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diff --git a/Documentation/filesystems/xfs-self-describing-metadata.txt b/Documentation/filesystems/xfs-self-describing-metadata.txt new file mode 100644 index 000000000..05aa45516 --- /dev/null +++ b/Documentation/filesystems/xfs-self-describing-metadata.txt @@ -0,0 +1,350 @@ +XFS Self Describing Metadata +---------------------------- + +Introduction +------------ + +The largest scalability problem facing XFS is not one of algorithmic +scalability, but of verification of the filesystem structure. Scalabilty of the +structures and indexes on disk and the algorithms for iterating them are +adequate for supporting PB scale filesystems with billions of inodes, however it +is this very scalability that causes the verification problem. + +Almost all metadata on XFS is dynamically allocated. The only fixed location +metadata is the allocation group headers (SB, AGF, AGFL and AGI), while all +other metadata structures need to be discovered by walking the filesystem +structure in different ways. While this is already done by userspace tools for +validating and repairing the structure, there are limits to what they can +verify, and this in turn limits the supportable size of an XFS filesystem. + +For example, it is entirely possible to manually use xfs_db and a bit of +scripting to analyse the structure of a 100TB filesystem when trying to +determine the root cause of a corruption problem, but it is still mainly a +manual task of verifying that things like single bit errors or misplaced writes +weren't the ultimate cause of a corruption event. It may take a few hours to a +few days to perform such forensic analysis, so for at this scale root cause +analysis is entirely possible. + +However, if we scale the filesystem up to 1PB, we now have 10x as much metadata +to analyse and so that analysis blows out towards weeks/months of forensic work. +Most of the analysis work is slow and tedious, so as the amount of analysis goes +up, the more likely that the cause will be lost in the noise. Hence the primary +concern for supporting PB scale filesystems is minimising the time and effort +required for basic forensic analysis of the filesystem structure. + + +Self Describing Metadata +------------------------ + +One of the problems with the current metadata format is that apart from the +magic number in the metadata block, we have no other way of identifying what it +is supposed to be. We can't even identify if it is the right place. Put simply, +you can't look at a single metadata block in isolation and say "yes, it is +supposed to be there and the contents are valid". + +Hence most of the time spent on forensic analysis is spent doing basic +verification of metadata values, looking for values that are in range (and hence +not detected by automated verification checks) but are not correct. Finding and +understanding how things like cross linked block lists (e.g. sibling +pointers in a btree end up with loops in them) are the key to understanding what +went wrong, but it is impossible to tell what order the blocks were linked into +each other or written to disk after the fact. + +Hence we need to record more information into the metadata to allow us to +quickly determine if the metadata is intact and can be ignored for the purpose +of analysis. We can't protect against every possible type of error, but we can +ensure that common types of errors are easily detectable. Hence the concept of +self describing metadata. + +The first, fundamental requirement of self describing metadata is that the +metadata object contains some form of unique identifier in a well known +location. This allows us to identify the expected contents of the block and +hence parse and verify the metadata object. IF we can't independently identify +the type of metadata in the object, then the metadata doesn't describe itself +very well at all! + +Luckily, almost all XFS metadata has magic numbers embedded already - only the +AGFL, remote symlinks and remote attribute blocks do not contain identifying +magic numbers. Hence we can change the on-disk format of all these objects to +add more identifying information and detect this simply by changing the magic +numbers in the metadata objects. That is, if it has the current magic number, +the metadata isn't self identifying. If it contains a new magic number, it is +self identifying and we can do much more expansive automated verification of the +metadata object at runtime, during forensic analysis or repair. + +As a primary concern, self describing metadata needs some form of overall +integrity checking. We cannot trust the metadata if we cannot verify that it has +not been changed as a result of external influences. Hence we need some form of +integrity check, and this is done by adding CRC32c validation to the metadata +block. If we can verify the block contains the metadata it was intended to +contain, a large amount of the manual verification work can be skipped. + +CRC32c was selected as metadata cannot be more than 64k in length in XFS and +hence a 32 bit CRC is more than sufficient to detect multi-bit errors in +metadata blocks. CRC32c is also now hardware accelerated on common CPUs so it is +fast. So while CRC32c is not the strongest of possible integrity checks that +could be used, it is more than sufficient for our needs and has relatively +little overhead. Adding support for larger integrity fields and/or algorithms +does really provide any extra value over CRC32c, but it does add a lot of +complexity and so there is no provision for changing the integrity checking +mechanism. + +Self describing metadata needs to contain enough information so that the +metadata block can be verified as being in the