# Re: \[PATCH 07/20\] bcache: add initial data structures for nvm pages

- From: Coly Li \<hidden\>
- Date: 2021-02-11 03:59:12 
- List: linux-bcache
- Page: https://ratatoskr.run/linux-bcache/2021/02/6400958
- Thread: https://ratatoskr.run/linux-bcache/2021/02/6400921/t.md

```text
On 2/10/21 11:09 PM, Jens Axboe wrote:
> On 2/9/21 10:07 PM, Coly Li wrote:
>> +struct bch_nvm_pgalloc_recs {
>> +union {
>> +	struct {
>> +		struct bch_nvm_pages_owner_head	*owner;
>> +		struct bch_nvm_pgalloc_recs	*next;
>> +		__u8				magic[16];
>> +		__u8				owner_uuid[16];
>> +		__u32				size;
>> +		__u32				used;
>> +		__u64				_pad[4];
>> +		struct bch_pgalloc_rec		recs[];
>> +	};
>> +	__u8	pad[8192];
>> +};
>> +};
> 

Hi Jens,

> This doesn't look right in a user header, any user API should be 32-bit
> and 64-bit agnostic.

The above data structure is stored in NVDIMM as allocator's meta data.
It is designed to be directly accessed (in future update) as in-memory
object, but stored on non-volatiled memory like on-disk data structure.

To me, it is fine to use unsigned int/long/long long to define the
members, because nvdimm driver only works on 64bit platform. It is just
unclear to me which form/style I should use to define such data
structure. On one side they are stores as non-volatiled media, on other
side they are accessed directly as in-memory object...


> 
>> +struct bch_nvm_pages_owner_head {
>> +	__u8			uuid[16];
>> +	char			label[BCH_NVM_PAGES_LABEL_SIZE];
>> +	/* Per-namespace own lists */
>> +	struct bch_nvm_pgalloc_recs	*recs[BCH_NVM_PAGES_NAMESPACES_MAX];
>> +};
> 
> Same here.

For the above pointer, it is the same reason. In later version, such
object on NVDIMM will be referenced directly by an in-memory pointer
like we normally do for an in-memory object.

Therefore I do treat the data structure as in-memory object after the
DAX mapping accomplished. If not define it as an in-memory pointer, I
have to cast it into (void *) every time when I use it.


> 
>> +/* heads[0] is always for nvm_pages internal usage */
>> +struct bch_owner_list_head {
>> +union {
>> +	struct {
>> +		__u32				size;
>> +		__u32				used;
>> +		__u64				_pad[4];
>> +		struct bch_nvm_pages_owner_head	heads[];
>> +	};
>> +	__u8	pad[8192];
>> +};
>> +};
> 
> And here.
> 
>> +#define BCH_MAX_OWNER_LIST				\
>> +	((sizeof(struct bch_owner_list_head) -		\
>> +	 offsetof(struct bch_owner_list_head, heads)) /	\
>> +	 sizeof(struct bch_nvm_pages_owner_head))
>> +
>> +/* The on-media bit order is local CPU order */
>> +struct bch_nvm_pages_sb {
>> +	__u64			csum;
>> +	__u64			ns_start;
>> +	__u64			sb_offset;
>> +	__u64			version;
>> +	__u8			magic[16];
>> +	__u8			uuid[16];
>> +	__u32			page_size;
>> +	__u32			total_namespaces_nr;
>> +	__u32			this_namespace_nr;
>> +	union {
>> +		__u8		set_uuid[16];
>> +		__u64		set_magic;
>> +	};
> 
> This doesn't look like it packs right either.

This is my mimicry from bcache code, which uses the least significant 8
bytes from the randomly generated UUID as a magic number. It is solid
and not changed during the whole life cycle for the nvm pages set.


> 
>> +
>> +	__u64			flags;
>> +	__u64			seq;
>> +
>> +	__u64			feature_compat;
>> +	__u64			feature_incompat;
>> +	__u64			feature_ro_compat;
>> +
>> +	/* For allocable nvm pages from buddy systems */
>> +	__u64			pages_offset;
>> +	__u64			pages_total;
>> +
>> +	__u64			pad[8];
>> +
>> +	/* Only on the first name space */
>> +	struct bch_owner_list_head	*owner_list_head;
> 
> And here's another pointer...
> 

Same reason for I use it as an in-memory pointer.

The above definition is just using all the structures as in-memory
object, the difference is just they are non-volatiled after reboot.

Thanks.

Coly Li
```