Subject: Re: [RFC v8 PATCH 08/20] memory-hotplug: remove /sys/firmware/memmap/X sysfs

From: Andrew Morton <[email-masked]>
Date: Fri, 31 Aug 2012 14:06:23 -0700

On Tue, 28 Aug 2012 18:00:15 +0800
[email-masked] wrote:

> From: Yasuaki Ishimatsu <[email-masked]>
> 
> When (hot)adding memory into system, /sys/firmware/memmap/X/{end, start, type}
> sysfs files are created. But there is no code to remove these files. The patch
> implements the function to remove them.
> 
> Note : The code does not free firmware_map_entry since there is no way to free
>        memory which is allocated by bootmem.
> 
> ....
>
> +#define to_memmap_entry(obj) container_of(obj, struct firmware_map_entry, kobj)

It would be better to implement this as an inlined C function.  That
has improved type safety and improved readability.

> +static void release_firmware_map_entry(struct kobject *kobj)
> +{
> +	struct firmware_map_entry *entry = to_memmap_entry(kobj);
> +	struct page *page;
> +
> +	page = virt_to_page(entry);
> +	if (PageSlab(page) || PageCompound(page))

That PageCompound() test looks rather odd.  Why is this done?

> +		kfree(entry);
> +
> +	/* There is no way to free memory allocated from bootmem*/
> +}

This function is a bit ugly - poking around in page flags to determine
whether or not the memory came from bootmem.  It would be cleaner to
use a separate boolean.  Although I guess we can live with it as you
have it here.

>  static struct kobj_type memmap_ktype = {
> +	.release	= release_firmware_map_entry,
>  	.sysfs_ops	= &memmap_attr_ops,
>  	.default_attrs	= def_attrs,
>  };
> @@ -123,6 +139,16 @@ static int firmware_map_add_entry(u64 start, u64 end,
>  	return 0;
>  }
>  
> +/**
> + * firmware_map_remove_entry() - Does the real work to remove a firmware
> + * memmap entry.
> + * @entry: removed entry.
> + **/
> +static inline void firmware_map_remove_entry(struct firmware_map_entry *entry)
> +{
> +	list_del(&entry->list);
> +}

Is there no locking  to protect that list?

>  /*
>   * Add memmap entry on sysfs
>   */
> @@ -144,6 +170,31 @@ static int add_sysfs_fw_map_entry(struct firmware_map_entry *entry)
>  	return 0;
>  }
>  
> +/*
> + * Remove memmap entry on sysfs
> + */
> +static inline void remove_sysfs_fw_map_entry(struct firmware_map_entry *entry)
> +{
> +	kobject_put(&entry->kobj);
> +}
> +
> +/*
> + * Search memmap entry
> + */
> +
> +struct firmware_map_entry * __meminit
> +find_firmware_map_entry(u64 start, u64 end, const char *type)

A better name would be firmware_map_find_entry().  To retain the (good)
convention that symbols exported from here all start with
"firmware_map_".

> +{
> +	struct firmware_map_entry *entry;
> +
> +	list_for_each_entry(entry, &map_entries, list)
> +		if ((entry->start == start) && (entry->end == end) &&
> +		    (!strcmp(entry->type, type)))
> +			return entry;
> +
> +	return NULL;
> +}
> +
>  /**
>   * firmware_map_add_hotplug() - Adds a firmware mapping entry when we do
>   * memory hotplug.
> @@ -196,6 +247,32 @@ int __init firmware_map_add_early(u64 start, u64 end, const char *type)
>  	return firmware_map_add_entry(start, end, type, entry);
>  }
>  
> +/**
> + * firmware_map_remove() - remove a firmware mapping entry
> + * @start: Start of the memory range.
> + * @end:   End of the memory range.
> + * @type:  Type of the memory range.
> + *
> + * removes a firmware mapping entry.
> + *
> + * Returns 0 on success, or -EINVAL if no entry.
> + **/
> +int __meminit firmware_map_remove(u64 start, u64 end, const char *type)
> +{
> +	struct firmware_map_entry *entry;
> +
> +	entry = find_firmware_map_entry(start, end - 1, type);
> +	if (!entry)
> +		return -EINVAL;
> +
> +	firmware_map_remove_entry(entry);
> +
> +	/* remove the memmap entry */
> +	remove_sysfs_fw_map_entry(entry);
> +
> +	return 0;
> +}

Again, the lack of locking looks bad.

> ...
>
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1052,9 +1052,9 @@ int offline_memory(u64 start, u64 size)
>  	return 0;
>  }
>  
> -int remove_memory(int nid, u64 start, u64 size)
> +int __ref remove_memory(int nid, u64 start, u64 size)

Why was __ref added?

>  {
> -	int ret = -EBUSY;
> +	int ret = 0;
>  	lock_memory_hotplug();
>  	/*
>  	 * The memory might become online by other task, even if you offine it.
>
> ...
>