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Date: Thu, 15 Sep 2011 10:19:34 -0700
From: "Darrick J. Wong" <djwong@...ibm.com>
To: "Theodore Ts'o" <tytso@....edu>
Cc: Amir Goldstein <amir73il@...il.com>, linux-ext4@...r.kernel.org
Subject: Re: On-disk field assignments for metadata checksum and snapshots
On Thu, Sep 15, 2011 at 09:55:12AM -0700, Darrick J. Wong wrote:
> On Thu, Sep 15, 2011 at 11:12:22AM -0400, Theodore Ts'o wrote:
> >
> > Hi Darrick, Amir,
> >
> > Could you please take a look at the changes here and make sure they look
> > sane to you? I just want to make sure we're all on the same page as
> > far as on-disk field assignments are concerned.
> >
> > Also, one thought for Darrick. I note that with the assignment of
> > l_i_checksum, we've exhausted the very last field in the base 128-byte
> > inode. Given that the checksum is protecting a 128 byte or 256 byte
> > inode, I wonder if we need to use a full 32 bit checksum. Maybe we
> > should just use 16 bits of a crc32c?
> >
> > I did some web searching on the issue, and the recommendation I've come
> > across is the following:
> >
> > "Generally speaking, an n-bit CRC's error detection properties
> > degrade after 2**(n-1)-1 data bits."
> >
> > So a CRC-16 will be good up to just under 4k. A 16-bit truncated crc32c
> > will presumably not be as good as a crc16, but seems to me that it's
> > probably fine for a 128-256 byte inode. Especially since the main thing
> > we're generally worried about is detecting a block getting written to
> > the wrong location, overwriting an existing inode table block. So if we
> > were really paranoid we could verify the checksums for all of the inodes
> > in a particular inode table block when we read in the inode table block
> > in question.
>
> On the other hand, you can set inode_size = block_size, which means that with a
> 4k inode + 32-bit inode number + 16-byte UUID you actually could run afoul of
> that degradation. But that seems like an extreme argument for an infrequent
> case.
>
> Actually, I've started wondering if we could split the 4 bytes of the crc32c
> among the first few inodes of the block, and compute the checksums at block
> size granularity. Though that would make inode updates particularly more
> expensive... but if I'm going to shift the write-time checksum to a journal
> callback then it's not going to matter (for the journal-using users, anyway).
>
> Though with that scheme, you'd probably lose more inodes for any given
> integrity error. It also means that the checksum size in each inode becomes
> variable (32 bits if inode=blocksize, 16 if inode=blocksize/2, and 8
> otherwise), which is a somewhat confusing schema.
>
> <shrug> Do you anticipate a need to add more fields to 128-byte inode
> filesystems? I think most of those would be former ext2/3 filesystems,
> floppies, and "small" filesystems, correct?
>
> Or does this second scheme sound more attractive?
I forgot to say, "as opposed to storing the lower 16 bits below 128 bytes and
the upper 16 bits somewhere above it."
--D
>
> --D
> >
> > - Ted
> >
> >
> > commit ceade753f14f2697d329f71b5277b49fd46fcb55
> > Author: Theodore Ts'o <tytso@....edu>
> > Date: Thu Sep 15 10:38:55 2011 -0400
> >
> > libext2fs: add metadata checksum and snapshot feature flags
> >
> > Reserve EXT4_FEATURE_RO_COMPAT_METADATA_CSUM and
> > EXT2_FEATURE_COMPAT_EXCLUDE_BITMAP. Also reserve fields in the
> > superblock and the inode for the checksums. In the block group
> > descriptor, reserve the exclude bitmap field for the snapshot feature,
> > and checksums for the inode and block allocation bitmaps.
> >
> > With this commit, the metadata checksum and exclude bitmap features
> > should have reserved all of the fields they need in ext4's on-disk
> > format.
> >
> > This commit also fixes an a missing byte swap for s_overhead_blocks.
