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Date:	Wed, 22 Feb 2012 18:51:37 -0600
From:	Eric Sandeen <sandeen@...hat.com>
To:	Lukas Czerner <lczerner@...hat.com>
CC:	linux-ext4@...r.kernel.org, tytso@....edu, psusi@...ntu.com
Subject: Re: [PATCH 1/2] e2fsck: Discard only unused parts of inode table

On 2/21/12 10:02 AM, Lukas Czerner wrote:
> When calling e2fsck with '-E discard' option it might happen that
> valid inodes are discarded accidentally. This is because we just
> discard the part of inode table which lies past the highest used
> inode. This is terribly wrong (sorry!).
> 
> This patch fixes it so only the free parts of an inode table
> is discarded, leaving used inodes intact. This was tested with highly
> fragmented inode tables with block size 4k and 1k.
> 
> Signed-off-by: Lukas Czerner <lczerner@...hat.com>
> Reported-by: Phillip Susi <psusi@...ntu.com>
> ---
>  e2fsck/pass5.c |   59 +++++++++++++++++++++++++++++++++++--------------------
>  1 files changed, 37 insertions(+), 22 deletions(-)
> 
> diff --git a/e2fsck/pass5.c b/e2fsck/pass5.c
> index 1e836e3..9cc4a20 100644
> --- a/e2fsck/pass5.c
> +++ b/e2fsck/pass5.c
> @@ -94,6 +94,26 @@ static void e2fsck_discard_blocks(e2fsck_t ctx, io_manager manager,
>  		ctx->options &= ~E2F_OPT_DISCARD;
>  }

I could really use a comment here.  Is "start" the inode nr within
the group?  Is the first inode in the group 0 or 1?  The calling
function's loop starts at inode 1 ... but based on some testing
it looks like you call the first free inode in an empty group
"inode 0" so I'll assume that, but comments please.


> +static void e2fsck_discard_inodes(e2fsck_t ctx, int group,
> +				  int start, int count)

I'm not sure if ints are good here, but it's what the calling
function uses, I guess.

> +{
> +	ext2_filsys fs = ctx->fs;
> +	blk64_t blk, num;
> +
> +	/*
> +	 * We can discard only blocks containing only unused
> +	 * inodes in the table.
> +	 */
> +	blk = DIV_ROUND_UP(start,
> +		EXT2_INODES_PER_BLOCK(fs->super));
> +	count -= (blk * EXT2_INODES_PER_BLOCK(fs->super) - start);
> +	blk += ext2fs_inode_table_loc(fs, group);
> +	num = count / EXT2_INODES_PER_BLOCK(fs->super);
> +
> +	if (num > 0)
> +		e2fsck_discard_blocks(ctx, fs->io->manager, blk, num);
> +}

I can't make intuitive sense of that at all, and had to count on my fingers
and draw diagrams.  Maybe it's just my old brain.  :(

Let's say we have 4 inode blocks in this table, with
1 == used, 0 == unused, and we start counting inodes at 0.

|1111111100000000|0000000000000000|0000000000000000|0000000000000001|
 012345678 <- start

so start = 8, count = 55 (note one inode used at the end)

In this case we should get:

blk = 1 (DIV_ROUND_UP(8, 16)) - ok, that's next block after the partial block
count = 55 - ((1 * 16) - 8) = 47
num = 47 / 16 = 2

Ok, so that works, assuming the first inode in the group is "0"

We can make a filesystem kind of like that, to test it:

# mkfs.ext4 -I 256 -b 4096 -N 256 -G 1 -g 256 /dev/loop0 1024

with 4 groups each having 64 inodes.

If I run a e2fsck -E discard on that fresh filesystem, with some debugging, I see:

Pass 5: Checking group summary information
got start 12 count 51; count reduced to 47 inodes, discarding 2 blocks starting at block 36

Ok, that is a little weird; on a fresh filesystem lost+found/ is 11, so the first free
inode is 12; but now we seem to have started counting at 1, not 0?

got start 0 count 63; count reduced to 63 inodes, discarding 3 blocks starting at block 291
got start 0 count 63; count reduced to 63 inodes, discarding 3 blocks starting at block 514
got start 0 count 63; count reduced to 63 inodes, discarding 3 blocks starting at block 803

Oh, now we're back to "the first inode in the group is 0"  :(

And here it looks like we are not discarding all the free blocks in the completely-free tables;
the last group has:

  Inode table at 803-806 (+35)
  217 free blocks, 64 free inodes, 0 directories, 64 unused inodes

but we only discarded 803 through 805, for example, because we were only told to discard
63 inodes, not 64 as I'd have expected since 64 are free.

I can't really make heads or tails out of how we got here, the calling function is so
messy it's really hard for me to keep track of.  I think perhaps a clean up series
for the calling function first, and then we can try to properly extend it...?

