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

Yafang Shao
2025-04-02T08:42:47Z

On Wed, Apr 2, 2025 at 12:15 PM Harry Yoo <harry.yoo@oracle.com> wrote:
>
> 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?

Modifying the existing kvmalloc functions risks performance regressions.
Could we instead introduce a new variant like vkmalloc() (favoring
vmalloc over kmalloc) or kvmalloc_costless()?

>
> 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.
>

--
Regards
Yafang