linux-kernel - Re: [PATCH v8 01/11] btrfs: add raid stripe tree definitions

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Message-ID: <50cfa5a0-c209-430f-8c00-54ba41c3791d@wdc.com>
Date:   Wed, 13 Sep 2023 06:02:09 +0000
From:   Johannes Thumshirn <Johannes.Thumshirn@....com>
To:     "dsterba@...e.cz" <dsterba@...e.cz>
CC:     Chris Mason <clm@...com>, Josef Bacik <josef@...icpanda.com>,
        David Sterba <dsterba@...e.com>,
        Christoph Hellwig <hch@....de>,
        Naohiro Aota <Naohiro.Aota@....com>, Qu Wenruo <wqu@...e.com>,
        Damien Le Moal <dlemoal@...nel.org>,
        "linux-btrfs@...r.kernel.org" <linux-btrfs@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v8 01/11] btrfs: add raid stripe tree definitions

On 12.09.23 22:32, David Sterba wrote:
>> @@ -306,6 +306,16 @@ BTRFS_SETGET_FUNCS(timespec_nsec, struct btrfs_timespec, nsec, 32);
>>   BTRFS_SETGET_STACK_FUNCS(stack_timespec_sec, struct btrfs_timespec, sec, 64);
>>   BTRFS_SETGET_STACK_FUNCS(stack_timespec_nsec, struct btrfs_timespec, nsec, 32);
>>   
>> +BTRFS_SETGET_FUNCS(stripe_extent_encoding, struct btrfs_stripe_extent, encoding, 8);
> 
> What is encoding referring to?

At the moment (only) the RAID type. But in the future it can be expanded 
to all kinds of encodings, like Reed-Solomon, Butterfly-Codes, etc...

>>   static struct btrfs_lockdep_keyset {
>>   	u64			id;		/* root objectid */
>> -	/* Longest entry: btrfs-block-group-00 */
>> -	char			names[BTRFS_MAX_LEVEL][24];
>> +	/* Longest entry: btrfs-raid-stripe-tree-00 */
>> +	char			names[BTRFS_MAX_LEVEL][25];
> 
> Length of "btrfs-raid-stripe-tree-00" is 25, there should be +1 for the
> NUL, also length aligned to at least 4 is better.
> 

OK.

>>   	struct lock_class_key	keys[BTRFS_MAX_LEVEL];
>>   } btrfs_lockdep_keysets[] = {
>>   	{ .id = BTRFS_ROOT_TREE_OBJECTID,	DEFINE_NAME("root")	},
>> @@ -74,6 +74,7 @@ static struct btrfs_lockdep_keyset {
>>   	{ .id = BTRFS_UUID_TREE_OBJECTID,	DEFINE_NAME("uuid")	},
>>   	{ .id = BTRFS_FREE_SPACE_TREE_OBJECTID,	DEFINE_NAME("free-space") },
>>   	{ .id = BTRFS_BLOCK_GROUP_TREE_OBJECTID, DEFINE_NAME("block-group") },
>> +	{ .id = BTRFS_RAID_STRIPE_TREE_OBJECTID,DEFINE_NAME("raid-stripe-tree") },
> 
> The naming is without the "tree"

OK

>> @@ -73,6 +72,9 @@
>>   /* Holds the block group items for extent tree v2. */
>>   #define BTRFS_BLOCK_GROUP_TREE_OBJECTID 11ULL
>>   
>> +/* tracks RAID stripes in block groups. */
> 
> 	Tracks ...
> 

OK

>> +#define BTRFS_RAID_STRIPE_TREE_OBJECTID 12ULL
>> +
>>   /* device stats in the device tree */
>>   #define BTRFS_DEV_STATS_OBJECTID 0ULL
>>   
>> @@ -285,6 +287,8 @@
>>    */
>>   #define BTRFS_QGROUP_RELATION_KEY       246
>>   
>> +#define BTRFS_RAID_STRIPE_KEY		247
> 
> Any particular reason you chose 247 for the key number? It does not
> leave any gap after BTRFS_QGROUP_RELATION_KEY and before
> BTRFS_BALANCE_ITEM_KEY. If this is related to extents then please find
> more suitable group of keys where to put it.

Nope, it was just the last free spot.

> 
>> +
>>   /*
>>    * Obsolete name, see BTRFS_TEMPORARY_ITEM_KEY.
>>    */
>> @@ -719,6 +723,31 @@ struct btrfs_free_space_header {
>>   	__le64 num_bitmaps;
>>   } __attribute__ ((__packed__));
>>   
>> +struct btrfs_raid_stride {
>> +	/* btrfs device-id this raid extent lives on */
> 
> Comments should be full sentences.

OK

> 
>> +	__le64 devid;
>> +	/* physical location on disk */
>> +	__le64 physical;
>> +	/* length of stride on this disk */
>> +	__le64 length;
>> +};
> 
> __attribute__ ((__packed__));

The structure doesn't have any holes in it so packed is not needed.

I might also be misinformed, but doesn't packed potentially lead to bad 
code generation on some platforms?  I've always been under the 
impression that packed forces the compiler to do byte-wise loads and 
stores. But as I've said, I might be misinformed.

> 
>> +
>> +#define BTRFS_STRIPE_DUP	0
>> +#define BTRFS_STRIPE_RAID0	1
>> +#define BTRFS_STRIPE_RAID1	2
>> +#define BTRFS_STRIPE_RAID1C3	3
>> +#define BTRFS_STRIPE_RAID1C4	4
>> +#define BTRFS_STRIPE_RAID5	5
>> +#define BTRFS_STRIPE_RAID6	6
>> +#define BTRFS_STRIPE_RAID10	7
> 
> This is probably defining the on-disk format so some consistency is
> desired, there are already the BTRFS_BLOCK_GROUP_* types, from which the
> BTRFS_RAID_* are derive, so the BTRFS_STRIPE_* values should match the
> order and ideally the values themselves if possible.
> 
>> +
>> +struct btrfs_stripe_extent {
>> +	__u8 encoding;
>> +	__u8 reserved[7];
>> +	/* array of raid strides this stripe is composed of */
>> +	__DECLARE_FLEX_ARRAY(struct btrfs_raid_stride, strides);
> 
> Do we really whant to declare that as __DECLARE_FLEX_ARRAY? It's not a
> standard macro and obscures the definition.
> 

Indeed we do not anymore, as this version does introduce another u64 
before the strides array! I'll gladly get rid of it.