I think we need ultra-careful attention to which variables are 0-based and which
are 1-based, for starters.

For e2fsck_discard_inodes() I wonder if it could be made more intuitive by:

1) find first totally free block based on start
2) find last totally free block based on the last free inode (start+count-1)
3) find free block length, free_length = last_blk - first_blk + 1
4) e2fsck_discard_blocks(..., ..., first_blk, free_length);

i.e. get rid of the count -= ...

_assuming_ we start counting at 0:

	/* round up to next block after a partially free one */
	first_blk = DIV_ROUND_UP(start,
		EXT2_INODES_PER_BLOCK(fs->super));
	first_blk += ext2fs_inode_table_loc(fs, group);

	last_free_inode = start + count - 1;

	/* round down to block before a partially free one */
	last_blk = last_free_inode / EXT2_INODES_PER_BLOCK(fs->super));
	free_blks = last_blk - first_blk + 1;
	if (free_blks > 0)
		e2fsck_discard_blocks(ctx, fs->io->manager, first_blk, free_blks);

I probably have off-by-ones there too, but you get the idea...

-Eric

> +
>  #define NO_BLK ((blk64_t) -1)
>  
>  static void print_bitmap_problem(e2fsck_t ctx, int problem,
> @@ -422,6 +442,7 @@ static void check_inode_bitmaps(e2fsck_t ctx)
>  	ext2_ino_t	i;
>  	unsigned int	free_inodes = 0;
>  	int		group_free = 0;
> +	int		first_free = fs->super->s_inodes_per_group;
>  	int		dirs_count = 0;
>  	int		group = 0;
>  	unsigned int	inodes = 0;
> @@ -497,6 +518,7 @@ redo_counts:
>  				 * are 0, count the free inode,
>  				 * skip the current block group.
>  				 */
> +				first_free = 0;
>  				inodes = fs->super->s_inodes_per_group - 1;
>  				group_free = inodes;
>  				free_inodes += inodes;
> @@ -561,37 +583,27 @@ redo_counts:
>  		ctx->options &= ~E2F_OPT_DISCARD;
>  
>  do_counts:
> +		inodes++;
>  		if (bitmap) {
>  			if (ext2fs_test_inode_bitmap2(ctx->inode_dir_map, i))
>  				dirs_count++;
> +			if (inodes - 1 > first_free) {
> +				e2fsck_discard_inodes(ctx, group, first_free,
> +						      inodes - first_free - 1);
> +				first_free = fs->super->s_inodes_per_group;
> +			}
>  		} else if (!skip_group || csum_flag) {
>  			group_free++;
>  			free_inodes++;
> +			if (first_free > inodes)
> +				first_free = inodes;
>  		}
>  
> -		inodes++;
>  		if ((inodes == fs->super->s_inodes_per_group) ||
>  		    (i == fs->super->s_inodes_count)) {
> -
> -			free_array[group] = group_free;
> -			dir_array[group] = dirs_count;
> -
> -			/* Discard inode table */
> -			if (ctx->options & E2F_OPT_DISCARD) {
> -				blk64_t used_blks, blk, num;
> -
> -				used_blks = DIV_ROUND_UP(
> -					(EXT2_INODES_PER_GROUP(fs->super) -
> -					group_free),
> -					EXT2_INODES_PER_BLOCK(fs->super));
> -
> -				blk = ext2fs_inode_table_loc(fs, group) +
> -				      used_blks;
> -				num = fs->inode_blocks_per_group -
> -				      used_blks;
> -				e2fsck_discard_blocks(ctx, manager, blk, num);
> -			}
> -
> +			if (inodes - 1 > first_free)
> +				e2fsck_discard_inodes(ctx, group, first_free,
> +						      inodes - first_free - 1);
>  			/*
>  			 * If discard zeroes data and the group inode table
>  			 * was not zeroed yet, set itable as zeroed
> @@ -599,12 +611,15 @@ do_counts:
>  			if ((ctx->options & E2F_OPT_DISCARD) &&
>  			    (io_channel_discard_zeroes_data(fs->io)) &&
>  			    !(ext2fs_bg_flags_test(fs, group,
> -						  EXT2_BG_INODE_ZEROED))) {
> +						   EXT2_BG_INODE_ZEROED))) {
>  				ext2fs_bg_flags_set(fs, group,
>  						    EXT2_BG_INODE_ZEROED);
>  				ext2fs_group_desc_csum_set(fs, group);
>  			}
>  
> +			first_free = fs->super->s_inodes_per_group;
> +			free_array[group] = group_free;
> +			dir_array[group] = dirs_count;
>  			group ++;
>  			inodes = 0;
>  			skip_group = 0;

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