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Date:   Mon, 5 Jun 2023 12:15:32 +0800
From:   Chao Yu <chao@...nel.org>
To:     Theodore Ts'o <tytso@....edu>
Cc:     Andreas Dilger <adilger.kernel@...ger.ca>,
        linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jan Kara <jack@...e.cz>
Subject: Re: [PATCH] ext4: fix wrong calculation of minlen in ext4_trim_fs()

Ping,

On 2023/4/21 17:35, Jan Kara wrote:
> On Thu 06-04-23 23:06:21, Chao Yu wrote:
>> As Ted pointed out as below:
>>
>> "
>> However the minlen variable in ext4_trim_fs is in units of *clusters*.
>> And so it gets rounded up two places.  The first time is when it is
>> converted into units of a cluster:
>>
>> 	minlen = EXT4_NUM_B2C(EXT4_SB(sb),
>> 			      range->minlen >> sb->s_blocksize_bits);
>>
>> Oh, and by the way, that first conversion is not correct as currently
>> written in ext4_fs_trim().   It should be
>>
>> 	minlen = (range->minlen + EXT4_CLUSTER_SIZE(sb) - 1) >>
>> 		(sb->s_blocksize_bits + EXT4_CLUSTER_BITS(sb));
>> "
>>
>> The reason is if range->minlen is smaller than block size of ext4,
>> original calculation "range->minlen >> sb->s_blocksize_bits" may
>> return zero, but since EXT4_NUM_B2C() expects a non-zero in-parameter,
>> so it needs to round up range->minlen to cluster size directly as above.
>>
>> Link: https://lore.kernel.org/lkml/20230311031843.GF860405@mit.edu/
>> Suggested-by: Theodore Ts'o <tytso@....edu>
>> Signed-off-by: Chao Yu <chao@...nel.org>
> 
> Looks good to me. Feel free to add:
> 
> Reviewed-by: Jan Kara <jack@...e.cz>
> 
> 								Honza
> 
>> ---
>>   fs/ext4/mballoc.c | 4 ++--
>>   1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
>> index 5b2ae37a8b80..d8b9d6a83d1e 100644
>> --- a/fs/ext4/mballoc.c
>> +++ b/fs/ext4/mballoc.c
>> @@ -6478,8 +6478,8 @@ int ext4_trim_fs(struct super_block *sb, struct fstrim_range *range)
>>   
>>   	start = range->start >> sb->s_blocksize_bits;
>>   	end = start + (range->len >> sb->s_blocksize_bits) - 1;
>> -	minlen = EXT4_NUM_B2C(EXT4_SB(sb),
>> -			      range->minlen >> sb->s_blocksize_bits);
>> +	minlen = (range->minlen + EXT4_CLUSTER_SIZE(sb) - 1) >>
>> +			(sb->s_blocksize_bits + EXT4_CLUSTER_BITS(sb));
>>   
>>   	if (minlen > EXT4_CLUSTERS_PER_GROUP(sb) ||
>>   	    start >= max_blks ||
>> -- 
>> 2.25.1
>>

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