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From:   Fariya F <fariya.fatima03@gmail.com>
Date:   Fri, 25 Mar 2022 12:12:30 +0530
Message-ID: <CACA3K+i8nZRBxeTfdy7Uq5LHAsbZEHTNati7-RRybsj_4ckUyw@mail.gmail.com>
Subject: df returns incorrect size of partition due to huge overhead block
 count in ext4 partition
To:     linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org
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My eMMC partition is ext4 formatted and has about 100MB size. df -h
command lists the size of the partition and the used percentage as
below.
    Filesystem           Size  Used Avail Use% Mounted on

    /dev/mmcblk2p4    16Z   16Z   79M 100% /data

For your reference, the returned values for statfs64( ) are

statfs64("/data", 88, {f_type="EXT2_SUPER_MAGIC", f_bsize=1024,
f_blocks=18446744073659310077, f_bfree=87628, f_bavail=80460,
f_files=25688, f_ffree=25189, f_fsid={-1446355608, 1063639410},
f_namelen=255, f_frsize=1024, f_flags=4128}) = 0

The output dumpe2fs returns the following


    Block count:              102400

    Reserved block count:     5120

    Overhead blocks:          50343939

As per my kernel (4.9.31) code, the f_blocks is block_count - overhead
blocks. Considering the subtraction with the above values results in a
negative value this is interpreted as the huge value of
18446744073659310077.

I have a script which monitors the used percentage of the partition
using df -h command and when the used percentage is greater than 70%,
it deletes files until the used percentage comes down. Considering df
is reporting all the time 100% usage, all my files get deleted.

My questions are:

a) Where does overhead blocks get set?

b) Why is this value huge for my partition and how to correct it
considering fsck is also not correcting this

Please note fsck on this partition doesn't report any issues at all.

I am also able to create files in this partition.