linux-ext4 - offline shrink bug w/ testcase & image

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Date:	Tue, 18 Jun 2013 19:24:36 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	ext4 development <linux-ext4@...r.kernel.org>
Subject: offline shrink bug w/ testcase & image

This fs image is problematic for offline resize2fs shrink.
(sorry, it expands to 11G)

# resize2fs <image> 2507776

will lead to corruption[1] because the new size is in the middle
of a block group which has its inode & block bitmaps towards
the end; the bitmaps are left beyond the end of the new filesystem.

I've been trying to find my way around resize2fs, but bleah,
I must admit to being perpetually lost.

I'm not sure there's any simple functionality to handle
this case; does anything else ever need to move bitmaps?

I was looking at blocks_to_move(), and within the "shrinking" loop
I added logic to detect this case:

                /* If this (first) group will remain but bitmaps are past EOF, move them */
                if (g == ext2fs_group_of_blk2(fs, ext2fs_blocks_count(fs->super))) {
                        int retval;
                        blk64_t new_size = ext2fs_blocks_count(fs->super);

                        if ((IS_BLOCK_BM(fs, g, blk) || IS_INODE_BM(fs, g, blk)) &&
                            (blk >= new_size)) {

                            <do something>
                        }

but at this point I'm not sure what to do.  The magic around identifying
& marking & reserving & accounting for moved blocks has me stumped.

I'm happy to keep looking at it but could use some pointers, or if the
fix is obvious to someone, by all means have at it.  :)

Thanks,
-Eric

[1] - like this -

e2fsck: Group descriptors look bad... trying backup blocks...
Block bitmap for group 76 is not in group.  (block 2508258)
Relocate? yes

Inode bitmap for group 76 is not in group.  (block 2508259)
Relocate? yes

One or more block group descriptor checksums are invalid.  Fix? yes

Group descriptor 76 checksum is invalid.  FIXED.
Pass 1: Checking inodes, blocks, and sizes
Relocating group 76's block bitmap to 2097161...
Relocating group 76's inode bitmap to 2097162...
Pass 2: Checking directory structure
Pass 3: Checking directory connectivity
Pass 4: Checking reference counts
Pass 5: Checking group summary information
Block bitmap differences:  +(2097161--2097162) +(2490368--2490874)
Fix? yes

Free blocks count wrong for group #64 (28187, counted=28185).
Fix? yes

Free blocks count wrong (2430591, counted=2430589).
Fix? yes

testfile: ***** FILE SYSTEM WAS MODIFIED *****
testfile: 11/624624 files (0.0% non-contiguous), 77187/2507776 blocks

Download attachment "testcase.img.bz2" of type "application/x-bzip2" (25126 bytes)