linux-kernel - Re: [PATCH] bcache: fix wrong BITMASK offset value for BDEV_CACHE

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Date:   Mon, 16 May 2022 09:50:22 +0800
From:   Lin Feng <linf@...gsu.com>
To:     Coly Li <colyli@...e.de>
Cc:     axboe@...nel.dk, linux-kernel@...r.kernel.org,
        linux-bcache@...r.kernel.org
Subject: Re: [PATCH] bcache: fix wrong BITMASK offset value for
 BDEV_CACHE_MODE



On 5/13/22 23:20, Coly Li wrote:
> 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.
> 

Yes, you are right, this patch happened as I started reading bcache codes,
I did not get a whole picture of bcache at that time, I'm sorry for making
noise.

Thank you, Coly!

linfeng
> 
> 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@...gsu.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
>