Date:	Tue, 26 Nov 2013 16:55:05 -0800
From:	"Darrick J. Wong" <darrick.wong@...cle.com>
To:	Akira Fujita <a-fujita@...jp.nec.com>
Cc:	"'Theodore Tso'" <tytso@....edu>,
	"'ext4 development'" <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH] mke2fs: Fix block bitmaps initalization with -O
 ^resize_inode

On Tue, Nov 26, 2013 at 05:10:56PM +0900, Akira Fujita wrote:
> Hi Darrick,
> 
> With your patch, e2fsck can't fix the filesystem created with existing mke2fs without
> resize_inode.
> Because "fs->block_map == bmap" condition is in ext2fs_reserve_super_and_bgd().

Hmm, I think I see what's going on here.

First, I think you're arguing that BLOCK_UNINIT should only be set on a group
if it's completely empty.  I think you're also arguing that even if the group
contains only its own superblocks, group descriptors, bitmaps, or inode tables,
BLOCK_UNINIT should not be set.  Let's call that condition (block group
contains only group data, but BLOCK_UNINIT is set) "(1)".

Note: If a group contains some other group's metadata, BLOCK_UNINIT should not
(and does not seem to) be set.

(If I'm wrong, please let me know.)

The kernel, when it needs to load a bitmap, checks for BLOCK_UNINIT; if set, it
calls ext4_init_block_bitmap() to zero the bitmap and mark the relevant
sb/gdt/bmap/table bits, if necessary.  That takes care of (1) in the kernel.

e2fsck pass5 also checks for BLOCK_UNINIT; if it's set, then it first compares
block_found_map against a zeroed bitmap (which falls back to no_optimize mode),
and then it secondly compares each block individually against the ranges that
ext2fs_super_and_bgd_loc2(), ext2fs_*_bitmap_loc(), and
ext2fs_inode_table_loc() return.  That takes care of (1) in e2fsck.

libext2fs, however, checks for BLOCK_UNINIT; if it's set, it zeroes that part
of the bitmap and moves on without bothering to mark the sb/gdt/bmap/table
bits.  This does NOT take care of (1), and opens a window where callers can
allocate blocks that already hold group metadata!  (Let's call this (2).)

The fuse2fs bug that I was seeing was that if (2) happens, then fuse2fs sees a
zero bitmap and blindly hands away group metadata, which is broken.  With
yesterday's patch, (1) doesn't happen and all seems ok.

I think in your e2fsck test, e2fsck tests block_found_map against a correctly
constructed bitmap (with ext2fs_super_and_bgd_loc2) if BLOCK_UNINIT is set,
which is why it can't fix (1) unless e2fsck is fed a backup superblock (or
there's a corrupt a group descriptor checksum), either of which clears
BLOCK_UNINIT before pass5).

So I'm wondering, is there more to the bug report than just "I ran mkfs and
prodded e2fsck into reporting errors"?

Given that both the kernel and e2fsck know how to calculate the proper block
bitmap when BLOCK_UNINIT is set, it doesn't seem to be an error case if
BLOCK_UNINIT is set on a group that contains superblocks, group descriptors,
bitmaps, or inode tables.  It seems to me that if we can calculate a bitmap,
then there's no need to record it, and we can use BLOCK_UNINIT.

What do you think of this solution: Change read_bitmaps to mark the location of
sb/gdt/bmap/inode blocks in the in-core block bitmap for BLOCK_UNINIT groups.
This fixes the problem that fs->block_bitmap doesn't accurately reflect the
real state of the filesystem (as evidenced by dumpe2fs/debugfs reporting more
free blocks than the summar.  I think we can also eliminate the skip_group code
from the block bitmap check in pass5.  That should speed up pass5 since we can
also use the fast bitmap compare.

I'll send out a test patch.

Of course, the tests seem coded to pass things like:

> Group 2: (Blocks 16385-24576) [INODE_UNINIT, BLOCK_UNINIT, ITABLE_ZEROED]
>   Block bitmap at 16385 (+0), Inode bitmap at 16386 (+1)
                    ^^^^^                       ^^^^^
>   Inode table at 16387-16642 (+2)
                   ^^^^^^^^^^^
>   7934 free blocks, 2048 free inodes, 0 directories, 2048 unused inodes
>   Free blocks: 16385-24576
                 ^^^^^
>   Free inodes: 4097-6144

