Re: [PATCH] bcache: stop using the deprecated get_seconds()

[Coly Li <colyli@suse.de>]

On 2018/6/20 5:51 PM, Arnd Bergmann wrote:
> bcache uses get_seconds() to read the current system time and store it in
> the superblock as well as in uuid_entry structures that are user visible.
> 
> This changes over from the deprecated function to
> ktime_get_real_seconds(), which returns a 64-bit timestamp as it
> should. Unfortunately, the two structures are still limited to 32 bits,
> so this won't fix any real problems. Let's at least document that
> properly, in case we get an updated format in the future it can be
> fixed. Until then, we still have some time, and checking the tools
> at https://github.com/koverstreet/bcache-tools reveals no access to
> any of them.
> 
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Hi Arnd,

Firstly thanks to your patch, especially the detailed information in
patch log, it helps me to understand the problem more easier.

From the information, it seems the problem is current 32bit time stamp
will be overflow in 2106. So it will be 88 years later, which I have to
say I don't care.

Also for get_seconds() which works well for current code as many other
places call it, I would like to keep it.

But the code comments in bcache.h looks good, if you want to send a
single patch for the code comments for bcache.h, I would like to see it.

Thanks.

Coly Li

> ---
>  drivers/md/bcache/super.c   | 23 +++++++++++++++++------
>  include/uapi/linux/bcache.h |  4 ++--
>  2 files changed, 19 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
> index fa4058e43202..aa9790ee5cb5 100644
> --- a/drivers/md/bcache/super.c
> +++ b/drivers/md/bcache/super.c
> @@ -53,6 +53,17 @@ struct workqueue_struct *bcache_wq;
>  /* limitation of bcache devices number on single system */
>  #define BCACHE_DEVICE_IDX_MAX	((1U << MINORBITS)/BCACHE_MINORS)
>  
> +/*
> + * various timestamp fields in the superblock are unfortunately
> + * limited to 32 bits, which will lead to overflow in year 2106.
> + *
> + * If we ever get a new superblock format, that should be fixed.
> + */
> +static inline u32 bcache_get_realtime32(void)
> +{
> +	return (u32)ktime_get_real_seconds();
> +}
> +
>  /* Superblock */
>  
>  static const char *read_super(struct cache_sb *sb, struct block_device *bdev,
> @@ -181,7 +192,7 @@ static const char *read_super(struct cache_sb *sb, struct block_device *bdev,
>  		goto err;
>  	}
>  
> -	sb->last_mount = get_seconds();
> +	sb->last_mount = bcache_get_realtime32();
>  	err = NULL;
>  
>  	get_page(bh->b_page);
> @@ -701,7 +712,7 @@ static void bcache_device_detach(struct bcache_device *d)
>  
>  		SET_UUID_FLASH_ONLY(u, 0);
>  		memcpy(u->uuid, invalid_uuid, 16);
> -		u->invalidated = cpu_to_le32(get_seconds());
> +		u->invalidated = cpu_to_le32(bcache_get_realtime32());
>  		bch_uuid_write(d->c);
>  	}
>  
> @@ -1027,7 +1038,7 @@ void bch_cached_dev_detach(struct cached_dev *dc)
>  int bch_cached_dev_attach(struct cached_dev *dc, struct cache_set *c,
>  			  uint8_t *set_uuid)
>  {
> -	uint32_t rtime = cpu_to_le32(get_seconds());
> +	uint32_t rtime = cpu_to_le32(bcache_get_realtime32());
>  	struct uuid_entry *u;
>  	struct cached_dev *exist_dc, *t;
>  
> @@ -1070,7 +1081,7 @@ int bch_cached_dev_attach(struct cached_dev *dc, struct cache_set *c,
>  	    (BDEV_STATE(&dc->sb) == BDEV_STATE_STALE ||
>  	     BDEV_STATE(&dc->sb) == BDEV_STATE_NONE)) {
>  		memcpy(u->uuid, invalid_uuid, 16);
> -		u->invalidated = cpu_to_le32(get_seconds());
> +		u->invalidated = cpu_to_le32(bcache_get_realtime32());
>  		u = NULL;
>  	}
>  
> @@ -1390,7 +1401,7 @@ int bch_flash_dev_create(struct cache_set *c, uint64_t size)
>  
>  	get_random_bytes(u->uuid, 16);
>  	memset(u->label, 0, 32);
> -	u->first_reg = u->last_reg = cpu_to_le32(get_seconds());
> +	u->first_reg = u->last_reg = cpu_to_le32(bcache_get_realtime32());
>  
>  	SET_UUID_FLASH_ONLY(u, 1);
>  	u->sectors = size >> 9;
> @@ -1894,7 +1905,7 @@ static void run_cache_set(struct cache_set *c)
>  		goto err;
>  
>  	closure_sync(&cl);
> -	c->sb.last_mount = get_seconds();
> +	c->sb.last_mount = bcache_get_realtime32();
>  	bcache_write_super(c);
>  
>  	list_for_each_entry_safe(dc, t, &uncached_devices, list)
> diff --git a/include/uapi/linux/bcache.h b/include/uapi/linux/bcache.h
> index 821f71a2e48f..8d19e02d752a 100644
> --- a/include/uapi/linux/bcache.h
> +++ b/include/uapi/linux/bcache.h
> @@ -195,7 +195,7 @@ struct cache_sb {
>  	};
>  	};
>  
> -	__u32			last_mount;	/* time_t */
> +	__u32			last_mount;	/* time overflow in y2106 */
>  
>  	__u16			first_bucket;
>  	union {
> @@ -318,7 +318,7 @@ struct uuid_entry {
>  		struct {
>  			__u8	uuid[16];
>  			__u8	label[32];
> -			__u32	first_reg;
> +			__u32	first_reg; /* time overflow in y2106 */
>  			__u32	last_reg;
>  			__u32	invalidated;
>  
>