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Date: Fri, 2 Sep 2011 12:18:28 -0700
From: "Darrick J. Wong" <djwong@...ibm.com>
To: Andreas Dilger <adilger.kernel@...ger.ca>
Cc: Theodore Tso <tytso@....edu>,
Sunil Mushran <sunil.mushran@...cle.com>,
Amir Goldstein <amir73il@...il.com>,
linux-kernel <linux-kernel@...r.kernel.org>,
Andi Kleen <andi@...stfloor.org>,
Mingming Cao <cmm@...ibm.com>,
Joel Becker <jlbec@...lplan.org>,
linux-fsdevel <linux-fsdevel@...r.kernel.org>,
linux-ext4@...r.kernel.org, Coly Li <colyli@...il.com>
Subject: Re: [PATCH 08/16] ext4: Calculate and verify checksums for inode
bitmaps
On Wed, Aug 31, 2011 at 10:49:05PM -0600, Andreas Dilger wrote:
> On 2011-08-31, at 6:31 PM, Darrick J. Wong wrote:
> > Compute and verify the checksum of the inode bitmap; the checkum is stored in
> > the block group descriptor.
> >
> > Signed-off-by: Darrick J. Wong <djwong@...ibm.com>
> > ---
> > fs/ext4/ext4.h | 3 ++-
> > fs/ext4/ialloc.c | 33 ++++++++++++++++++++++++++++++---
> > 2 files changed, 32 insertions(+), 4 deletions(-)
> >
> >
> > diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> > index bc7ace1..248cbd2 100644
> > --- a/fs/ext4/ext4.h
> > +++ b/fs/ext4/ext4.h
> > @@ -279,7 +279,8 @@ struct ext4_group_desc
> > __le16 bg_free_inodes_count_hi;/* Free inodes count MSB */
> > __le16 bg_used_dirs_count_hi; /* Directories count MSB */
> > __le16 bg_itable_unused_hi; /* Unused inodes count MSB */
> > - __u32 bg_reserved2[3];
> > + __le32 bg_inode_bitmap_csum; /* crc32c(uuid+group+ibitmap) */
> > + __u32 bg_reserved2[2];
> > };
>
> I would prefer if there was a 16-bit checksum for the (most common)
> 32-byte group descriptors, and this was extended to a 32-bit checksum
> for the (much less common) 64-byte+ group descriptors. For filesystems
> that are newly formatted with the 64bit feature it makes no difference,
> but virtually all ext3/4 filesystems have only the smaller group descriptors.
>
> Regardless of whether using half of the crc32c is better or worse than
> using crc16 for the bitmap blocks, storing _any_ checksum is better than
> storing nothing at all. I would propose the following:
That's an interesting reframing of the argument that I hadn't considered. I'd
fallen into the idea of needing crc32c because of its bit error guarantees (all
corruptions of odd numbers of bits and all corruptions of fewer than ...4?
bits) that I hadn't quite realized that even if crc16 can't guarantee to find
any corruption at all, it still _might_, and that's better than nothing.
Ok, let's split the 32-bit fields and use crc16 for the case of 32-byte block
group descriptors.
> struct ext4_group_desc
> {
> __le32 bg_block_bitmap_lo; /* Blocks bitmap block */
> __le32 bg_inode_bitmap_lo; /* Inodes bitmap block */
> __le32 bg_inode_table_lo; /* Inodes table block */
> __le16 bg_free_blocks_count_lo; /* Free blocks count */
> __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) */
> __le32 bg_exclude_bitmap_lo; /* Exclude bitmap block */
> __le16 bg_block_bitmap_csum_lo; /* Block bitmap checksum */
> __le16 bg_inode_bitmap_csum_lo; /* Inode bitmap checksum */
> __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 */
> __le32 bg_inode_bitmap_hi; /* Inodes bitmap block MSB */
> __le32 bg_inode_table_hi; /* Inodes table block MSB */
> __le16 bg_free_blocks_count_hi; /* Free blocks count MSB */
> __le16 bg_free_inodes_count_hi; /* Free inodes count MSB */
> __le16 bg_used_dirs_count_hi; /* Directories count MSB */
> __le16 bg_itable_unused_hi; /* Unused inodes count MSB */
> __le32 bg_exclude_bitmap_hi; /* Exclude bitmap block MSB */
> __le16 bg_block_bitmap_csum_hi; /* Blocks bitmap checksum MSB */
> __le16 bg_inode_bitmap_csum_hi; /* Inodes bitmap checksum MSB */
> __le32 bg_reserved2;
> };
>
> This is also different from your layout because it locates the block bitmap
> checksum field before the inode bitmap checksum, to more closely match the
> order of other fields in this structure.
