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+----------------------------------------------------------------------
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+1. INTRODUCTION
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+
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+Modern filesystems feature checksumming of data and metadata to
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+protect against data corruption. However, the detection of the
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+corruption is done at read time which could potentially be months
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+after the data was written. At that point the original data that the
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+application tried to write is most likely lost.
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+
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+The solution is to ensure that the disk is actually storing what the
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+application meant it to. Recent additions to both the SCSI family
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+protocols (SBC Data Integrity Field, SCC protection proposal) as well
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+as SATA/T13 (External Path Protection) try to remedy this by adding
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+support for appending integrity metadata to an I/O. The integrity
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+metadata (or protection information in SCSI terminology) includes a
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+checksum for each sector as well as an incrementing counter that
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+ensures the individual sectors are written in the right order. And
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+for some protection schemes also that the I/O is written to the right
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+place on disk.
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+
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+Current storage controllers and devices implement various protective
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+measures, for instance checksumming and scrubbing. But these
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+technologies are working in their own isolated domains or at best
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+between adjacent nodes in the I/O path. The interesting thing about
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+DIF and the other integrity extensions is that the protection format
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+is well defined and every node in the I/O path can verify the
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+integrity of the I/O and reject it if corruption is detected. This
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+allows not only corruption prevention but also isolation of the point
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+of failure.
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+
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+----------------------------------------------------------------------
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+2. THE DATA INTEGRITY EXTENSIONS
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+
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+As written, the protocol extensions only protect the path between
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+controller and storage device. However, many controllers actually
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+allow the operating system to interact with the integrity metadata
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+(IMD). We have been working with several FC/SAS HBA vendors to enable
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+the protection information to be transferred to and from their
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+controllers.
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+
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+The SCSI Data Integrity Field works by appending 8 bytes of protection
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+information to each sector. The data + integrity metadata is stored
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+in 520 byte sectors on disk. Data + IMD are interleaved when
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+transferred between the controller and target. The T13 proposal is
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+similar.
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+
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+Because it is highly inconvenient for operating systems to deal with
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+520 (and 4104) byte sectors, we approached several HBA vendors and
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+encouraged them to allow separation of the data and integrity metadata
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+scatter-gather lists.
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+
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+The controller will interleave the buffers on write and split them on
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+read. This means that the Linux can DMA the data buffers to and from
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+host memory without changes to the page cache.
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+
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+Also, the 16-bit CRC checksum mandated by both the SCSI and SATA specs
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+is somewhat heavy to compute in software. Benchmarks found that
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+calculating this checksum had a significant impact on system
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+performance for a number of workloads. Some controllers allow a
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+lighter-weight checksum to be used when interfacing with the operating
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+system. Emulex, for instance, supports the TCP/IP checksum instead.
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+The IP checksum received from the OS is converted to the 16-bit CRC
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+when writing and vice versa. This allows the integrity metadata to be
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+generated by Linux or the application at very low cost (comparable to
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+software RAID5).
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+
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+The IP checksum is weaker than the CRC in terms of detecting bit
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+errors. However, the strength is really in the separation of the data
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+buffers and the integrity metadata. These two distinct buffers much
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+match up for an I/O to complete.
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+
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+The separation of the data and integrity metadata buffers as well as
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+the choice in checksums is referred to as the Data Integrity
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+Extensions. As these extensions are outside the scope of the protocol
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+bodies (T10, T13), Oracle and its partners are trying to standardize
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+them within the Storage Networking Industry Association.
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+
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+----------------------------------------------------------------------
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+3. KERNEL CHANGES
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+
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+The data integrity framework in Linux enables protection information
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+to be pinned to I/Os and sent to/received from controllers that
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+support it.
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+
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+The advantage to the integrity extensions in SCSI and SATA is that
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+they enable us to protect the entire path from application to storage
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+device. However, at the same time this is also the biggest
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+disadvantage. It means that the protection information must be in a
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+format that can be understood by the disk.
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+
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+Generally Linux/POSIX applications are agnostic to the intricacies of
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+the storage devices they are accessing. The virtual filesystem switch
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+and the block layer make things like hardware sector size and
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+transport protocols completely transparent to the application.
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+
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+However, this level of detail is required when preparing the
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+protection information to send to a disk. Consequently, the very
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+concept of an end-to-end protection scheme is a layering violation.
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+It is completely unreasonable for an application to be aware whether
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+it is accessing a SCSI or SATA disk.
