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Message-ID: <4F4D0159.1090409@sandeen.net>
Date: Tue, 28 Feb 2012 10:31:21 -0600
From: Eric Sandeen <sandeen@...deen.net>
To: Lukas Czerner <lczerner@...hat.com>
CC: linux-ext4@...r.kernel.org, tytso@....edu, psusi@...ntu.com
Subject: Re: [PATCH 1/2 v2] e2fsck: Discard only unused parts of inode table
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@...hat.com>
> Reported-by: Phillip Susi <psusi@...ntu.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;
--
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