correct place without needing to +look at any other metadata. This means it needs to contain location information. +Just adding a block number to the metadata is not sufficient to protect against +mis-directed writes - a write might be misdirected to the wrong LUN and so be +written to the "correct block" of the wrong filesystem. Hence location +information must contain a filesystem identifier as well as a block number. + +Another key information point in forensic analysis is knowing who the metadata +block belongs to. We already know the type, the location, that it is valid +and/or corrupted, and how long ago that it was last modified. Knowing the owner +of the block is important as it allows us to find other related metadata to +determine the scope of the corruption. For example, if we have a extent btree +object, we don't know what inode it belongs to and hence have to walk the entire +filesystem to find the owner of the block. Worse, the corruption could mean that +no owner can be found (i.e. it's an orphan block), and so without an owner field +in the metadata we have no idea of the scope of the corruption. If we have an +owner field in the metadata object, we can immediately do top down validation to +determine the scope of the problem. + +Different types of metadata have different owner identifiers. For example, +directory, attribute and extent tree blocks are all owned by an inode, whilst +freespace btree blocks are owned by an allocation group. Hence the size and +contents of the owner field are determined by the type of metadata object we are +looking at. The owner information can also identify misplaced writes (e.g. +freespace btree block written to the wrong AG). + +Self describing metadata also needs to contain some indication of when it was +written to the filesystem. One of the key information points when doing forensic +analysis is how recently the block was modified. Correlation of set of corrupted +metadata blocks based on modification times is important as it can indicate +whether the corruptions are related, whether there's been multiple corruption +events that lead to the eventual failure, and even whether there are corruptions +present that the run-time verification is not detecting. + +For example, we can determine whether a metadata object is supposed to be free +space or still allocated if it is still referenced by its owner by looking at +when the free space btree block that contains the block was last written +compared to when the metadata object itself was last written. If the free space +block is more recent than the object and the object's owner, then there is a +very good chance that the block should have been removed from the owner. + +To provide this "written timestamp", each metadata block gets the Log Sequence +Number (LSN) of the most recent transaction it was modified on written into it. +This number will always increase over the life of the filesystem, and the only +thing that resets it is running xfs_repair on the filesystem. Further, by use of +the LSN we can tell if the corrupted metadata all belonged to the same log +checkpoint and hence have some idea of how much modification occurred between +the first and last instance of corrupt metadata on disk and, further, how much +modification occurred between the corruption being written and when it was +detected. + +Runtime Validation +------------------ + +Validation of self-describing metadata takes place at runtime in two places: + + - immediately after a successful read from disk + - immediately prior to write IO submission + +The verification is completely stateless - it is done independently of the +modification process, and seeks only to check that the metadata is what it says +it is and that the metadata fields are within bounds and internally consistent. +As such, we cannot catch all types of corruption that can occur within a block +as there may be certain limitations that operational state enforces of the +metadata, or there may be corruption of interblock relationships (e.g. corrupted +sibling pointer lists). Hence we still need stateful checking in the main code +body, but in general most of the per-field validation is handled by the +verifiers. + +For read verification, the caller needs to specify the expected type of metadata +that it should see, and the IO completion process verifies that the metadata +object matches what was expected. If the verification process fails, then it +marks the object being read as EFSCORRUPTED. The caller needs to catch this +error (same as for IO errors), and if it needs to take special action due to a +verification error it can do so by catching the EFSCORRUPTED error value. If we +need more discrimination of error type at higher levels, we can define new +error numbers for different errors as necessary. + +The first step in read verification is checking the magic number and determining +whether CRC validating is necessary. If it is, the CRC32c is calculated and +compared against the value stored in the object itself. Once this is validated, +further checks are made against the location information, followed by extensive +object specific metadata validation. If any of these checks fail, then the +buffer is considered corrupt and the EFSCORRUPTED error is set appropriately. + +Write verification is the opposite of the read verification - first the object +is extensively verified and if it is OK we then update the LSN from the last +modification made to the object, After this, we calculate the CRC and insert it +into the object. Once this is done the write IO is allowed to continue. If any +error occurs during this process, the buffer is again marked with a EFSCORRUPTED +error for the higher layers to catch. + +Structures +---------- + +A typical on-disk structure needs to contain the following information: + +struct xfs_ondisk_hdr { + __be32 magic; /* magic number */ + __be32 crc; /* CRC, not logged */ + uuid_t uuid; /* filesystem identifier */ + __be64 owner; /* parent object */ + __be64 blkno; /* location on disk */ + __be64 lsn; /* last modification in log, not logged */ +}; + +Depending on the metadata, this information may be part of a header structure +separate to the metadata contents, or may be distributed through an existing +structure. The latter occurs with metadata that already contains some of this +information, such as the superblock and AG headers. + +Other metadata may have different formats for the information, but the same +level of information is generally provided. For example: + + - short btree blocks have a 32 bit owner (ag number) and a 32 bit block + number for location. The two of these combined provide the same + information as @owner and @blkno in eh above structure, but using 8 + bytes less space on disk. + + - directory/attribute node blocks have a 16 bit magic number, and the + header that contains the magic number has other information in it as + well. hence the additional metadata headers change the overall format + of the metadata. + +A typical buffer read verifier is structured as follows: + +#define XFS_FOO_CRC_OFF offsetof(struct xfs_ondisk_hdr, crc) + +static void +xfs_foo_read_verify( + struct xfs_buf *bp) +{ + struct xfs_mount *mp = bp->b_target->bt_mount; + + if ((xfs_sb_version_hascrc(&mp->m_sb) && + !xfs_verify_cksum(bp->b_addr, BBTOB(bp->b_length), + XFS_FOO_CRC_OFF)) || + !xfs_foo_verify(bp)) { + XFS_CORRUPTION_ERROR(__func__, XFS_ERRLEVEL_LOW, mp, bp->b_addr); + xfs_buf_ioerror(bp, EFSCORRUPTED); + } +} + +The code ensures that the CRC is only checked if the filesystem has CRCs enabled +by checking the superblock of the feature bit, and then if the CRC verifies OK +(or is not needed) it verifies the actual contents of the block. + +The verifier function will take a couple of different forms, depending on +whether the magic number can be used to determine the format of the block. In +the case it can't, the code is structured as follows: + +static bool +xfs_foo_verify( + struct xfs_buf *bp) +{ + struct xfs_mount *mp = bp->b_target->bt_mount; + struct xfs_ondisk_hdr *hdr = bp->b_addr; + + if (hdr->magic != cpu_to_be32(XFS_FOO_MAGIC)) + return false; + + if (!xfs_sb_version_hascrc(&mp->m_sb)) { + if (!uuid_equal(&hdr->uuid, &mp->m_sb.sb_uuid)) + return false; + if (bp->b_bn != be64_to_cpu(hdr->blkno)) + return false; + if (hdr->owner == 0) + return false; + } + + /* object specific verification checks here */ + + return true; +} + +If there are different magic numbers for the different formats, the verifier +will look like: + +static bool +xfs_foo_verify( + struct xfs_buf *bp) +{ + struct xfs_mount *mp = bp->b_target->bt_mount; + struct xfs_ondisk_hdr *hdr = bp->b_addr; + + if (hdr->magic == cpu_to_be32(XFS_FOO_CRC_MAGIC)) { + if (!uuid_equal(&hdr->uuid, &mp->m_sb.sb_uuid)) + return false; + if (bp->b_bn != be64_to_cpu(hdr->blkno)) + return false; + if (hdr->owner == 0) + return false; + } else if (hdr->magic != cpu_to_be32(XFS_FOO_MAGIC)) + return false; + + /* object specific verification checks here */ + + return true; +} + +Write verifiers are very similar to the read verifiers, they just do things in +the opposite order to the read verifiers. A typical write verifier: + +static void +xfs_foo_write_verify( + struct xfs_buf *bp) +{ + struct xfs_mount *mp = bp->b_target->bt_mount; + struct xfs_buf_log_item *bip = bp->b_fspriv; + + if (!xfs_foo_verify(bp)) { + XFS_CORRUPTION_ERROR(__func__, XFS_ERRLEVEL_LOW, mp, bp->b_addr); + xfs_buf_ioerror(bp, EFSCORRUPTED); + return; + } + + if (!xfs_sb_version_hascrc(&mp->m_sb)) + return; + + + if (bip) { + struct xfs_ondisk_hdr *hdr = bp->b_addr; + hdr->lsn = cpu_to_be64(bip->bli_item.li_lsn); + } + xfs_update_cksum(bp->b_addr, BBTOB(bp->b_length), XFS_FOO_CRC_OFF); +} + +This will verify the internal structure of the metadata before we go any +further, detecting corruptions that have occurred as the metadata has been +modified in memory. If the metadata verifies OK, and CRCs are enabled, we then +update the LSN field (when it was last modified) and calculate the CRC on the +metadata. Once this is done, we can issue the IO. + +Inodes and Dquots +----------------- + +Inodes and dquots are special snowflakes. They have per-object CRC and +self-identifiers, but they are packed so that there are multiple objects per +buffer. Hence we do not use per-buffer verifiers to do the work of per-object +verification and CRC calculations. The per-buffer verifiers simply perform basic +identification of the buffer - that they contain inodes or dquots, and that +there are magic numbers in all the expected spots. All further CRC and +verification checks are done when each inode is read from or written back to the +buffer. + +The structure of the verifiers and the identifiers checks is very similar to the +buffer code described above. The only difference is where they are called. For +example, inode read verification is done in xfs_iread() when the inode is first +read out of the buffer and the struct xfs_inode is instantiated. The inode is +already extensively verified during writeback in xfs_iflush_int, so the only +addition here is to add the LSN and CRC to the inode as it is copied back into +the buffer. + +XXX: inode unlinked list modification doesn't recalculate the inode CRC! None of +the unlinked list modifications check or update CRCs, neither during unlink nor +log recovery. So, it's gone unnoticed until now. This won't matter immediately - +repair will probably complain about it - but it needs to be fixed. + |