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+
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+The data integrity support implemented in Linux attempts to hide this
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+from the application. As far as the application (and to some extent
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+the kernel) is concerned, the integrity metadata is opaque information
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+that's attached to the I/O.
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+
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+The current implementation allows the block layer to automatically
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+generate the protection information for any I/O. Eventually the
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+intent is to move the integrity metadata calculation to userspace for
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+user data. Metadata and other I/O that originates within the kernel
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+will still use the automatic generation interface.
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+
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+Some storage devices allow each hardware sector to be tagged with a
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+16-bit value. The owner of this tag space is the owner of the block
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+device. I.e. the filesystem in most cases. The filesystem can use
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+this extra space to tag sectors as they see fit. Because the tag
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+space is limited, the block interface allows tagging bigger chunks by
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+way of interleaving. This way, 8*16 bits of information can be
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+attached to a typical 4KB filesystem block.
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+
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+This also means that applications such as fsck and mkfs will need
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+access to manipulate the tags from user space. A passthrough
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+interface for this is being worked on.
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+
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+
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+----------------------------------------------------------------------
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+4. BLOCK LAYER IMPLEMENTATION DETAILS
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+
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+4.1 BIO
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+
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+The data integrity patches add a new field to struct bio when
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+CONFIG_BLK_DEV_INTEGRITY is enabled. bio->bi_integrity is a pointer
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+to a struct bip which contains the bio integrity payload. Essentially
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+a bip is a trimmed down struct bio which holds a bio_vec containing
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+the integrity metadata and the required housekeeping information (bvec
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+pool, vector count, etc.)
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+
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+A kernel subsystem can enable data integrity protection on a bio by
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+calling bio_integrity_alloc(bio). This will allocate and attach the
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+bip to the bio.
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+
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+Individual pages containing integrity metadata can subsequently be
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+attached using bio_integrity_add_page().
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+
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+bio_free() will automatically free the bip.
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+
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+
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+4.2 BLOCK DEVICE
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+
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+Because the format of the protection data is tied to the physical
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+disk, each block device has been extended with a block integrity
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+profile (struct blk_integrity). This optional profile is registered
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+with the block layer using blk_integrity_register().
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+
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+The profile contains callback functions for generating and verifying
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+the protection data, as well as getting and setting application tags.
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+The profile also contains a few constants to aid in completing,
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+merging and splitting the integrity metadata.
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+
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+Layered block devices will need to pick a profile that's appropriate
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+for all subdevices. blk_integrity_compare() can help with that. DM
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+and MD linear, RAID0 and RAID1 are currently supported. RAID4/5/6
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+will require extra work due to the application tag.
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+
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+
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+----------------------------------------------------------------------
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+5.0 BLOCK LAYER INTEGRITY API
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+
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+5.1 NORMAL FILESYSTEM
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+
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+ The normal filesystem is unaware that the underlying block device
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+ is capable of sending/receiving integrity metadata. The IMD will
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+ be automatically generated by the block layer at submit_bio() time
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+ in case of a WRITE. A READ request will cause the I/O integrity
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+ to be verified upon completion.
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+
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+ IMD generation and verification can be toggled using the
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+
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+ /sys/block/<bdev>/integrity/write_generate
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+
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+ and
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+
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+ /sys/block/<bdev>/integrity/read_verify
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+
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+ flags.
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+
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+
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+5.2 INTEGRITY-AWARE FILESYSTEM
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+
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+ A filesystem that is integrity-aware can prepare I/Os with IMD
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+ attached. It can also use the application tag space if this is
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+ supported by the block device.
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+
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+
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+ int bdev_integrity_enabled(block_device, int rw);
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+
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+ bdev_integrity_enabled() will return 1 if the block device
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+ supports integrity metadata transfer for the data direction
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+ specified in 'rw'.
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+
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+ bdev_integrity_enabled() honors the write_generate and
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+ read_verify flags in sysfs and will respond accordingly.
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+
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+
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+ int bio_integrity_prep(bio);
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+
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+ To generate IMD for WRITE and to set up buffers for READ, the
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+ filesystem must call bio_integrity_prep(bio).
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+
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+ Prior to calling this function, the bio data direction and start
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+ sector must be set, and the bio should have all data pages
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+ added. It is up to the caller to ensure that the bio does not
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+ change while I/O is in progress.
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+
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+ bio_integrity_prep() should only be called if
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+ bio_integrity_enabled() returned 1.
