linux-kernel - Re: [RFC PATCH V3 01/17] mm: Add kscand kthread for PTE A bit scan

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Message-ID: <20251002141207.00007b58@huawei.com>
Date: Thu, 2 Oct 2025 14:12:07 +0100
From: Jonathan Cameron <jonathan.cameron@...wei.com>
To: Raghavendra K T <raghavendra.kt@....com>
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Subject: Re: [RFC PATCH V3 01/17] mm: Add kscand kthread for PTE A bit scan

On Thu, 14 Aug 2025 15:32:51 +0000
Raghavendra K T <raghavendra.kt@....com> wrote:

> Also add a config option for the same.
> 
> High level design:
> 
> While (1):
>   Scan the slowtier pages belonging to VMAs of a task.
>   Add to migation list.
> 
> A separate thread:
>   Migrate scanned pages to a toptier node based on heuristics.
> 
> The overall code is influenced by khugepaged design.
> 
> Signed-off-by: Raghavendra K T <raghavendra.kt@....com>

Hi Raghavendra,

A few comments inline.

Thanks,

Jonathan

> diff --git a/mm/kscand.c b/mm/kscand.c
> new file mode 100644
> index 000000000000..f7bbbc70c86a
> --- /dev/null
> +++ b/mm/kscand.c
> @@ -0,0 +1,163 @@
> +// SPDX-License-Identifier: GPL-2.0
> +#include <linux/mm.h>

Good to pick an order for includes.   Various options, my personal
preference is alphabetical with groups of linux/* asm/* and local headers.

> +#include <linux/mm_types.h>
> +#include <linux/sched.h>
> +#include <linux/sched/mm.h>
> +#include <linux/mmu_notifier.h>
> +#include <linux/swap.h>
> +#include <linux/mm_inline.h>
> +#include <linux/kthread.h>
> +#include <linux/string.h>
> +#include <linux/delay.h>
> +#include <linux/cleanup.h>
> +
> +#include <asm/pgalloc.h>
> +#include "internal.h"
...


> +static void kscand_wait_work(void)
> +{
> +	const unsigned long scan_sleep_jiffies =
> +		msecs_to_jiffies(kscand_scan_sleep_ms);
> +
> +	if (!scan_sleep_jiffies)
> +		return;
> +
> +	kscand_sleep_expire = jiffies + scan_sleep_jiffies;
> +
> +	/* Allows kthread to pause scanning */
> +	wait_event_timeout(kscand_wait, kscand_should_wakeup(),
> +			scan_sleep_jiffies);
> +}

Trivial but more consistent I think to have a blank line here.

> +static void kscand_do_scan(void)
> +{
> +	unsigned long iter = 0, mms_to_scan;
> +
> +	mms_to_scan = READ_ONCE(kscand_mms_to_scan);
Slightly nicer perhaps as

	unsigned long iter = 0;
	unsigned long mms_to_scan = READ_ONCE(kscand_mms_to_scan);

> +
> +	while (true) {
> +		if (unlikely(kthread_should_stop()) ||
> +			!READ_ONCE(kscand_scan_enabled))
> +			break;
> +
> +		if (kscand_has_work())
> +			msleep(100);
> +
> +		iter++;
> +
> +		if (iter >= mms_to_scan)
> +			break;
> +		cond_resched();
> +	}
> +}
> +

> +
> +static int start_kscand(void)
> +{
> +	struct task_struct *kthread;
> +
> +	guard(mutex)(&kscand_mutex);
> +
> +	if (kscand_thread)
> +		return 0;
> +
> +	kthread = kthread_run(kscand, NULL, "kscand");
> +	if (IS_ERR(kscand_thread)) {

That looks like the wrong variable to check.

> +		pr_err("kscand: kthread_run(kscand) failed\n");
> +		return PTR_ERR(kthread);
> +	}
> +
> +	kscand_thread = kthread;
> +	pr_info("kscand: Successfully started kscand");
> +
> +	if (!list_empty(&kscand_scan.mm_head))

You have kscand_has_work() for this.

> +		wake_up_interruptible(&kscand_wait);
> +
> +	return 0;
> +}
> +
> +static int stop_kscand(void)
> +{
> +	guard(mutex)(&kscand_mutex);
> +
> +	if (kscand_thread) {
> +		kthread_stop(kscand_thread);
> +		kscand_thread = NULL;
> +	}
> +
> +	return 0;
> +}
> +
> +static int __init kscand_init(void)
> +{
> +	int err;
> +
> +	err = start_kscand();
> +	if (err)
> +		goto err_kscand;
This breaks the nice principle of side effect free calls
on error. If start_kscand() has failed, I'd not really expect to have
have stop explicitly.   I'm not sure you actually do need to do so.


> +
> +	return 0;
> +
> +err_kscand:
> +	stop_kscand();
> +
> +	return err;
> +}
> +subsys_initcall(kscand_init);