ext4: avoid trim error on fs with small groups

A user reported FITRIM ioctl failing for him on ext4 on some devices
without apparent reason.  After some debugging we've found out that
these devices (being LVM volumes) report rather large discard
granularity of 42MB and the filesystem had 1k blocksize and thus group
size of 8MB. Because ext4 FITRIM implementation puts discard
granularity into minlen, ext4_trim_fs() declared the trim request as
invalid. However just silently doing nothing seems to be a more
appropriate reaction to such combination of parameters since user did
not specify anything wrong.

CC: Lukas Czerner <lczerner@redhat.com>
Fixes: 5c2ed62f ("ext4: Adjust minlen with discard_granularity in the FITRIM ioctl")
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20211112152202.26614-1-jack@suse.czSigned-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/ioctl.c

@@ -1114,8 +1114,6 @@ static long __ext4_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
 		    sizeof(range)))
 			return -EFAULT;
 
-		range.minlen = max((unsigned int)range.minlen,
-				   q->limits.discard_granularity);
 		ret = ext4_trim_fs(sb, &range);
 		if (ret < 0)
 			return ret;

fs/ext4/mballoc.c

@@ -6400,6 +6400,7 @@ ext4_trim_all_free(struct super_block *sb, ext4_group_t group,
  */
 int ext4_trim_fs(struct super_block *sb, struct fstrim_range *range)
 {
+	struct request_queue *q = bdev_get_queue(sb->s_bdev);
 	struct ext4_group_info *grp;
 	ext4_group_t group, first_group, last_group;
 	ext4_grpblk_t cnt = 0, first_cluster, last_cluster;
@@ -6418,6 +6419,13 @@ int ext4_trim_fs(struct super_block *sb, struct fstrim_range *range)
 	    start >= max_blks ||
 	    range->len < sb->s_blocksize)
 		return -EINVAL;
+	/* No point to try to trim less than discard granularity */
+	if (range->minlen < q->limits.discard_granularity) {
+		minlen = EXT4_NUM_B2C(EXT4_SB(sb),
+			q->limits.discard_granularity >> sb->s_blocksize_bits);
+		if (minlen > EXT4_CLUSTERS_PER_GROUP(sb))
+			goto out;
+	}
 	if (end >= max_blks)
 		end = max_blks - 1;
 	if (end <= first_data_blk)