Re: [PATCH 1/2 v2] e2fsck: Discard only unused parts of inode table

[Eric Sandeen <sandeen@sandeen.net>]

On 02/23/2012 10:29 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@redhat.com>
> Reported-by: Phillip Susi <psusi@ubuntu.com>

Apologies for being so iterative about this review but it's taken a while
for this function to make sense to me.

This patch does seem to fix the original bug, but I have a few more
nitpicks below.

> ---
> v2: reworked so that we comply with inode number counting and adjust
>     start in the e2fsck_discard_inodes(). Add some comments

>  e2fsck/pass5.c |   82 ++++++++++++++++++++++++++++++++++++++++---------------
>  1 files changed, 59 insertions(+), 23 deletions(-)
> 
> diff --git a/e2fsck/pass5.c b/e2fsck/pass5.c
> index 1e836e3..57a207d 100644
> --- a/e2fsck/pass5.c
> +++ b/e2fsck/pass5.c
> @@ -94,6 +94,43 @@ static void e2fsck_discard_blocks(e2fsck_t ctx, io_manager manager,
>  		ctx->options &= ~E2F_OPT_DISCARD;
>  }
>  
> +/*
> + * This will try to discard number 'count' starting at inode
> + * number 'start'. Note that 'start' is a inode number and hence
> + * it is counted from 1, it means that we need to adjust it
> + * by -1 in this function to compute right offset in the
> + * inode table.
> + */

But that's not quite right... a casual reader would assume that we
pass in the inode number of the starting inode we want to free;
that's true for the first group, but not the rest:

$ e2fsck/e2fsck -f -E discard fsfile 
e2fsck 1.42 (29-Nov-2011)
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
discard starting at 12
discard starting at 1
discard starting at 1
discard starting at 1
discard starting at 1
discard starting at 1

...

we are not discarding inode 1 that many times ;) 

The comment should reflect that the function expects to receive
the inode number within this group, i.e. the Nth inode, starting
with the 1st, i.e. 1-based.  (which is a little weird, but I don't
see a better way).

The calling loop _does_ keep track of the actual inode number
within the system ('i') - but that's not what's passed in here,
and I don't think ctx->fs has the info we need to be able to
do it that way.

> +static void e2fsck_discard_inodes(e2fsck_t ctx, int group,
> +				  int start, int count)
> +{
> +	ext2_filsys fs = ctx->fs;
> +	blk64_t blk, num;
> +	int orig = count;
> +
> +	if ((ctx->options & E2F_OPT_NO) || !(ctx->options & E2F_OPT_DISCARD))
> +		return;
> +
> +	/*
> +	 * Start is inode number which starts counting from 1,

... inode number within the group ...

> +	 * so we need to adjust it.
> +	 */
> +	start -= 1;

I wonder if we need defense against programming errors here; if start
inadvertently comes in at 0, what happens?  I suppose blk goes off the end
and other checks hopefully catch it... probably not 'til it hits the kernel
though?

> +	/*
> +	 * 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);
> +}
> +
>  #define NO_BLK ((blk64_t) -1)
>  
>  static void print_bitmap_problem(e2fsck_t ctx, int problem,
> @@ -435,6 +472,7 @@ static void check_inode_bitmaps(e2fsck_t ctx)
>  	int		skip_group = 0;
>  	int		redo_flag = 0;
>  	io_manager	manager = ctx->fs->io->manager;
> +	unsigned long long	first_free = fs->super->s_inodes_per_group + 1;

>  
>  	clear_problem_context(&pctx);
>  	free_array = (int *) e2fsck_allocate_memory(ctx,
> @@ -497,6 +535,7 @@ redo_counts:
>  				 * are 0, count the free inode,
>  				 * skip the current block group.
>  				 */
> +				first_free = 1;
>  				inodes = fs->super->s_inodes_per_group - 1;
>  				group_free = inodes;
>  				free_inodes += inodes;
> @@ -561,50 +600,47 @@ 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 > first_free) {
> +				e2fsck_discard_inodes(ctx, group, first_free,
> +						      inodes - first_free);
> +				first_free = fs->super->s_inodes_per_group + 1;
> +			}
>  		} 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 the last inode is free, we can discard it as well.
> +			 */
> +			if (inodes >= first_free)
> +				e2fsck_discard_inodes(ctx, group, first_free,
> +						      inodes - first_free + 1);

This is unrelated to the changelog

>  			/*
>  			 * If discard zeroes data and the group inode table
>  			 * was not zeroed yet, set itable as zeroed
>  			 */
>  			if ((ctx->options & E2F_OPT_DISCARD) &&
> -			    (io_channel_discard_zeroes_data(fs->io)) &&
> +			    !(ctx->options & E2F_OPT_NO) &&

This is unrelated as well...

Rather than continuing to check _OPT_NO here and in e2fsck_discard_blocks(), it
might be better (in a separate patch) ;) to simply turn off OPT_DISCARD right
after options processing if -n was specified, and not worry about it down here.

> +			    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 + 1;
> +			free_array[group] = group_free;
> +			dir_array[group] = dirs_count;
>  			group ++;
>  			inodes = 0;
>  			skip_group = 0;