<grumble>

--D

> But if the condition removed (like my previous patch) we can detect and
> fix filesystem inconsistency as follows.
> 
> 1.Create filesystem with existing mke2fs command
> # mke2fs -t ext4 -O ^resize_inode DEVICE
> 
> 2. (With my patch) e2fsck removes uninit_bg flag then fixes metadata inconsistency
> # ./e2fsck/e2fsck -f DEVICE
> e2fsck 1.43-WIP (8-Jul-2013)
> One or more block group descriptor checksums are invalid.  Fix<y>? yes
> Group descriptor 1 checksum is 0x0343, should be 0xc4ba.  FIXED.
> Group descriptor 3 checksum is 0x17f2, should be 0xd00b.  FIXED.
> Group descriptor 5 checksum is 0xf36c, should be 0x3495.  FIXED.
> Group descriptor 7 checksum is 0x3e90, should be 0xf969.  FIXED.
> Group descriptor 9 checksum is 0xa31e, should be 0x64e7.  FIXED.
> Group descriptor 25 checksum is 0x165a, should be 0xd1a3.  FIXED.
> Group descriptor 27 checksum is 0xce4c, should be 0x09b5.  FIXED.
> Group descriptor 49 checksum is 0x502b, should be 0x97d2.  FIXED.
> 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:  +(32768--32769) +(98304--98305) +(163840--163841) +(229376--229377)
> +(294912--294913) +(819200--819201) +(884736--884737) +(1605632--1605633)
> Fix<y>? yes
> 
> /dev/loop0: ***** FILE SYSTEM WAS MODIFIED *****
> /dev/loop0: 11/656640 files (0.0% non-contiguous), 73991/2621440 blocks
> 
> Regards,
> Akira Fujita
> 
> >-----Original Message-----
> >From: Darrick J. Wong [mailto:darrick.wong@...cle.com]
> >Sent: Tuesday, November 26, 2013 10:27 AM
> >To: Akira Fujita
> >Cc: Theodore Tso; ext4 development
> >Subject: Re: [PATCH] mke2fs: Fix block bitmaps initalization with -O ^resize_inode
> >
> >Does the following patch fix your error?  I think I've started to hit it too, now that I added
> >^resize_inode,^meta_bg to the mix.
> >
> >--D
> >
> >libext2fs: clear BLOCK_UNINIT any time a group has metadata blocks
> >
> >If ext2fs_reserve_super_and_bgd() is called with the filesystem's block bitmap, we clear the
> group's
> >BLOCK_UNINIT flag if the group has an old-style (pre-metabg) group descriptor block with
> reserved
> >descriptor blocks prior to marking the old-style descriptor blocks in use.
> >
> >Unfortunately, this is not sufficient -- if the group has no reserved blocks yet isn't a
> meta_bg
> >filesystem (^resize_inode,^meta_bg), we forget to clear the flag, so the filesystem thinks all
> >blocks are free, which is disastrous.
> >
> >Therefore, if we know that we're going to mark any of the bitmaps, make sure that we clear
> >BLOCK_UNINIT.
> >
> >This is based on an earlier patch by Akira Fujita.
> >
> >> Steps to reproduce:
> >> 1. mke2fs -t ext4 -b 4096 -O ^resize_inode device 2. e2fsck -b 32768
> >> DEV
> >>   <snip>
> >>   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:  +(32768--32769) +(98304--98305) +(163840--163841)
> >>   Fix<y>?
> >
> >Reported-by: Akira Fujita <a-fujita@...jp.nec.com>
> >Signed-off-by: Darrick J. Wong <darrick.wong@...cle.com>
> >---
> > lib/ext2fs/alloc_sb.c |   11 +++++++++--
> > 1 file changed, 9 insertions(+), 2 deletions(-)
> >
> >diff --git a/lib/ext2fs/alloc_sb.c b/lib/ext2fs/alloc_sb.c index 223ec51..f1898df 100644
> >--- a/lib/ext2fs/alloc_sb.c
> >+++ b/lib/ext2fs/alloc_sb.c
> >@@ -52,6 +52,15 @@ int ext2fs_reserve_super_and_bgd(ext2_filsys fs,
> > 	ext2fs_super_and_bgd_loc2(fs, group, &super_blk,
> > 				  &old_desc_blk, &new_desc_blk, &used_blks);
> >
> >+	/*
> >+	 * If we're marking blocks in the main block bitmap, be sure that we
> >+	 * clear the group's BLOCK_UNINIT flag if this group has metadata
> >+	 * blocks allocated.
> >+	 */
> >+	if (fs->block_map == bmap &&
> >+	    (new_desc_blk || old_desc_blk || super_blk || (group == 0)))
> >+		ext2fs_bg_flags_clear(fs, group, EXT2_BG_BLOCK_UNINIT);
> >+
> > 	if (fs->super->s_feature_incompat & EXT2_FEATURE_INCOMPAT_META_BG)
> > 		old_desc_blocks = fs->super->s_first_meta_bg;
> > 	else
> >@@ -65,8 +74,6 @@ int ext2fs_reserve_super_and_bgd(ext2_filsys fs,
> > 		ext2fs_mark_block_bitmap2(bmap, 0);
> >
> > 	if (old_desc_blk) {
> >-		if (fs->super->s_reserved_gdt_blocks && fs->block_map == bmap)
> >-			ext2fs_bg_flags_clear(fs, group, EXT2_BG_BLOCK_UNINIT);
> > 		num_blocks = old_desc_blocks;
> > 		if (old_desc_blk + num_blocks >= ext2fs_blocks_count(fs->super))
> > 			num_blocks = ext2fs_blocks_count(fs->super) -
> 
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