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SubjectRe: [PATCH 09/16] ext4: Calculate and verify block bitmap checksum
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On 2011-09-02, at 1:08 PM, Darrick J. Wong wrote:
> On Thu, Sep 01, 2011 at 12:08:44AM -0600, Andreas Dilger wrote:
>> On 2011-08-31, at 6:31 PM, Darrick J. Wong wrote:
>>> Compute and verify the checksum of the block bitmap; this checksum is stored in the block group descriptor.
>>
>> Since this is CPU intensive, it might make sense to start computing the
>> block bitmap checksums as soon as the buffer is uptodate, instead of
>> waiting for all of the buffers to be read and _then_ doing the checksums.
>>
>> Even better might be to do move all of the above code to do the checksum
>> to be in a new the b_end_io callback, so that it can start as soon as each
>> buffer is read from disk, to maximize CPU and IO overlap, like:
>
> Good suggestion. I'll put it into the next rev.
>
> --D
>
>> struct ext4_csum_data {
>> struct superblock *cd_sb;
>> ext4_group_t cd_group;
>> };
>>
>> static void ext4_end_buffered_read_sync_csum(struct buffer_head *bh,
>> int uptodate)
>> {
>> struct superblock *sb = (struct ext4_csum_data *)(bh->b_private)->cd_sb;
>> ext4_group_t group = (struct ext4_csum_data *)(bh->b_private)->cd_group;
>>
>> end_buffer_read_sync(bh, uptodate);

Actually, the call to end_buffer_read_sync() should go _after_ the
checksum is calculated, so that no other thread can start using
the buffer before the checksum is verified (i.e. any checks on
buffer_uptodate() could incorrectly succeed before the checksum
is computed and set_buffer_verified(bh) is called, and it would
incorrectly return an IO error thinking the checksum failed).

Also, end_buffer_sync_read() drops the reference to bh, but this
code is accessing bh->b_data, so another reason to move it after
the checksum is computed.

Cheers, Andreas

>> if (uptodate) {
>> struct ext4_group_desc *desc;
>>
>> desc = ext4_get_group_desc(sb, group, NULL);
>> if (!desc)
>> return;
>>
>> ext4_lock_group(sb, group);
>> if (ext4_valid_block_bitmap(sb, desc, group, bh) &&
>> ext4_bitmap_csum_verify(sb, group,
>> desc->bg_block_bitmap_csum, bh,
>> (EXT4_BLOCKS_PER_GROUP(sb) + 7) / 8))
>> set_buffer_verified(bh);
>>
>> ext4_unlock_group(rcd->rcd_sb, rcd->rcd_group);
>> }
>> }
>>
>> Then later in the code can just check buffer_verified() in the caller:
>>
>> ext4_read_block_bitmap()
>> {
>> /* read all groups the page covers into the cache */
>> for (i = 0; i < groups_per_page; i++) {
>> :
>> :
>> set_bitmap_uptodate(bh[i]);
>> ecd[i].cd_sb = sb;
>> ecd[i].cd_group = first_group + i;
>> bh[i]->b_end_io = ext4_end_buffer_read_sync_csum;
>> submit_bh(READ, bh[i]);
>> mb_debug(1, "read bitmap for group %u\n", first_group + i);
>> }
>>
>> err = 0;
>> /* always wait for I/O completion before returning */
>> for (i = 0; i < groups_per_page; i++) {
>> if (bh[i]) {
>> wait_on_buffer(bh[i]);
>> if (!buffer_uptodate(bh[i]) ||
>> !buffer_verified(bh[i]))
>> err = -EIO;
>> }
>> }
>>
>>
>>> err = 0;
>>> first_block = page->index * blocks_per_page;
>>> for (i = 0; i < blocks_per_page; i++) {
>>> @@ -2829,6 +2856,9 @@ ext4_mb_mark_diskspace_used(struct ext4_allocation_context *ac,
>>> }
>>> len = ext4_free_blks_count(sb, gdp) - ac->ac_b_ex.fe_len;
>>> ext4_free_blks_set(sb, gdp, len);
>>> + ext4_bitmap_csum_set(sb, ac->ac_b_ex.fe_group,
>>> + &gdp->bg_block_bitmap_csum, bitmap_bh,
>>> + (EXT4_BLOCKS_PER_GROUP(sb) + 7) / 8);
>>> gdp->bg_checksum = ext4_group_desc_csum(sbi, ac->ac_b_ex.fe_group, gdp);
>>>
>>> ext4_unlock_group(sb, ac->ac_b_ex.fe_group);
>>> @@ -4638,6 +4668,8 @@ do_more:
>>>
>>> ret = ext4_free_blks_count(sb, gdp) + count;
>>> ext4_free_blks_set(sb, gdp, ret);
>>> + ext4_bitmap_csum_set(sb, block_group, &gdp->bg_block_bitmap_csum,
>>> + bitmap_bh, (EXT4_BLOCKS_PER_GROUP(sb) + 7) / 8);
>>> gdp->bg_checksum = ext4_group_desc_csum(sbi, block_group, gdp);
>>> ext4_unlock_group(sb, block_group);
>>> percpu_counter_add(&sbi->s_freeblocks_counter, count);
>>> @@ -4780,6 +4812,8 @@ int ext4_group_add_blocks(handle_t *handle, struct super_block *sb,
>>> mb_free_blocks(NULL, &e4b, bit, count);
>>> blk_free_count = blocks_freed + ext4_free_blks_count(sb, desc);
>>> ext4_free_blks_set(sb, desc, blk_free_count);
>>> + ext4_bitmap_csum_set(sb, block_group, &desc->bg_block_bitmap_csum,
>>> + bitmap_bh, (EXT4_BLOCKS_PER_GROUP(sb) + 7) / 8);
>>> desc->bg_checksum = ext4_group_desc_csum(sbi, block_group, desc);
>>> ext4_unlock_group(sb, block_group);
>>> percpu_counter_add(&sbi->s_freeblocks_counter, blocks_freed);
>>>
>>



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