[PATCH] ext4: fix wrong calculation of minlen in ext4_trim_fs()

[PATCH] ext4: fix wrong calculation of minlen in ext4_trim_fs()

[Chao Yu]

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@mit.edu>
Signed-off-by: Chao Yu <chao@kernel.org>
---
 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

Re: [PATCH] ext4: fix wrong calculation of minlen in ext4_trim_fs()

[Jan Kara]

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@mit.edu>
> Signed-off-by: Chao Yu <chao@kernel.org>

Looks good to me. Feel free to add:

Reviewed-by: Jan Kara <jack@suse.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
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

Re: [PATCH] ext4: fix wrong calculation of minlen in ext4_trim_fs()

[Chao Yu]

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@mit.edu>
>> Signed-off-by: Chao Yu <chao@kernel.org>
> 
> Looks good to me. Feel free to add:
> 
> Reviewed-by: Jan Kara <jack@suse.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
>>