Re: [PATCH v3] mm/page_reporting: Add page_reporting_delay_ms module parameter

[Pratyush Mallick]

> Of course we'd prefer some sort of self-tuning so the kernel
> automatically avoids the situation.  But the user's expectation that
> reporting occurs at a fixed frequency messes up that concept.

That's definitely a good direction to ponder for future iterations. :')
Thanks for handling the merge resolution.

Regards,
Pratyush

On Mon, Jul 27, 2026 at 6:13 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Mon, 27 Jul 2026 23:05:45 +0000 pratmal@xxxxxxxxxx wrote:
>
> > From: Pratyush Mallick <pratmal@xxxxxxxxxx>
> >
> > Currently, the free page reporting uses a hardcoded delay of
> > (2 HZ) between reporting intervals. While this is a reasonable
> > default, it lacks the flexibility to adapt to varying guest workloads.
> >
> > A low delay allows aggressive memory reclamation, returning unused
> > pages to the host as quickly as possible. However, during spiky
> > allocation/free churn, this immediate reporting can lead to a severe
> > performance penalty (nested page faults) as the guest re-allocates memory
> > that the host has just unmapped. In these scenarios, there is benefit
> > from increasing the delay to batch free pages over a longer window,
> > absorbing the churn without hypercall and re-fault overhead.
> >
> > This patch exposes the delay as a module parameter:
> > /sys/module/page_reporting/parameters/page_reporting_delay_ms, measured
> > in milliseconds and defaults to 2000ms.
>
> Of course we'd prefer some sort of self-tuning so the kernel
> automatically avoids the situation.  But the user's expectation that
> reporting occurs at a fixed frequency messes up that concept.
>
>
> > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
>
> lgtm.  It conflicts with Link Lin's "mm/page_reporting: use
> system_freezable_wq to fix UAF during suspend".  Easy resolution:
>
>
>  Documentation/admin-guide/kernel-parameters.txt |    6 ++++
>  mm/page_reporting.c                             |   19 ++++++++------
>  2 files changed, 17 insertions(+), 8 deletions(-)
>
> --- a/Documentation/admin-guide/kernel-parameters.txt~mm-page_reporting-add-page_reporting_delay_ms-module-parameter
> +++ a/Documentation/admin-guide/kernel-parameters.txt
> @@ -4813,6 +4813,12 @@ Kernel parameters
>                         Adjust the minimal page reporting order. The page
>                         reporting is disabled when it exceeds MAX_PAGE_ORDER.
>
> +       page_reporting.page_reporting_delay_ms=
> +                       [KNL] Free page reporting delay in milliseconds
> +                       Format: <unsigned integer>
> +                       Adjust the delay in milliseconds between free page
> +                       reporting intervals. Default is 2000 (2 seconds).
> +
>         panic=          [KNL] Kernel behaviour on panic: delay <timeout>
>                         timeout > 0: seconds before rebooting
>                         timeout = 0: wait forever
> --- a/mm/page_reporting.c~mm-page_reporting-add-page_reporting_delay_ms-module-parameter
> +++ a/mm/page_reporting.c
> @@ -48,7 +48,11 @@ MODULE_PARM_DESC(page_reporting_order, "
>   */
>  EXPORT_SYMBOL_GPL(page_reporting_order);
>
> -#define PAGE_REPORTING_DELAY   (2 * HZ)
> +static unsigned int page_reporting_delay_ms = 2 * MSEC_PER_SEC;
> +module_param(page_reporting_delay_ms, uint, 0644);
> +MODULE_PARM_DESC(page_reporting_delay_ms,
> +                "Set page reporting delay in milliseconds");
> +
>  static struct page_reporting_dev_info __rcu *pr_dev_info __read_mostly;
>
>  enum {
> @@ -77,12 +81,11 @@ __page_reporting_request(struct page_rep
>                 return;
>
>         /*
> -        * Delay the start of work to allow a sizable queue to build. For
> -        * now we are limiting this to running no more than once every
> -        * couple of seconds.
> +        * Delay the start of work to allow a sizable queue to build.
> +        * We limit this based on page_reporting_delay_ms.
>          */
>         queue_delayed_work(system_freezable_wq, &prdev->work,
> -                          PAGE_REPORTING_DELAY);
> +                          msecs_to_jiffies(page_reporting_delay_ms));
>  }
>
>  /* notify prdev of free page reporting request */
> @@ -337,13 +340,13 @@ static void page_reporting_process(struc
>  err_out:
>         /*
>          * If the state has reverted back to requested then there may be
> -        * additional pages to be processed. We will defer for 2s to allow
> -        * more pages to accumulate.
> +        * additional pages to be processed. We will defer by
> +        * page_reporting_delay_ms to allow more pages to accumulate.
>          */
>         state = atomic_cmpxchg(&prdev->state, state, PAGE_REPORTING_IDLE);
>         if (state == PAGE_REPORTING_REQUESTED)
>                 queue_delayed_work(system_freezable_wq, &prdev->work,
> -                                  PAGE_REPORTING_DELAY);
> +                                  msecs_to_jiffies(page_reporting_delay_ms));
>  }
>
>  static DEFINE_MUTEX(page_reporting_mutex);
> _
>