Re: [RFC PATCH v2 2/5] fs: add RWF_ENCODED for reading/writing compressed data

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Tue, Oct 15, 2019 at 11:42:40AM -0700, Omar Sandoval wrote:
> From: Omar Sandoval <osandov@fb.com>
> 
> Btrfs supports transparent compression: data written by the user can be
> compressed when written to disk and decompressed when read back.
> However, we'd like to add an interface to write pre-compressed data
> directly to the filesystem, and the matching interface to read
> compressed data without decompressing it. This adds support for
> so-called "encoded I/O" via preadv2() and pwritev2().
> 
> A new RWF_ENCODED flags indicates that a read or write is "encoded". If
> this flag is set, iov[0].iov_base points to a struct encoded_iov which
> is used for metadata: namely, the compression algorithm, unencoded
> (i.e., decompressed) length, and what subrange of the unencoded data
> should be used (needed for truncated or hole-punched extents and when
> reading in the middle of an extent). For reads, the filesystem returns
> this information; for writes, the caller provides it to the filesystem.
> iov[0].iov_len must be set to sizeof(struct encoded_iov), which can be
> used to extend the interface in the future. The remaining iovecs contain
> the encoded extent.
> 
> Filesystems must indicate that they support encoded writes by setting
> FMODE_ENCODED_IO in ->file_open().
> 
> Signed-off-by: Omar Sandoval <osandov@fb.com>
> ---
>  include/linux/fs.h      | 14 +++++++
>  include/uapi/linux/fs.h | 26 ++++++++++++-
>  mm/filemap.c            | 82 ++++++++++++++++++++++++++++++++++-------
>  3 files changed, 108 insertions(+), 14 deletions(-)
> 
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index e0d909d35763..54681f21e05e 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -175,6 +175,9 @@ typedef int (dio_iodone_t)(struct kiocb *iocb, loff_t offset,
>  /* File does not contribute to nr_files count */
>  #define FMODE_NOACCOUNT		((__force fmode_t)0x20000000)
>  
> +/* File supports encoded IO */
> +#define FMODE_ENCODED_IO	((__force fmode_t)0x40000000)
> +
>  /*
>   * Flag for rw_copy_check_uvector and compat_rw_copy_check_uvector
>   * that indicates that they should check the contents of the iovec are
> @@ -314,6 +317,7 @@ enum rw_hint {
>  #define IOCB_SYNC		(1 << 5)
>  #define IOCB_WRITE		(1 << 6)
>  #define IOCB_NOWAIT		(1 << 7)
> +#define IOCB_ENCODED		(1 << 8)
>  
>  struct kiocb {
>  	struct file		*ki_filp;
> @@ -3088,6 +3092,11 @@ extern int sb_min_blocksize(struct super_block *, int);
>  extern int generic_file_mmap(struct file *, struct vm_area_struct *);
>  extern int generic_file_readonly_mmap(struct file *, struct vm_area_struct *);
>  extern ssize_t generic_write_checks(struct kiocb *, struct iov_iter *);
> +struct encoded_iov;
> +extern int generic_encoded_write_checks(struct kiocb *, struct encoded_iov *);
> +extern ssize_t check_encoded_read(struct kiocb *, struct iov_iter *);
> +extern int import_encoded_write(struct kiocb *, struct encoded_iov *,
> +				struct iov_iter *);
>  extern int generic_remap_checks(struct file *file_in, loff_t pos_in,
>  				struct file *file_out, loff_t pos_out,
>  				loff_t *count, unsigned int remap_flags);
> @@ -3403,6 +3412,11 @@ static inline int kiocb_set_rw_flags(struct kiocb *ki, rwf_t flags)
>  			return -EOPNOTSUPP;
>  		ki->ki_flags |= IOCB_NOWAIT;
>  	}
> +	if (flags & RWF_ENCODED) {
> +		if (!(ki->ki_filp->f_mode & FMODE_ENCODED_IO))
> +			return -EOPNOTSUPP;
> +		ki->ki_flags |= IOCB_ENCODED;
> +	}
>  	if (flags & RWF_HIPRI)
>  		ki->ki_flags |= IOCB_HIPRI;
>  	if (flags & RWF_DSYNC)
> diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
> index 379a612f8f1d..ed92a8a257cb 100644
> --- a/include/uapi/linux/fs.h
> +++ b/include/uapi/linux/fs.h
> @@ -284,6 +284,27 @@ struct fsxattr {
>  
>  typedef int __bitwise __kernel_rwf_t;
>  
> +enum {
> +	ENCODED_IOV_COMPRESSION_NONE,
> +	ENCODED_IOV_COMPRESSION_ZLIB,
> +	ENCODED_IOV_COMPRESSION_LZO,
> +	ENCODED_IOV_COMPRESSION_ZSTD,
> +	ENCODED_IOV_COMPRESSION_TYPES = ENCODED_IOV_COMPRESSION_ZSTD,
> +};
> +
> +enum {
> +	ENCODED_IOV_ENCRYPTION_NONE,
> +	ENCODED_IOV_ENCRYPTION_TYPES = ENCODED_IOV_ENCRYPTION_NONE,
> +};
> +
> +struct encoded_iov {
> +	__u64 len;
> +	__u64 unencoded_len;
> +	__u64 unencoded_offset;
> +	__u32 compression;
> +	__u32 encryption;

Can we add some must-be-zero padding space at the end here for whomever
comes along next wanting to add more encoding info?

(And maybe a manpage and some basic testing, to reiterate Dave...)

