Re: [PATCH v10 07/14] btrfs: add definitions + documentation for encoded I/O ioctls

Nikolay Borisov
2021-08-20T08:56:41Z

On 18.08.21 г. 0:06, Omar Sandoval wrote:
> From: Omar Sandoval <osandov@fb.com>
>
> In order to allow sending and receiving compressed data without
> decompressing it, we need an interface to write pre-compressed data
> directly to the filesystem and the matching interface to read compressed
> data without decompressing it. This adds the definitions for ioctls to
> do that and detailed explanations of how to use them.
>
> Signed-off-by: Omar Sandoval <osandov@fb.com>
> ---
> include/uapi/linux/btrfs.h | 132 +++++++++++++++++++++++++++++++++++++
> 1 file changed, 132 insertions(+)
>
> diff --git a/include/uapi/linux/btrfs.h b/include/uapi/linux/btrfs.h
> index d7d3cfead056..95da52955894 100644
> --- a/include/uapi/linux/btrfs.h
> +++ b/include/uapi/linux/btrfs.h
> @@ -861,6 +861,134 @@ struct btrfs_ioctl_get_subvol_rootref_args {
> __u8 align[7];
> };
>
> +/*
> + * Data and metadata for an encoded read or write.
> + *
> + * Encoded I/O bypasses any encoding automatically done by the filesystem (e.g.,
> + * compression). This can be used to read the compressed contents of a file or
> + * write pre-compressed data directly to a file.
> + *
> + * BTRFS_IOC_ENCODED_READ and BTRFS_IOC_ENCODED_WRITE are essentially
> + * preadv/pwritev with additional metadata about how the data is encoded and the
> + * size of the unencoded data.
> + *
> + * BTRFS_IOC_ENCODED_READ fills the given iovecs with the encoded data, fills
> + * the metadata fields, and returns the size of the encoded data. It reads one
> + * extent per call. It can also read data which is not encoded.
> + *
> + * BTRFS_IOC_ENCODED_WRITE uses the metadata fields, writes the encoded data
> + * from the iovecs, and returns the size of the encoded data. Note that the
> + * encoded data is not validated when it is written; if it is not valid (e.g.,
> + * it cannot be decompressed), then a subsequent read may return an error.
> + *
> + * Since the filesystem page cache contains decoded data, encoded I/O bypasses
> + * the page cache. Encoded I/O requires CAP_SYS_ADMIN.
> + */
> +struct btrfs_ioctl_encoded_io_args {
> + /* Input parameters for both reads and writes. */
> +
> + /*
> + * iovecs containing encoded data.
> + *
> + * For reads, if the size of the encoded data is larger than the sum of
> + * iov[n].iov_len for 0 <= n < iovcnt, then the ioctl fails with
> + * ENOBUFS.
> + *
> + * For writes, the size of the encoded data is the sum of iov[n].iov_len
> + * for 0 <= n < iovcnt. This must be less than 128 KiB (this limit may
> + * increase in the future). This must also be less than or equal to
> + * unencoded_len.
> + */
> + const struct iovec __user *iov;
> + /* Number of iovecs. */
> + unsigned long iovcnt;
> + /*
> + * Offset in file.
> + *
> + * For writes, must be aligned to the sector size of the filesystem.
> + */
> + __s64 offset;
> + /* Currently must be zero. */
> + __u64 flags;
> +
> + /*
> + * For reads, the following members are filled in with the metadata for
> + * the encoded data.
> + * For writes, the following members must be set to the metadata for the
> + * encoded data.
> + */
> +
> + /*
> + * Length of the data in the file.
> + *
> + * Must be less than or equal to unencoded_len - unencoded_offset. For
> + * writes, must be aligned to the sector size of the filesystem unless
> + * the data ends at or beyond the current end of the file.
> + */
> + __u64 len;
> + /*
> + * Length of the unencoded (i.e., decrypted and decompressed) data.
> + *
> + * For writes, must be no more than 128 KiB (this limit may increase in
> + * the future). If the unencoded data is actually longer than
> + * unencoded_len, then it is truncated; if it is shorter, then it is
> + * extended with zeroes.
> + */
> + __u64 unencoded_len;
> + /*
> + * Offset from the first byte of the unencoded data to the first byte of
> + * logical data in the file.
> + *
> + * Must be less than unencoded_len.
> + */
> + __u64 unencoded_offset;
> + /*
> + * BTRFS_ENCODED_IO_COMPRESSION_* type.
> + *
> + * For writes, must not be BTRFS_ENCODED_IO_COMPRESSION_NONE.
> + */
> + __u32 compression;
> + /* Currently always BTRFS_ENCODED_IO_ENCRYPTION_NONE. */
> + __u32 encryption;
> + /*
> + * Reserved for future expansion.
> + *
> + * For reads, always returned as zero. Users should check for non-zero
> + * bytes. If there are any, then the kernel has a newer version of this
> + * structure with additional information that the user definition is
> + * missing.
> + *
> + * For writes, must be zeroed.
> + */
> + __u8 reserved[32];
> +};
> +
> +/* Data is not compressed. */
> +#define BTRFS_ENCODED_IO_COMPRESSION_NONE 0
> +/* Data is compressed as a single zlib stream. */
> +#define BTRFS_ENCODED_IO_COMPRESSION_ZLIB 1
> +/*
> + * Data is compressed as a single zstd frame with the windowLog compression
> + * parameter set to no more than 17.
> + */
> +#define BTRFS_ENCODED_IO_COMPRESSION_ZSTD 2
> +/*
> + * Data is compressed page by page (using the page size indicated by the name of
> + * the constant) with LZO1X and wrapped in the format documented in
> + * fs/btrfs/lzo.c. For writes, the compression page size must match the
> + * filesystem page size.
> + */
> +#define BTRFS_ENCODED_IO_COMPRESSION_LZO_4K 3
> +#define BTRFS_ENCODED_IO_COMPRESSION_LZO_8K 4
> +#define BTRFS_ENCODED_IO_COMPRESSION_LZO_16K 5
> +#define BTRFS_ENCODED_IO_COMPRESSION_LZO_32K 6
> +#define BTRFS_ENCODED_IO_COMPRESSION_LZO_64K 7
> +#define BTRFS_ENCODED_IO_COMPRESSION_TYPES 8
> +
> +/* Data is not encrypted. */
> +#define BTRFS_ENCODED_IO_ENCRYPTION_NONE 0
> +#define BTRFS_ENCODED_IO_ENCRYPTION_TYPES 1

How about an enums for encryption/compression.

> +
> /* Error codes as returned by the kernel */
> enum btrfs_err_code {
> BTRFS_ERROR_DEV_RAID1_MIN_NOT_MET = 1,
> @@ -989,5 +1117,9 @@ enum btrfs_err_code {
> struct btrfs_ioctl_ino_lookup_user_args)
> #define BTRFS_IOC_SNAP_DESTROY_V2 _IOW(BTRFS_IOCTL_MAGIC, 63, \
> struct btrfs_ioctl_vol_args_v2)
> +#define BTRFS_IOC_ENCODED_READ _IOR(BTRFS_IOCTL_MAGIC, 64, \
> + struct btrfs_ioctl_encoded_io_args)
> +#define BTRFS_IOC_ENCODED_WRITE _IOW(BTRFS_IOCTL_MAGIC, 64, \
> + struct btrfs_ioctl_encoded_io_args)
>
> #endif /* _UAPI_LINUX_BTRFS_H */
>