Block bitmap differences

Hi!

After cleaning up some other file system errors, I got the following
problem: e2fsck finished (apparently) successfully. But to be sure, I ran another e2fsck, which still complained about block bitmap differences:

Block bitmap differences: +(107151362--107151397) +(107151603--107184127) [...] Fix<y>? yes

/dev/vg1/pool1: ***** FILE SYSTEM WAS MODIFIED *****
/dev/vg1/pool1: 195275/28581920 files (3.9% non-contiguous), 122154001/142753792 blocks

To be honest, the file system was mounted read-only at that moment. Now I umounted the filesystem, and mounted it again read-only (I do not want to leave users without read access to the files, if possible).

But then, another e2fsck gave the same errors again:

bigspace2:~# ./e2fsck.static -f /dev/vg1/pool1 
e2fsck 1.34 (25-Jul-2003)
/dev/vg1/pool1 is mounted.  

WARNING!!! Running e2fsck on a mounted filesystem may cause SEVERE filesystem damage.

Do you really want to continue (y/n)? yes

Pass 1: Checking inodes, blocks, and sizes
Pass 2: Checking directory structure
Pass 3: Checking directory connectivity
Pass 4: Checking reference counts
Pass 5: Checking group summary information
Block bitmap differences: +(107151362--107151397) +(107151603--107184127) [...] Fix<y>? yes

How can that be? Bug in e2fsck? Or does the kernel write to the filesystem even though it's mounted read-only? (this is stock linux 2.4.21)

Jan