> >
> > Signed-off-by: "Theodore Ts'o" <tytso@....edu>
> > Cc: Darrick J. Wong <djwong@...ibm.com>
> > Cc: Amir Goldstein <amir73il@...il.com>
> >
> > diff --git a/debugfs/set_fields.c b/debugfs/set_fields.c
> > index ac6bc25..cba9c12 100644
> > --- a/debugfs/set_fields.c
> > +++ b/debugfs/set_fields.c
> > @@ -144,6 +144,7 @@ static struct field_set_info super_fields[] = {
> > { "usr_quota_inum", &set_sb.s_usr_quota_inum, 4, parse_uint },
> > { "grp_quota_inum", &set_sb.s_grp_quota_inum, 4, parse_uint },
> > { "overhead_blocks", &set_sb.s_overhead_blocks, 4, parse_uint },
> > + { "checksum", &set_sb.s_checksum, 2, parse_uint },
> > { 0, 0, 0, 0 }
> > };
> >
> > @@ -179,6 +180,7 @@ static struct field_set_info inode_fields[] = {
> > { "fsize", &set_inode.osd2.hurd2.h_i_fsize, 1, parse_uint },
> > { "uid_high", &set_inode.osd2.linux2.l_i_uid_high, 2, parse_uint },
> > { "gid_high", &set_inode.osd2.linux2.l_i_gid_high, 2, parse_uint },
> > + { "checksum", &set_inode.osd2.linux2.l_i_checksum, 4, parse_uint },
> > { "author", &set_inode.osd2.hurd2.h_i_author, 4, parse_uint },
> > { "bmap", NULL, 4, parse_bmap, FLAG_ARRAY },
> > { 0, 0, 0, 0 }
> > @@ -192,7 +194,6 @@ static struct field_set_info ext2_bg_fields[] = {
> > { "free_inodes_count", &set_gd.bg_free_inodes_count, 2, parse_uint },
> > { "used_dirs_count", &set_gd.bg_used_dirs_count, 2, parse_uint },
> > { "flags", &set_gd.bg_flags, 2, parse_uint },
> > - { "reserved", &set_gd.bg_reserved, 2, parse_uint, FLAG_ARRAY, 2 },
> > { "itable_unused", &set_gd.bg_itable_unused, 2, parse_uint },
> > { "checksum", &set_gd.bg_checksum, 2, parse_gd_csum },
> > { 0, 0, 0, 0 }
> > diff --git a/lib/e2p/feature.c b/lib/e2p/feature.c
> > index 16fba53..965fc16 100644
> > --- a/lib/e2p/feature.c
> > +++ b/lib/e2p/feature.c
> > @@ -40,6 +40,8 @@ static struct feature feature_list[] = {
> > "resize_inode" },
> > { E2P_FEATURE_COMPAT, EXT2_FEATURE_COMPAT_LAZY_BG,
> > "lazy_bg" },
> > + { E2P_FEATURE_COMPAT, EXT2_FEATURE_COMPAT_EXCLUDE_BITMAP,
> > + "snapshot" },
> >
> > { E2P_FEATURE_RO_INCOMPAT, EXT2_FEATURE_RO_COMPAT_SPARSE_SUPER,
> > "sparse_super" },
> > @@ -59,6 +61,8 @@ static struct feature feature_list[] = {
> > "quota" },
> > { E2P_FEATURE_RO_INCOMPAT, EXT4_FEATURE_RO_COMPAT_BIGALLOC,
> > "bigalloc"},
> > + { E2P_FEATURE_RO_INCOMPAT, EXT4_FEATURE_RO_COMPAT_METADATA_CSUM,
> > + "metadata_csum"},
> >
> > { E2P_FEATURE_INCOMPAT, EXT2_FEATURE_INCOMPAT_COMPRESSION,
> > "compression" },
> > diff --git a/lib/e2p/ls.c b/lib/e2p/ls.c
> > index 0f36f40..aaacdaa 100644
> > --- a/lib/e2p/ls.c
> > +++ b/lib/e2p/ls.c
> > @@ -413,6 +413,10 @@ void list_super2(struct ext2_super_block * sb, FILE *f)