--D

> +};
> +
>  /* high priority request, poll if possible */
>  #define RWF_HIPRI	((__force __kernel_rwf_t)0x00000001)
>  
> @@ -299,8 +320,11 @@ typedef int __bitwise __kernel_rwf_t;
>  /* per-IO O_APPEND */
>  #define RWF_APPEND	((__force __kernel_rwf_t)0x00000010)
>  
> +/* encoded (e.g., compressed or encrypted) IO */
> +#define RWF_ENCODED	((__force __kernel_rwf_t)0x00000020)
> +
>  /* mask of flags supported by the kernel */
>  #define RWF_SUPPORTED	(RWF_HIPRI | RWF_DSYNC | RWF_SYNC | RWF_NOWAIT |\
> -			 RWF_APPEND)
> +			 RWF_APPEND | RWF_ENCODED)
>  
>  #endif /* _UAPI_LINUX_FS_H */
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 1146fcfa3215..d2e6d9caf353 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2948,24 +2948,15 @@ static int generic_write_check_limits(struct file *file, loff_t pos,
>  	return 0;
>  }
>  
> -/*
> - * Performs necessary checks before doing a write
> - *
> - * Can adjust writing position or amount of bytes to write.
> - * Returns appropriate error code that caller should return or
> - * zero in case that write should be allowed.
> - */
> -inline ssize_t generic_write_checks(struct kiocb *iocb, struct iov_iter *from)
> +static int generic_write_checks_common(struct kiocb *iocb, loff_t *count)
>  {
>  	struct file *file = iocb->ki_filp;
>  	struct inode *inode = file->f_mapping->host;
> -	loff_t count;
> -	int ret;
>  
>  	if (IS_SWAPFILE(inode))
>  		return -ETXTBSY;
>  
> -	if (!iov_iter_count(from))
> +	if (!*count)
>  		return 0;
>  
>  	/* FIXME: this is for backwards compatibility with 2.4 */
> @@ -2975,8 +2966,21 @@ inline ssize_t generic_write_checks(struct kiocb *iocb, struct iov_iter *from)
>  	if ((iocb->ki_flags & IOCB_NOWAIT) && !(iocb->ki_flags & IOCB_DIRECT))
>  		return -EINVAL;
>  
> -	count = iov_iter_count(from);
> -	ret = generic_write_check_limits(file, iocb->ki_pos, &count);
> +	return generic_write_check_limits(iocb->ki_filp, iocb->ki_pos, count);
> +}
> +
> +/*
> + * Performs necessary checks before doing a write
> + *
> + * Can adjust writing position or amount of bytes to write.
> + * Returns a negative errno or the new number of bytes to write.
> + */
> +inline ssize_t generic_write_checks(struct kiocb *iocb, struct iov_iter *from)
> +{
> +	loff_t count = iov_iter_count(from);
> +	int ret;
> +
> +	ret = generic_write_checks_common(iocb, &count);
>  	if (ret)
>  		return ret;
>  
> @@ -2985,6 +2989,58 @@ inline ssize_t generic_write_checks(struct kiocb *iocb, struct iov_iter *from)
>  }
>  EXPORT_SYMBOL(generic_write_checks);
>  
> +int generic_encoded_write_checks(struct kiocb *iocb,
> +				 struct encoded_iov *encoded)
> +{
> +	loff_t count = encoded->unencoded_len;
> +	int ret;
> +
> +	ret = generic_write_checks_common(iocb, &count);
> +	if (ret)
> +		return ret;
> +
> +	if (count != encoded->unencoded_len) {
> +		/*
> +		 * The write got truncated by generic_write_checks_common(). We
> +		 * can't do a partial encoded write.
> +		 */
> +		return -EFBIG;
> +	}
> +	return 0;
> +}
> +EXPORT_SYMBOL(generic_encoded_write_checks);
> +
> +ssize_t check_encoded_read(struct kiocb *iocb, struct iov_iter *iter)
> +{
> +	if (!(iocb->ki_filp->f_flags & O_ENCODED))
> +		return -EPERM;
> +	if (iov_iter_single_seg_count(iter) != sizeof(struct encoded_iov))
> +		return -EINVAL;
> +	return iov_iter_count(iter) - sizeof(struct encoded_iov);
> +}
> +EXPORT_SYMBOL(check_encoded_read);
> +
> +int import_encoded_write(struct kiocb *iocb, struct encoded_iov *encoded,
> +			 struct iov_iter *from)
> +{
> +	if (!(iocb->ki_filp->f_flags & O_ENCODED))
> +		return -EPERM;
> +	if (iov_iter_single_seg_count(from) != sizeof(*encoded))
> +		return -EINVAL;
> +	if (copy_from_iter(encoded, sizeof(*encoded), from) != sizeof(*encoded))
> +		return -EFAULT;
> +	if (encoded->compression == ENCODED_IOV_COMPRESSION_NONE &&
> +	    encoded->encryption == ENCODED_IOV_ENCRYPTION_NONE)
> +		return -EINVAL;
> +	if (encoded->compression > ENCODED_IOV_COMPRESSION_TYPES ||
> +	    encoded->encryption > ENCODED_IOV_ENCRYPTION_TYPES)
> +		return -EINVAL;
> +	if (encoded->unencoded_offset >= encoded->unencoded_len)
> +		return -EINVAL;
> +	return 0;
> +}
> +EXPORT_SYMBOL(import_encoded_write);
> +
>  /*
>   * Performs necessary checks before doing a clone.
>   *
> -- 
> 2.23.0
>