Re: [PATCH] bcache: fix wrong BITMASK offset value for BDEV_CACHE_MODE

Coly Li
2022-05-13T15:21:39Z

On 7/20/21 6:32 PM, Lin Feng wrote:
> Original codes:
> BITMASK(CACHE_SYNC, struct cache_sb, flags, 0, 1);
> BITMASK(CACHE_DISCARD, struct cache_sb, flags, 1, 1);
> BITMASK(CACHE_REPLACEMENT, struct cache_sb, flags, 2, 3);
> ...
> BITMASK(BDEV_CACHE_MODE, struct cache_sb, flags, 0, 4);
>
> Should BDEV_CACHE_MODE bits start from bit-nr 5(2+3) else it overlaps
> with previous defined bit chunks, since we have 4 types of cache modes,
> BDEV_CACHE_MODE will overwrite CACHE_SYNC and CACHE_DISCARD bits.

The overlap won't happen, previous lines are for cache device, and what
you modified is for backing device.

And your patch changes the on-disk format, which is unacceptable anyway.

Coly Li

>
> This bug stays there since first upstream version of bcache, don't know
> why it lives so long, or am i missing something, please point me out
> if I'm wrong, thanks!
>
> Signed-off-by: Lin Feng <linf@wangsu.com>
> ---
> include/uapi/linux/bcache.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/uapi/linux/bcache.h b/include/uapi/linux/bcache.h
> index cf7399f03b71..dccd89756451 100644
> --- a/include/uapi/linux/bcache.h
> +++ b/include/uapi/linux/bcache.h
> @@ -288,7 +288,7 @@ BITMASK(CACHE_REPLACEMENT, struct cache_sb, flags, 2, 3);
> #define CACHE_REPLACEMENT_FIFO 1U
> #define CACHE_REPLACEMENT_RANDOM 2U
>
> -BITMASK(BDEV_CACHE_MODE, struct cache_sb, flags, 0, 4);
> +BITMASK(BDEV_CACHE_MODE, struct cache_sb, flags, 5, 4);
> #define CACHE_MODE_WRITETHROUGH 0U
> #define CACHE_MODE_WRITEBACK 1U
> #define CACHE_MODE_WRITEAROUND 2U