[PATCH] proc: Avoid costly high-order page allocations when reading proc files

[Yafang Shao <laoar.shao@gmail.com>]

While investigating a kcompactd 100% CPU utilization issue in production, I
observed frequent costly high-order (order-6) page allocations triggered by
proc file reads from monitoring tools. This can be reproduced with a simple
test case:

  fd = open(PROC_FILE, O_RDONLY);
  size = read(fd, buff, 256KB);
  close(fd);

Although we should modify the monitoring tools to use smaller buffer sizes,
we should also enhance the kernel to prevent these expensive high-order
allocations.

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Cc: Josef Bacik <josef@toxicpanda.com>
---
 fs/proc/proc_sysctl.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index cc9d74a06ff0..c53ba733bda5 100644
--- a/fs/proc/proc_sysctl.c
+++ b/fs/proc/proc_sysctl.c
@@ -581,7 +581,15 @@ static ssize_t proc_sys_call_handler(struct kiocb *iocb, struct iov_iter *iter,
 	error = -ENOMEM;
 	if (count >= KMALLOC_MAX_SIZE)
 		goto out;
-	kbuf = kvzalloc(count + 1, GFP_KERNEL);
+
+	/*
+	 * Use vmalloc if the count is too large to avoid costly high-order page
+	 * allocations.
+	 */
+	if (count < (PAGE_SIZE << PAGE_ALLOC_COSTLY_ORDER))
+		kbuf = kvzalloc(count + 1, GFP_KERNEL);
+	else
+		kbuf = vmalloc(count + 1);
 	if (!kbuf)
 		goto out;
 
-- 
2.43.5