Er.. I reversed the order in the structure definition just prior to publishing,
and forgot to update the wiki page. Well I guess I'm about to update it again.
:)
> > /*
> > diff --git a/fs/ext4/ialloc.c b/fs/ext4/ialloc.c
> > index 9c63f27..53faffc 100644
> > --- a/fs/ext4/ialloc.c
> > +++ b/fs/ext4/ialloc.c
> > @@ -82,12 +82,18 @@ static unsigned ext4_init_inode_bitmap(struct super_block *sb,
> > 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_inode_bitmap_csum, bh,
> > + (EXT4_INODES_PER_GROUP(sb) + 7) / 8);
>
> The number of inodes per group is already always a multiple of 8.
Ok. I suppose we can fix that in the lines below too.
> > return 0;
> > }
> >
> > memset(bh->b_data, 0, (EXT4_INODES_PER_GROUP(sb) + 7) / 8);
> > ext4_mark_bitmap_end(EXT4_INODES_PER_GROUP(sb), sb->s_blocksize * 8,
> > bh->b_data);
> > + ext4_bitmap_csum_set(sb, block_group, &gdp->bg_inode_bitmap_csum, bh,
> > + (EXT4_INODES_PER_GROUP(sb) + 7) / 8);
> > + gdp->bg_checksum = ext4_group_desc_csum(sbi, block_group, gdp);
> >
> > return EXT4_INODES_PER_GROUP(sb);
> > }
> > @@ -118,12 +124,12 @@ ext4_read_inode_bitmap(struct super_block *sb, ext4_group_t block_group)
> > return NULL;
> > }
> > 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);
> > @@ -131,6 +137,7 @@ ext4_read_inode_bitmap(struct super_block *sb, ext4_group_t block_group)
> > ext4_init_inode_bitmap(sb, bh, block_group, desc);
> > set_bitmap_uptodate(bh);
> > set_buffer_uptodate(bh);
> > + set_buffer_verified(bh);
> > ext4_unlock_group(sb, block_group);
> > unlock_buffer(bh);
> > return bh;
> > @@ -144,7 +151,7 @@ ext4_read_inode_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
> > @@ -161,6 +168,21 @@ ext4_read_inode_bitmap(struct super_block *sb, ext4_group_t block_group)
> > block_group, bitmap_blk);
> > return NULL;
> > }
> > +
> > +verify:
> > + ext4_lock_group(sb, block_group);
> > + if (!buffer_verified(bh) &&
> > + !ext4_bitmap_csum_verify(sb, block_group,
> > + desc->bg_inode_bitmap_csum, bh,
> > + (EXT4_INODES_PER_GROUP(sb) + 7) / 8)) {
> > + ext4_unlock_group(sb, block_group);
> > + put_bh(bh);
> > + ext4_error(sb, "Corrupt inode bitmap - block_group = %u, "
> > + "inode_bitmap = %llu", block_group, bitmap_blk);
> > + return NULL;
>
> At some point we should add a flag like EXT4_BG_INODE_ERROR so that the
> group can be marked in error on disk, and skipped for future allocations,
> but the whole filesystem does not need to be remounted read-only. That's
> for another patch, however.
Agreed. :)
--D
> > + }
> > + ext4_unlock_group(sb, block_group);
> > + set_buffer_verified(bh);
> > return bh;
> > }
> >
> > @@ -265,6 +287,8 @@ void ext4_free_inode(handle_t *handle, struct inode *inode)
> > ext4_used_dirs_set(sb, gdp, count);
> > percpu_counter_dec(&sbi->s_dirs_counter);
> > }
> > + ext4_bitmap_csum_set(sb, block_group, &gdp->bg_inode_bitmap_csum,
> > + bitmap_bh, (EXT4_INODES_PER_GROUP(sb) + 7) / 8);
> > gdp->bg_checksum = ext4_group_desc_csum(sbi, block_group, gdp);
> > ext4_unlock_group(sb, block_group);
> >
> > @@ -784,6 +808,9 @@ static int ext4_claim_inode(struct super_block *sb,
> > atomic_inc(&sbi->s_flex_groups[f].used_dirs);
> > }
> > }
> > + ext4_bitmap_csum_set(sb, group, &gdp->bg_inode_bitmap_csum,
> > + inode_bitmap_bh,
> > + (EXT4_INODES_PER_GROUP(sb) + 7) / 8);
> > gdp->bg_checksum = ext4_group_desc_csum(sbi, group, gdp);
> > err_ret:
> > ext4_unlock_group(sb, group);
> >
>
> --
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