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

[Kees Cook]

On April 1, 2025 12:30:46 AM PDT, Yafang Shao <laoar.shao@xxxxxxxxx> wrote:
>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@xxxxxxxxx>
>Cc: Josef Bacik <josef@xxxxxxxxxxxxxx>
>---
> 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);

Why not move this check into kvmalloc family?

>+	else
>+		kbuf = vmalloc(count + 1);

You dropped the zeroing. This must be vzalloc.

> 	if (!kbuf)
> 		goto out;
> 

Alternatively, why not force count to be <PAGE_SIZE? What uses >PAGE_SIZE writes in proc/sys?

-Kees

-- 
Kees Cook