Re: [PATCH 09/16] ext4: Calculate and verify block bitmap checksum

["Darrick J. Wong" <djwong@us.ibm.com>]

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.
> > 
> > Signed-off-by: Darrick J. Wong <djwong@us.ibm.com>
> > ---
> > fs/ext4/balloc.c  |   43 ++++++++++++++++++++++++++++++++++---------
> > fs/ext4/ext4.h    |    7 ++++++-
> > fs/ext4/ialloc.c  |    5 +++++
> > fs/ext4/mballoc.c |   34 ++++++++++++++++++++++++++++++++++
> > 4 files changed, 79 insertions(+), 10 deletions(-)
> > 
> > 
> > diff --git a/fs/ext4/balloc.c b/fs/ext4/balloc.c
> > index f8224ad..36d3020 100644
> > --- a/fs/ext4/balloc.c
> > +++ b/fs/ext4/balloc.c
> > @@ -105,6 +105,10 @@ unsigned ext4_init_block_bitmap(struct super_block *sb, struct buffer_head *bh,
> > 			ext4_free_inodes_set(sb, gdp, 0);
> > 			ext4_itable_unused_set(sb, gdp, 0);
> > 			memset(bh->b_data, 0xff, sb->s_blocksize);
> > +			ext4_bitmap_csum_set(sb, block_group,
> > +					     &gdp->bg_block_bitmap_csum, bh,
> > +					     (EXT4_BLOCKS_PER_GROUP(sb) + 7) /
> > +					     8);
> > 			return 0;
> > 		}
> > 		memset(bh->b_data, 0, sb->s_blocksize);
> > @@ -175,6 +179,11 @@ unsigned ext4_init_block_bitmap(struct super_block *sb, struct buffer_head *bh,
> > 		 */
> > 		ext4_mark_bitmap_end(group_blocks, sb->s_blocksize * 8,
> > 				     bh->b_data);
> > +		ext4_bitmap_csum_set(sb, block_group,
> > +				     &gdp->bg_block_bitmap_csum, bh,
> > +				     (EXT4_BLOCKS_PER_GROUP(sb) + 7) / 8);
> > +		gdp->bg_checksum = ext4_group_desc_csum(sbi, block_group,
> > +							gdp);
> > 	}
> > 	return free_blocks - ext4_group_used_meta_blocks(sb, block_group, gdp);
> > }
> > @@ -232,10 +241,10 @@ struct ext4_group_desc * ext4_get_group_desc(struct super_block *sb,
> > 	return desc;
> > }
> > 
> > -static int ext4_valid_block_bitmap(struct super_block *sb,
> > -					struct ext4_group_desc *desc,
> > -					unsigned int block_group,
> > -					struct buffer_head *bh)
> > +int ext4_valid_block_bitmap(struct super_block *sb,
> > +			    struct ext4_group_desc *desc,
> > +			    unsigned int block_group,
> > +			    struct buffer_head *bh)
> > {
> > 	ext4_grpblk_t offset;
> > 	ext4_grpblk_t next_zero_bit;
> > @@ -312,12 +321,12 @@ ext4_read_block_bitmap(struct super_block *sb, ext4_group_t block_group)
> > 	}
> > 
> > 	if (bitmap_uptodate(bh))
> > -		return bh;
> > +		goto verify;
> > 
> > 	lock_buffer(bh);
> > 	if (bitmap_uptodate(bh)) {
> > 		unlock_buffer(bh);
> > -		return bh;
> > +		goto verify;
> > 	}
> > 	ext4_lock_group(sb, block_group);
> > 	if (desc->bg_flags & cpu_to_le16(EXT4_BG_BLOCK_UNINIT)) {
> > @@ -336,7 +345,7 @@ ext4_read_block_bitmap(struct super_block *sb, ext4_group_t block_group)
> > 		 */
> > 		set_bitmap_uptodate(bh);
> > 		unlock_buffer(bh);
> > -		return bh;
> > +		goto verify;
> > 	}
> > 	/*
> > 	 * submit the buffer_head for read. We can
> > @@ -353,11 +362,27 @@ ext4_read_block_bitmap(struct super_block *sb, ext4_group_t block_group)
> > 			    block_group, bitmap_blk);
> > 		return NULL;
> > 	}
> > -	ext4_valid_block_bitmap(sb, desc, block_group, bh);
> > +
> > +verify:
> > +	if (buffer_verified(bh))
> > +		return bh;
> > 	/*
> > 	 * file system mounted not to panic on error,
> > -	 * continue with corrupt bitmap
> > +	 * -EIO with corrupt bitmap
> > 	 */
> > +	ext4_lock_group(sb, block_group);
> > +	if (!ext4_valid_block_bitmap(sb, desc, block_group, bh) ||
> > +	    !ext4_bitmap_csum_verify(sb, block_group,
> > +				     desc->bg_block_bitmap_csum, bh,
> > +				     (EXT4_BLOCKS_PER_GROUP(sb) + 7) / 8)) {
> > +		ext4_unlock_group(sb, block_group);
> > +		put_bh(bh);
> > +		ext4_error(sb, "Corrupt block bitmap - block_group = %u, "
> > +			   "block_bitmap = %llu", block_group, bitmap_blk);
> > +		return NULL;
> > +	}
> > +	ext4_unlock_group(sb, block_group);
> > +	set_buffer_verified(bh);
> > 	return bh;
> > }
> > 
> > diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> > index 248cbd2..df149b3 100644
> > --- a/fs/ext4/ext4.h
> > +++ b/fs/ext4/ext4.h
> > @@ -269,7 +269,8 @@ struct ext4_group_desc
> > 	__le16	bg_free_inodes_count_lo;/* Free inodes count */
> > 	__le16	bg_used_dirs_count_lo;	/* Directories count */
> > 	__le16	bg_flags;		/* EXT4_BG_flags (INODE_UNINIT, etc) */
> > -	__u32	bg_reserved[2];		/* Likely block/inode bitmap checksum */
> > +	__u32	bg_reserved[1];		/* unclaimed */
> > +	__le32	bg_block_bitmap_csum;	/* crc32c(uuid+group+bbitmap) */
> 
> Same comment as for the inode bitmap checksum - it should be split into
> two __le16 fields, so that we get at least some coverage for the vast
> majority of existing filesystems.
> 
> > 	__le16  bg_itable_unused_lo;	/* Unused inodes count */
> > 	__le16  bg_checksum;		/* crc16(sb_uuid+group+desc) */
> > 	__le32	bg_block_bitmap_hi;	/* Blocks bitmap block MSB */
> > @@ -1731,6 +1732,10 @@ void ext4_bitmap_csum_set(struct super_block *sb, ext4_group_t group,
> > 			  __le32 *csum, struct buffer_head *bh, int sz);
> > 
> > /* balloc.c */
> > +extern int ext4_valid_block_bitmap(struct super_block *sb,
> > +				   struct ext4_group_desc *desc,
> > +				   unsigned int block_group,
> > +				   struct buffer_head *bh);
> > extern unsigned int ext4_block_group(struct super_block *sb,
> > 			ext4_fsblk_t blocknr);
> > extern ext4_grpblk_t ext4_block_group_offset(struct super_block *sb,
> > diff --git a/fs/ext4/ialloc.c b/fs/ext4/ialloc.c
> > index 53faffc..a335d19 100644
> > --- a/fs/ext4/ialloc.c
> > +++ b/fs/ext4/ialloc.c
> > @@ -984,6 +984,11 @@ got:
> > 			free = ext4_free_blocks_after_init(sb, group, gdp);
> > 			gdp->bg_flags &= cpu_to_le16(~EXT4_BG_BLOCK_UNINIT);
> > 			ext4_free_blks_set(sb, gdp, free);
> > +			ext4_bitmap_csum_set(sb, group,
> > +					     &gdp->bg_block_bitmap_csum,
> > +					     block_bitmap_bh,
> > +					     (EXT4_BLOCKS_PER_GROUP(sb) + 7) /
> > +					     8);
> > 			gdp->bg_checksum = ext4_group_desc_csum(sbi, group,
> > 								gdp);
> > 		}
> > diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> > index 17a5a57..8dc3055 100644
> > --- a/fs/ext4/mballoc.c
> > +++ b/fs/ext4/mballoc.c
> > @@ -895,6 +895,33 @@ static int ext4_mb_init_cache(struct page *page, char *incore)
> > 		if (bh[i] && !buffer_uptodate(bh[i]))
> > 			goto out;
> > 
> > +	for (i = 0; i < groups_per_page; i++) {
> > +		struct ext4_group_desc *desc;
> > +
> > +		if (!bh[i] || !bh[i]->b_end_io)
> > +			continue;
> 
> Please don't treat pointers as boolean values.  I'd prefer to see a
> proper comparison like "bh[i] == NULL" here.
> 
> Also, it isn't obvious why the check for b_end_io is needed here?

