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Message-ID: <4F550D70.6060708@redhat.com>
Date: Mon, 05 Mar 2012 13:01:04 -0600
From: Eric Sandeen <sandeen@...hat.com>
To: Lukas Czerner <lczerner@...hat.com>
CC: linux-ext4@...r.kernel.org, tytso@....edu
Subject: Re: [PATCH 1/4] e2fsck: Discard only unused parts of inode table
On 03/05/2012 12:43 PM, Lukas Czerner wrote:
> On Mon, 5 Mar 2012, Eric Sandeen wrote:
>
>> On 03/05/2012 01:49 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 free inode count.
>>> 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.
>>
>> Every time I look at this I have new comments. The surrounding code
>> is confusing to read, IMHO, so don't take it too hard. ;)
>>
>>> Signed-off-by: Lukas Czerner <lczerner@...hat.com>
>>> Reported-by: Phillip Susi <psusi@...ntu.com>
>>
>> I'll go ahead and:
>>
>> Reviewed-by: Eric Sandeen <sandeen@...hat.com>
>>
>> so hopefully we can get this in & fix the bug.
>>
>> but I have one more suggestion, which could be done as a cleanup later
>> on (I think the check_*_bitmaps could use a fair bit of cleanup for
>> clarity):
>>
>> /*
>> * 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);
>>
>> could/should probably be something like:
>>
>> if (inodes == first_free)
>> e2fsck_discard_inodes(ctx, group, inodes, 1);
>>
>> I _think_ this case should only, ever, handle the last inode in the
>> group, right?
>
> Hi Eric,
>
> thanks for the review. It almost seems that I am not able to write
> understandable comments :).
It's as least as likely that I cannot understand understandable code ;)
I think you are right, and my suggestion was wrong. Sorry for the noise!
-Eric
> The reason the comment is there is that we
> usually do:
>
> if (inodes > first_free)
>
> but in this particular case we do
>
> if (inodes >= first_free)
>
> And we have '=' there because at that point we might have last inode
> which is free. But because we will not have another iteration on that
> group, there will not be another 'used' inode which will result the code
> to go in the 'if (bitmap)' branch discarding that last inode. So we have
> to discard the last inode as well.
>
> However, doing (inodes == first_free) would be wrong, because we might
> have more free inodes at the end of the inode table so first_free will
> be smaller than inodes (which is actually the usual case as we do in the
> 'if (bitmap)' branch).
>
> So the comment:
>
> If the last inode is free, we can discard it as well.
>
> is simply there 'because' we will only hit this if the last inode is
> free and we have to discard it as well (thus the '>=').
>
> Hope that makes sense, I am not very good at explaining things :)
>
> Thanks!
> -Lukas
>
>>
>> -Eric
>>
>>> ---
>>> e2fsck/pass5.c | 90 +++++++++++++++++++++++++++++++++++++++++--------------
>>> 1 files changed, 67 insertions(+), 23 deletions(-)
>>>
>>> diff --git a/e2fsck/pass5.c b/e2fsck/pass5.c
>>> index 1e836e3..ee73dd5 100644
>>> --- a/e2fsck/pass5.c
>>> +++ b/e2fsck/pass5.c
>>> @@ -94,6 +94,52 @@ static void e2fsck_discard_blocks(e2fsck_t ctx, io_manager manager,
>>> ctx->options &= ~E2F_OPT_DISCARD;
>>> }
>>>
>>> +/*
>>> + * This will try to discard number 'count' inodes starting at
>>> + * inode number 'start' within the 'group'. Note that 'start'
>>> + * is 1-based, it means that we need to adjust it by -1 in this
>>> + * function to compute right offset in the particular inode table.
>>> + */
>>> +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;
>>> +
>>> + /*
>>> + * Sanity check for 'start'
>>> + */
>>> + if ((start < 1) || (start > EXT2_INODES_PER_GROUP(fs->super))) {
>>> + printf("PROGRAMMING ERROR: Got start %d outside of group %d!"
>>> + " Disabling discard\n",
>>> + start, group);
>>> + ctx->options &= ~E2F_OPT_DISCARD;
>>> + }
>>> +
>>> + if ((ctx->options & E2F_OPT_NO) || !(ctx->options & E2F_OPT_DISCARD))
>>> + return;
>>> +
>>> + /*
>>> + * Start is inode number within the group which starts
>>> + * counting from 1, so we need to adjust it.
>>> + */
>>> + start -= 1;
>>> +
>>> + /*
>>> + * 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 +481,7 @@ 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 +544,7 @@ 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 +609,46 @@ 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);
>>> /*
>>> * 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)) &&
>>> + 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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