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

Harry Yoo
2025-04-02T04:15:32Z

On Tue, Apr 01, 2025 at 07:01:04AM -0700, Kees Cook wrote:
>
>
> On April 1, 2025 12:30:46 AM PDT, Yafang Shao <laoar.shao@gmail.com> 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@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);
>
> Why not move this check into kvmalloc family?

Hmm should this check really be in kvmalloc family?

I don't think users would expect kvmalloc() to implictly decide on using
vmalloc() without trying kmalloc() first, just because it's a high-order
allocation.

> >+ 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

--
Cheers,
Harry (formerly known as Hyeonggon)