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@xxxxxxx/
> > Suggested-by: Theodore Ts'o <tytso@xxxxxxx>
> > Signed-off-by: Chao Yu <chao@xxxxxxxxxx>
>
> Looks good to me. Feel free to add:
>
> Reviewed-by: Jan Kara <jack@xxxxxxx>
>
> 								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