_blk_trace_setup(struct request_queue *q, char *name, dev_t dev,
 	if (!buts->buf_size || !buts->buf_nr)
 		return -EINVAL;
 
+	/* make sure not allocate too much for userspace */
+	validate_and_adjust_buf(buts);
+
 	strncpy(buts->name, name, BLKTRACE_BDEV_SIZE);
 	buts->name[BLKTRACE_BDEV_SIZE - 1] = '\0';
 
-- 
2.29.2