linux-kernel - Re: [PATCH] bcache: kill macro MAX_CACHES_PER

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Date:   Sat, 30 Oct 2021 23:19:01 +0800
From:   Coly Li <colyli@...e.de>
To:     Lin Feng <linf@...gsu.com>
Cc:     linux-bcache@...r.kernel.org, linux-kernel@...r.kernel.org,
        kent.overstreet@...il.com
Subject: Re: [PATCH] bcache: kill macro MAX_CACHES_PER_SET

On 10/29/21 8:59 PM, Lin Feng wrote:
> Commit 697e23495c94f0380c1ed8b11f830b92b64c99ea
> ("bcache: explicitly make cache_set only have single cache")
> explicitly makes a cache_set only have single cache and based on the
> fact that historily only one cache is ever used in the cache set, so
> macro defination for MAX_CACHES_PER_SET as 8 is misleading now.
>
> In fact it should be redefined to 1 and valid number fo sb.nr_in_set
> should be 1 and sb.nr_this_dev should always be 0.
>
> But jset's disk layout replies on MAX_CACHES_PER_SET(8), so replace it
> with a hardcoded number 8.
>
> Signed-off-by: Lin Feng <linf@...gsu.com>
> ---
>   drivers/md/bcache/bcache.h  | 2 +-
>   drivers/md/bcache/super.c   | 4 +---
>   include/uapi/linux/bcache.h | 4 ++--
>   3 files changed, 4 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/md/bcache/bcache.h b/drivers/md/bcache/bcache.h
> index 5fc989a6d452..a4a410a178c0 100644
> --- a/drivers/md/bcache/bcache.h
> +++ b/drivers/md/bcache/bcache.h
> @@ -833,7 +833,7 @@ static inline uint8_t ptr_stale(struct cache_set *c, const struct bkey *k,
>   static inline bool ptr_available(struct cache_set *c, const struct bkey *k,
>   				 unsigned int i)
>   {
> -	return (PTR_DEV(k, i) < MAX_CACHES_PER_SET) && c->cache;
> +	return (PTR_DEV(k, i) == 0) && c->cache;
>   }
>   
>   /* Btree key macros */
> diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
> index f2874c77ff79..2253044c9289 100644
> --- a/drivers/md/bcache/super.c
> +++ b/drivers/md/bcache/super.c
> @@ -140,9 +140,7 @@ static const char *read_super_common(struct cache_sb *sb,  struct block_device *
>   		goto err;
>   
>   	err = "Bad cache device number in set";
> -	if (!sb->nr_in_set ||
> -	    sb->nr_in_set <= sb->nr_this_dev ||
> -	    sb->nr_in_set > MAX_CACHES_PER_SET)
> +	if (sb->nr_in_set != 1 || sb->nr_this_dev != 0)
>   		goto err;
>   
>   	err = "Journal buckets not sequential";

Hi Feng,

The above changes are fine to me. But let's keep the below part as what 
it is at this moment.

Thanks.

Coly Li

> diff --git a/include/uapi/linux/bcache.h b/include/uapi/linux/bcache.h
> index cf7399f03b71..4beb3e7826ca 100644
> --- a/include/uapi/linux/bcache.h
> +++ b/include/uapi/linux/bcache.h
> @@ -155,7 +155,6 @@ static inline struct bkey *bkey_idx(const struct bkey *k, unsigned int nr_keys)
>   #define SB_LABEL_SIZE			32
>   #define SB_JOURNAL_BUCKETS		256U
>   /* SB_JOURNAL_BUCKETS must be divisible by BITS_PER_LONG */
> -#define MAX_CACHES_PER_SET		8
>   
>   #define BDEV_DATA_START_DEFAULT		16	/* sectors */
>   
> @@ -356,7 +355,8 @@ struct jset {
>   	__u16			btree_level;
>   	__u16			pad[3];
>   
> -	__u64			prio_bucket[MAX_CACHES_PER_SET];
> +	/* only a single cache is available */
> +	__u64			prio_bucket[8];
>   
>   	union {
>   		struct bkey	start[0];