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Date:   Mon, 13 Jun 2022 10:10:08 +0200
From:   Oscar Salvador <osalvador@suse.de>
To:     Muchun Song <songmuchun@bytedance.com>
Cc:     mike.kravetz@oracle.com, david@redhat.com,
        akpm@linux-foundation.org, corbet@lwn.net, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org
Subject: Re: [PATCH 2/6] mm: hugetlb_vmemmap: optimize vmemmap_optimize_mode
 handling
Message-ID: <Yqbw4IYwtLQaoarB@localhost.localdomain>
References: <20220613063512.17540-1-songmuchun@bytedance.com>
 <20220613063512.17540-3-songmuchun@bytedance.com>
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On Mon, Jun 13, 2022 at 02:35:08PM +0800, Muchun Song wrote:
> We hold an another reference to hugetlb_optimize_vmemmap_key when
> making vmemmap_optimize_mode on, because we use static_key to tell
> memory_hotplug that memory_hotplug.memmap_on_memory should be
> overridden.  However, this rule has gone when we have introduced
> SECTION_CANNOT_OPTIMIZE_VMEMMAP.  Therefore, we could simplify
> vmemmap_optimize_mode handling by not holding an another reference
> to hugetlb_optimize_vmemmap_key.
> 
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>

LGTM, and it looks way nicer, so

Reviewed-by: Oscar Salvador <osalvador@suse.de>

One question below though

> -static enum vmemmap_optimize_mode vmemmap_optimize_mode =
> +static bool vmemmap_optimize_enabled =
>  	IS_ENABLED(CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP_DEFAULT_ON);

So, by default vmemmap_optimize_enabled will be on if we have
CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP_DEFAULT_ON, but we can always override that
via cmdline, as below, right?


>  
> -static void vmemmap_optimize_mode_switch(enum vmemmap_optimize_mode to)
> -{
> -	if (vmemmap_optimize_mode == to)
> -		return;
> -
> -	if (to == VMEMMAP_OPTIMIZE_OFF)
> -		static_branch_dec(&hugetlb_optimize_vmemmap_key);
> -	else
> -		static_branch_inc(&hugetlb_optimize_vmemmap_key);
> -	WRITE_ONCE(vmemmap_optimize_mode, to);
> -}
> -
>  static int __init hugetlb_vmemmap_early_param(char *buf)
>  {
> -	bool enable;
> -	enum vmemmap_optimize_mode mode;
> -
> -	if (kstrtobool(buf, &enable))
> -		return -EINVAL;
> -
> -	mode = enable ? VMEMMAP_OPTIMIZE_ON : VMEMMAP_OPTIMIZE_OFF;
> -	vmemmap_optimize_mode_switch(mode);
> -
> -	return 0;
> +	return kstrtobool(buf, &vmemmap_optimize_enabled);
>  }
>  early_param("hugetlb_free_vmemmap", hugetlb_vmemmap_early_param);
>  
> @@ -103,7 +76,7 @@ static unsigned int optimizable_vmemmap_pages(struct hstate *h,
>  	unsigned long pfn = page_to_pfn(head);
>  	unsigned long end = pfn + pages_per_huge_page(h);
>  
> -	if (READ_ONCE(vmemmap_optimize_mode) == VMEMMAP_OPTIMIZE_OFF)
> +	if (!READ_ONCE(vmemmap_optimize_enabled))
>  		return 0;
>  
>  	for (; pfn < end; pfn += PAGES_PER_SECTION) {
> @@ -155,7 +128,6 @@ void __init hugetlb_vmemmap_init(struct hstate *h)
>  
>  	if (!is_power_of_2(sizeof(struct page))) {
>  		pr_warn_once("cannot optimize vmemmap pages because \"struct page\" crosses page boundaries\n");
> -		static_branch_disable(&hugetlb_optimize_vmemmap_key);
>  		return;
>  	}
>  
> @@ -176,36 +148,13 @@ void __init hugetlb_vmemmap_init(struct hstate *h)
>  }
>  
>  #ifdef CONFIG_PROC_SYSCTL
> -static int hugetlb_optimize_vmemmap_handler(struct ctl_table *table, int write,
> -					    void *buffer, size_t *length,
> -					    loff_t *ppos)
> -{
> -	int ret;
> -	enum vmemmap_optimize_mode mode;
> -	static DEFINE_MUTEX(sysctl_mutex);
> -
> -	if (write && !capable(CAP_SYS_ADMIN))
> -		return -EPERM;
> -
> -	mutex_lock(&sysctl_mutex);
> -	mode = vmemmap_optimize_mode;
> -	table->data = &mode;
> -	ret = proc_dointvec_minmax(table, write, buffer, length, ppos);
> -	if (write && !ret)
> -		vmemmap_optimize_mode_switch(mode);
> -	mutex_unlock(&sysctl_mutex);
> -
> -	return ret;
> -}
> -
>  static struct ctl_table hugetlb_vmemmap_sysctls[] = {
>  	{
>  		.procname	= "hugetlb_optimize_vmemmap",
> -		.maxlen		= sizeof(enum vmemmap_optimize_mode),
> +		.data		= &vmemmap_optimize_enabled,
> +		.maxlen		= sizeof(int),
>  		.mode		= 0644,
> -		.proc_handler	= hugetlb_optimize_vmemmap_handler,
> -		.extra1		= SYSCTL_ZERO,
> -		.extra2		= SYSCTL_ONE,
> +		.proc_handler	= proc_dobool,
>  	},
>  	{ }
>  };
> -- 
> 2.11.0
> 
> 

-- 
Oscar Salvador
SUSE Labs