> > if (sb->s_grp_quota_inum)
> > fprintf(f, "Group quota inode: %u\n",
> > sb->s_grp_quota_inum);
> > +
> > + if (sb->s_feature_ro_compat & EXT4_FEATURE_RO_COMPAT_METADATA_CSUM)
> > + fprintf(f, "Checksum: 0x%08x\n",
> > + sb->s_checksum);
> > }
> >
> > void list_super (struct ext2_super_block * s)
> > diff --git a/lib/ext2fs/ext2_fs.h b/lib/ext2fs/ext2_fs.h
> > index 4fec5db..1b02054 100644
> > --- a/lib/ext2fs/ext2_fs.h
> > +++ b/lib/ext2fs/ext2_fs.h
> > @@ -142,7 +142,9 @@ struct ext2_group_desc
> > __u16 bg_free_inodes_count; /* Free inodes count */
> > __u16 bg_used_dirs_count; /* Directories count */
> > __u16 bg_flags;
> > - __u32 bg_reserved[2];
> > + __u32 bg_exclude_bitmap_lo; /* Exclude bitmap for snapshots */
> > + __u16 bg_block_bitmap_csum_lo;/* crc32c(s_uuid+grp_num+bitmap) LSB */
> > + __u16 bg_inode_bitmap_csum_lo;/* crc32c(s_uuid+grp_num+bitmap) LSB */
> > __u16 bg_itable_unused; /* Unused inodes count */
> > __u16 bg_checksum; /* crc16(s_uuid+grouo_num+group_desc)*/
> > };
> > @@ -159,7 +161,9 @@ struct ext4_group_desc
> > __u16 bg_free_inodes_count; /* Free inodes count */
> > __u16 bg_used_dirs_count; /* Directories count */
> > __u16 bg_flags; /* EXT4_BG_flags (INODE_UNINIT, etc) */
> > - __u32 bg_reserved[2]; /* Likely block/inode bitmap checksum */
> > + __u32 bg_exclude_bitmap_lo; /* Exclude bitmap for snapshots */
> > + __u16 bg_block_bitmap_csum_lo;/* crc32c(s_uuid+grp_num+bitmap) LSB */
> > + __u16 bg_inode_bitmap_csum_lo;/* crc32c(s_uuid+grp_num+bitmap) LSB */
> > __u16 bg_itable_unused; /* Unused inodes count */
> > __u16 bg_checksum; /* crc16(sb_uuid+group+desc) */
> > __u32 bg_block_bitmap_hi; /* Blocks bitmap block MSB */
> > @@ -169,7 +173,10 @@ struct ext4_group_desc
> > __u16 bg_free_inodes_count_hi;/* Free inodes count MSB */
> > __u16 bg_used_dirs_count_hi; /* Directories count MSB */
> > __u16 bg_itable_unused_hi; /* Unused inodes count MSB */
> > - __u32 bg_reserved2[3];
> > + __u32 bg_exclude_bitmap_hi; /* Exclude bitmap block MSB */
> > + __u16 bg_block_bitmap_csum_hi;/* crc32c(s_uuid+grp_num+bitmap) MSB */
> > + __u16 bg_inode_bitmap_csum_hi;/* crc32c(s_uuid+grp_num+bitmap) MSB */
> > + __u32 bg_reserved;
> > };
> >
> > #define EXT2_BG_INODE_UNINIT 0x0001 /* Inode table/bitmap not initialized */
> > @@ -363,7 +370,7 @@ struct ext2_inode {
> > __u16 l_i_file_acl_high;
> > __u16 l_i_uid_high; /* these 2 fields */
> > __u16 l_i_gid_high; /* were reserved2[0] */
> > - __u32 l_i_reserved2;
> > + __u32 l_i_checksum; /* crc32c(uuid+inum+inode) */
> > } linux2;
> > struct {
> > __u8 h_i_frag; /* Fragment number */
> > @@ -410,7 +417,7 @@ struct ext2_inode_large {
> > __u16 l_i_file_acl_high;
