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

From: Wen Congyang <[email-masked]>
Date: Mon, 03 Sep 2012 13:51:10 +0800

At 09/01/2012 05:06 AM, Andrew Morton Wrote:
> 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?

Liu Jiang and Christoph Lameter discussed how to find slab page
in this mail:
https://lkml.org/lkml/2012/7/6/333.

> 
>> +		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_".

OK.

> 
>> +{
>> +	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?

Hmm, firmware_map_remove() was put in meminit section, and we call it
in this function, so __ref is added here.

Thanks
Wen Congyang

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