Date:	Sat, 30 Nov 2013 12:06:24 -0800
From:	"Darrick J. Wong" <darrick.wong@...cle.com>
To:	"'Theodore Tso'" <tytso@....edu>,
	Akira Fujita <a-fujita@...jp.nec.com>
Cc:	"'ext4 development'" <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH] mke2fs: Fix block bitmaps initalization with -O
 ^resize_inode

Hi Ted, I was hoping you might resolve a question for us:

(please scroll down a bit)

On Thu, Nov 28, 2013 at 05:50:19PM +0900, Akira Fujita wrote:
> >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.)
> 
> Yes. Current ext4, it seems to me that BLOCK_UNINIT is set under the following rules:
> 
> 1 sb(backup sb)/gdt/bitmap/itable exist  (e.g. #BG0)   -> BLOCK_UNINIT off
> 2 backup sb/gdt exist                    (e.g. #BG5)   -> BLOCK_UNINIT off
> 3 bitmap/itable exist                    (e.g. #BG16)   -> BLOCK_UNINIT off
> 4 Completely empty                       (e.g. #BG6)   -> BLOCK_UNINIT on
>                             Note: #BGN is in ext4 created by default option
> 
> And there is a bug that block group which has backup super block
> and group descriptor block are not write out to device in wrtie_bitmaps()
> when BLOCK_UNINIT is set.
> So at this time, I attempted to fix this by cleaning BLOCK_UNIIT.

Is it the case that a group should only have BLOCK_UNINIT set if the group is
totally empty (no group metadata, no blocks allocated to files) as Akira says?

Or is it the case that a group can have BLOCK_UNINIT set if the group contains
group metadata but no blocks are allocated to files?

The kernel and e2fsck contain code to construct bitmaps at run time with the
group metadata blocks marked in-use (thus handling case #2).  Unfortunately,
libext2fs does not do this; it stuffs in a zeroed bitmap any time it sees
BLOCK_UNINIT (case #1).

It seems that you can mkfs -O ^resize_inode and it'll produce an fs containing
groups that have superblocks/group descriptors but BLOCK_UNINIT set.  The
kernel handles it correctly, but things like debugfs report that every block in
that group is free.

For sure I'd like to fix libext2fs to ensure that it marks the group metadata
blocks in-use in the in-memory bitmap when it's loading the block bitmap for a
BLOCK_UNINIT group.

As for whether or not a group can contain group data and have BLOCK_UNINIT set,
well, I defer to you. :)

--D

> 
> 
> >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"?
> 
> I guess there is another bug.
> 
> >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.
> 
> Speeding up bitmap check in pass5 sounds nice,
> but setting BLOCK_UNINIT for block groups which have metadata means
> we'll change the definition of BLOCK_UNIIT... :s
> 
> Regards,
> Akira Fujita
> 
> >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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