> > __u16 l_i_uid_high; /* these 2 fields */
> > __u16 l_i_gid_high; /* were reserved2[0] */
> > - __u32 l_i_reserved2;
> > + __u32 l_i_checksum; /* crc32c(uuid+inum+inode) */
> > } linux2;
> > struct {
> > __u8 h_i_frag; /* Fragment number */
> > @@ -441,7 +448,7 @@ struct ext2_inode_large {
> > #define i_gid_low i_gid
> > #define i_uid_high osd2.linux2.l_i_uid_high
> > #define i_gid_high osd2.linux2.l_i_gid_high
> > -#define i_reserved2 osd2.linux2.l_i_reserved2
> > +#define i_checksum osd2.linux2.l_i_checksum
> > #else
> > #if defined(__GNU__)
> >
> > @@ -623,7 +630,8 @@ struct ext2_super_block {
> > __u32 s_usr_quota_inum; /* inode number of user quota file */
> > __u32 s_grp_quota_inum; /* inode number of group quota file */
> > __u32 s_overhead_blocks; /* overhead blocks/clusters in fs */
> > - __u32 s_reserved[109]; /* Padding to the end of the block */
> > + __u32 s_checksum; /* crc32c(superblock) */
> > + __u32 s_reserved[108]; /* Padding to the end of the block */
> > };
> >
> > #define EXT4_S_ERR_LEN (EXT4_S_ERR_END - EXT4_S_ERR_START)
> > @@ -671,7 +679,9 @@ struct ext2_super_block {
> > #define EXT2_FEATURE_COMPAT_RESIZE_INODE 0x0010
> > #define EXT2_FEATURE_COMPAT_DIR_INDEX 0x0020
> > #define EXT2_FEATURE_COMPAT_LAZY_BG 0x0040
> > -#define EXT2_FEATURE_COMPAT_EXCLUDE_INODE 0x0080
> > +/* #define EXT2_FEATURE_COMPAT_EXCLUDE_INODE 0x0080 not used, legacy */
> > +#define EXT2_FEATURE_COMPAT_EXCLUDE_BITMAP 0x0100
> > +
> >
> > #define EXT2_FEATURE_RO_COMPAT_SPARSE_SUPER 0x0001
> > #define EXT2_FEATURE_RO_COMPAT_LARGE_FILE 0x0002
> > @@ -683,6 +693,7 @@ struct ext2_super_block {
> > #define EXT4_FEATURE_RO_COMPAT_HAS_SNAPSHOT 0x0080
> > #define EXT4_FEATURE_RO_COMPAT_QUOTA 0x0100
> > #define EXT4_FEATURE_RO_COMPAT_BIGALLOC 0x0200
> > +#define EXT4_FEATURE_RO_COMPAT_METADATA_CSUM 0x0400
> >
> > #define EXT2_FEATURE_INCOMPAT_COMPRESSION 0x0001
> > #define EXT2_FEATURE_INCOMPAT_FILETYPE 0x0002
> > diff --git a/lib/ext2fs/swapfs.c b/lib/ext2fs/swapfs.c
> > index 87b1a2e..d1c4a56 100644
> > --- a/lib/ext2fs/swapfs.c
> > +++ b/lib/ext2fs/swapfs.c
> > @@ -78,6 +78,8 @@ void ext2fs_swap_super(struct ext2_super_block * sb)
> > sb->s_snapshot_list = ext2fs_swab32(sb->s_snapshot_list);
> > sb->s_usr_quota_inum = ext2fs_swab32(sb->s_usr_quota_inum);
> > sb->s_grp_quota_inum = ext2fs_swab32(sb->s_grp_quota_inum);
> > + sb->s_overhead_blocks = ext2fs_swab32(sb->s_overhead_blocks);
> > + sb->s_checksum = ext2fs_swab32(sb->s_checksum);
> >
> > for (i=0; i < 4; i++)
> > sb->s_hash_seed[i] = ext2fs_swab32(sb->s_hash_seed[i]);
> > @@ -106,6 +108,11 @@ void ext2fs_swap_group_desc2(ext2_filsys fs, struct ext2_group_desc *gdp)
