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

Michal Hocko
2025-04-02T12:17:26Z

On Wed 02-04-25 11:25:12, Vlastimil Babka wrote:
> On 4/2/25 10:42, Yafang Shao wrote:
> > 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()?
>
> We have gfp flags and kmalloc_gfp_adjust() to moderate how aggressive
> kmalloc() is before the vmalloc() fallback. It does e.g.:
>
> if (!(flags & __GFP_RETRY_MAYFAIL))
> flags |= __GFP_NORETRY;
>
> However if your problem is kcompactd utilization then the kmalloc() attempt
> would have to avoid ___GFP_KSWAPD_RECLAIM to avoid waking up kswapd and then
> kcompactd. Should we remove the flag for costly orders? Dunno. Ideally the
> deferred compaction mechanism would limit the issue in the first place.

Yes, triggering heavy compation for costly allocations seems to be quite
bad. We have GFP_RETRY_MAYFAIL for that purpose if the caller really
needs the allocation to try really hard.

> The ad-hoc fixing up of a particular place (/proc files reading) or creating
> a new vkmalloc() and then spreading its use as you see other places
> triggering the issue seems quite suboptimal to me.

Yes I absolutely agree.
--
Michal Hocko
SUSE Labs