If b_end_io is set then the block is being read in and needs checking.
BH_Verified would work just as well here and be a little clearer.  I think I
had this patch written before I added the BH_Verified flag and forgot to update
this patch.  Oops. Good catch.

> > +		desc = ext4_get_group_desc(sb, first_group + i, NULL);
> > +		if (!desc)
> > +			goto out;
> > +
> > +		if (buffer_verified(bh[i]))
> > +			continue;
> > +		ext4_lock_group(sb, first_group + i);
> > +		if (!ext4_valid_block_bitmap(sb, desc, first_group + i,
> > +					     bh[i]) ||
> > +		    !ext4_bitmap_csum_verify(sb, first_group + i,
> > +					     desc->bg_block_bitmap_csum, bh[i],
> > +					     (EXT4_BLOCKS_PER_GROUP(sb) + 7) /
> > +					      8)) {
> > +			ext4_unlock_group(sb, first_group + i);
> > +			ext4_error(sb, "Corrupt block bitmap, group = %u",
> > +				   first_group + i);
> > +			goto out;
> > +		}
> > +		ext4_unlock_group(sb, first_group + i);
> > +		set_buffer_verified(bh[i]);
> > +	}
> 
> 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_buffered_read_sync(bh, uptodate);
> 
>         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);
> > 
>