> > gdp->bg_free_inodes_count = ext2fs_swab16(gdp->bg_free_inodes_count);
> > gdp->bg_used_dirs_count = ext2fs_swab16(gdp->bg_used_dirs_count);
> > gdp->bg_flags = ext2fs_swab16(gdp->bg_flags);
> > + gdp->bg_exclude_bitmap_lo = ext2fs_swab32(gdp->bg_exclude_bitmap_lo);
> > + gdp->bg_block_bitmap_csum_lo =
> > + ext2fs_swab16(gdp->bg_block_bitmap_csum_lo);
> > + gdp->bg_inode_bitmap_csum_lo =
> > + ext2fs_swab16(gdp->bg_inode_bitmap_csum_lo);
> > gdp->bg_itable_unused = ext2fs_swab16(gdp->bg_itable_unused);
> > gdp->bg_checksum = ext2fs_swab16(gdp->bg_checksum);
> > /* If we're 32-bit, we're done */
> > @@ -125,6 +132,11 @@ void ext2fs_swap_group_desc2(ext2_filsys fs, struct ext2_group_desc *gdp)
> > gdp4->bg_used_dirs_count_hi =
> > ext2fs_swab16(gdp4->bg_used_dirs_count_hi);
> > gdp4->bg_itable_unused_hi = ext2fs_swab16(gdp4->bg_itable_unused_hi);
> > + gdp->bg_exclude_bitmap_hi = ext2fs_swab16(gdp->bg_exclude_bitmap_hi);
> > + gdp->bg_block_bitmap_csum_hi =
> > + ext2fs_swab16(gdp->bg_block_bitmap_csum_hi);
> > + gdp->bg_inode_bitmap_csum_hi =
> > + ext2fs_swab16(gdp->bg_inode_bitmap_csum_hi);
> > }
> >
> > void ext2fs_swap_group_desc(struct ext2_group_desc *gdp)
> > @@ -244,8 +256,8 @@ void ext2fs_swap_inode_full(ext2_filsys fs, struct ext2_inode_large *t,
> > ext2fs_swab16 (f->osd2.linux2.l_i_uid_high);
> > t->osd2.linux2.l_i_gid_high =
> > ext2fs_swab16 (f->osd2.linux2.l_i_gid_high);
> > - t->osd2.linux2.l_i_reserved2 =
> > - ext2fs_swab32(f->osd2.linux2.l_i_reserved2);
> > + t->osd2.linux2.l_i_checksum =
> > + ext2fs_swab32(f->osd2.linux2.checksum);
> > break;
> > case EXT2_OS_HURD:
> > t->osd1.hurd1.h_i_translator =
> > diff --git a/lib/ext2fs/tst_inode_size.c b/lib/ext2fs/tst_inode_size.c
> > index 962f1cd..683b79c 100644
> > --- a/lib/ext2fs/tst_inode_size.c
> > +++ b/lib/ext2fs/tst_inode_size.c
> > @@ -61,7 +61,7 @@ void check_structure_fields()
> > check_field(osd2.linux2.l_i_file_acl_high);
> > check_field(osd2.linux2.l_i_uid_high);
> > check_field(osd2.linux2.l_i_gid_high);
> > - check_field(osd2.linux2.l_i_reserved2);
> > + check_field(osd2.linux2.l_i_checksum);
> > printf("Ending offset is %d\n\n", cur_offset);
> > #endif
> > }
> > diff --git a/lib/ext2fs/tst_super_size.c b/lib/ext2fs/tst_super_size.c
> > index 1e5a524..75659ae 100644
> > --- a/lib/ext2fs/tst_super_size.c
> > +++ b/lib/ext2fs/tst_super_size.c
> > @@ -126,6 +126,7 @@ void check_superblock_fields()
> > check_field(s_usr_quota_inum);
> > check_field(s_grp_quota_inum);
> > check_field(s_overhead_blocks);
> > + check_field(s_checksum);
> > check_field(s_reserved);
> > printf("Ending offset is %d\n\n", cur_offset);
> > #endif
> >
> --
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