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+
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+
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+ int bio_integrity_tag_size(bio);
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+
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+ If the filesystem wants to use the application tag space it will
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+ first have to find out how much storage space is available.
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+ Because tag space is generally limited (usually 2 bytes per
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+ sector regardless of sector size), the integrity framework
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+ supports interleaving the information between the sectors in an
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+ I/O.
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+
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+ Filesystems can call bio_integrity_tag_size(bio) to find out how
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+ many bytes of storage are available for that particular bio.
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+
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+ Another option is bdev_get_tag_size(block_device) which will
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+ return the number of available bytes per hardware sector.
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+
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+
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+ int bio_integrity_set_tag(bio, void *tag_buf, len);
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+
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+ After a successful return from bio_integrity_prep(),
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+ bio_integrity_set_tag() can be used to attach an opaque tag
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+ buffer to a bio. Obviously this only makes sense if the I/O is
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+ a WRITE.
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+
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+
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+ int bio_integrity_get_tag(bio, void *tag_buf, len);
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+
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+ Similarly, at READ I/O completion time the filesystem can
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+ retrieve the tag buffer using bio_integrity_get_tag().
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+
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+
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+6.3 PASSING EXISTING INTEGRITY METADATA
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+
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+ Filesystems that either generate their own integrity metadata or
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+ are capable of transferring IMD from user space can use the
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+ following calls:
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+
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+
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+ struct bip * bio_integrity_alloc(bio, gfp_mask, nr_pages);
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+
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+ Allocates the bio integrity payload and hangs it off of the bio.
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+ nr_pages indicate how many pages of protection data need to be
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+ stored in the integrity bio_vec list (similar to bio_alloc()).
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+
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+ The integrity payload will be freed at bio_free() time.
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+
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+
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+ int bio_integrity_add_page(bio, page, len, offset);
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+
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+ Attaches a page containing integrity metadata to an existing
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+ bio. The bio must have an existing bip,
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+ i.e. bio_integrity_alloc() must have been called. For a WRITE,
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+ the integrity metadata in the pages must be in a format
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+ understood by the target device with the notable exception that
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+ the sector numbers will be remapped as the request traverses the
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+ I/O stack. This implies that the pages added using this call
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+ will be modified during I/O! The first reference tag in the
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+ integrity metadata must have a value of bip->bip_sector.
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+
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+ Pages can be added using bio_integrity_add_page() as long as
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+ there is room in the bip bio_vec array (nr_pages).
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+
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+ Upon completion of a READ operation, the attached pages will
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+ contain the integrity metadata received from the storage device.
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+ It is up to the receiver to process them and verify data
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+ integrity upon completion.
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+
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+
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+6.4 REGISTERING A BLOCK DEVICE AS CAPABLE OF EXCHANGING INTEGRITY
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+ METADATA
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+
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+ To enable integrity exchange on a block device the gendisk must be
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+ registered as capable:
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+
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+ int blk_integrity_register(gendisk, blk_integrity);
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+
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+ The blk_integrity struct is a template and should contain the
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+ following:
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+
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+ static struct blk_integrity my_profile = {
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+ .name = "STANDARDSBODY-TYPE-VARIANT-CSUM",
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+ .generate_fn = my_generate_fn,
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+ .verify_fn = my_verify_fn,
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+ .get_tag_fn = my_get_tag_fn,
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+ .set_tag_fn = my_set_tag_fn,
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+ .tuple_size = sizeof(struct my_tuple_size),
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+ .tag_size = <tag bytes per hw sector>,
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+ };
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+
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+ 'name' is a text string which will be visible in sysfs. This is
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+ part of the userland API so chose it carefully and never change
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+ it. The format is standards body-type-variant.
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+ E.g. T10-DIF-TYPE1-IP or T13-EPP-0-CRC.
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+
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+ 'generate_fn' generates appropriate integrity metadata (for WRITE).
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+
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+ 'verify_fn' verifies that the data buffer matches the integrity
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+ metadata.
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+
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+ 'tuple_size' must be set to match the size of the integrity
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+ metadata per sector. I.e. 8 for DIF and EPP.
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+
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+ 'tag_size' must be set to identify how many bytes of tag space
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+ are available per hardware sector. For DIF this is either 2 or
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+ 0 depending on the value of the Control Mode Page ATO bit.
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+
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+ See 6.2 for a description of get_tag_fn and set_tag_fn.
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+
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+----------------------------------------------------------------------
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+2007-12-24 Martin K. Petersen <martin.petersen@oracle.com>
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Martin K. Petersen