linux-kernel - Re: [PATCH] bcache: stop using the deprecated get

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Date:   Wed, 20 Jun 2018 23:51:54 +0800
From:   Coly Li <colyli@...e.de>
To:     Arnd Bergmann <arnd@...db.de>,
        Kent Overstreet <kent.overstreet@...il.com>
Cc:     y2038@...ts.linaro.org, Jens Axboe <axboe@...nel.dk>,
        Michael Lyle <mlyle@...e.org>,
        Tang Junhui <tang.junhui@....com.cn>,
        Hannes Reinecke <hare@...e.com>,
        Bart Van Assche <bart.vanassche@....com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        linux-bcache@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] bcache: stop using the deprecated get_seconds()

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@...db.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;
>  
>