Re: [PATCH v6 03/11] fs: add RWF_ENCODED for reading/writing compressed data

[Amir Goldstein <amir73il@gmail.com>]

On Wed, Nov 18, 2020 at 9:18 PM Omar Sandoval <osandov@osandov.com> 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 a la copy_struct_from_user().
> The remaining iovecs contain the encoded extent.
>
> This adds the VFS helpers for supporting encoded I/O and documentation
> for filesystem support.
>
> Reviewed-by: Josef Bacik <josef@toxicpanda.com>
> Signed-off-by: Omar Sandoval <osandov@fb.com>
> ---
>  Documentation/filesystems/encoded_io.rst |  74 ++++++++++
>  Documentation/filesystems/index.rst      |   1 +
>  fs/read_write.c                          | 167 +++++++++++++++++++++--
>  include/linux/fs.h                       |  11 ++
>  include/uapi/linux/fs.h                  |  41 +++++-
>  5 files changed, 280 insertions(+), 14 deletions(-)
>  create mode 100644 Documentation/filesystems/encoded_io.rst
>
> diff --git a/Documentation/filesystems/encoded_io.rst b/Documentation/filesystems/encoded_io.rst
> new file mode 100644
> index 000000000000..50405276d866
> --- /dev/null
> +++ b/Documentation/filesystems/encoded_io.rst
> @@ -0,0 +1,74 @@
> +===========
> +Encoded I/O
> +===========
> +
> +Encoded I/O is a mechanism for reading and writing encoded (e.g., compressed
> +and/or encrypted) data directly from/to the filesystem. The userspace interface
> +is thoroughly described in the :manpage:`encoded_io(7)` man page; this document
> +describes the requirements for filesystem support.
> +
> +First of all, a filesystem supporting encoded I/O must indicate this by setting
> +the ``FMODE_ENCODED_IO`` flag in its ``file_open`` file operation::
> +

Should this be FMODE_ALLOW_ENCODED_IO?
How come I see no checks for this flag in vfs code?
You seem to only be checking the O_ flag.
Do we really want to allow setting the O_ flag after open or should we
deny that?

> +    static int foo_file_open(struct inode *inode, struct file *filp)
> +    {
> +            ...
> +            filep->f_mode |= FMODE_ENCODED_IO;
> +            ...
> +    }
> +
> +Encoded I/O goes through ``read_iter`` and ``write_iter``, designated by the
> +``IOCB_ENCODED`` flag in ``kiocb->ki_flags``.
> +
> +Reads
> +=====
> +
> +Encoded ``read_iter`` should:
> +
> +1. Call ``generic_encoded_read_checks()`` to validate the file and buffers
> +   provided by userspace.
> +2. Initialize the ``encoded_iov`` appropriately.
> +3. Copy it to the user with ``copy_encoded_iov_to_iter()``.
> +4. Copy the encoded data to the user.
> +5. Advance ``kiocb->ki_pos`` by ``encoded_iov->len``.
> +6. Return the size of the encoded data read, not including the ``encoded_iov``.
> +
> +There are a few details to be aware of:
> +
> +* Encoded ``read_iter`` should support reading unencoded data if the extent is
> +  not encoded.
> +* If the buffers provided by the user are not large enough to contain an entire
> +  encoded extent, then ``read_iter`` should return ``-ENOBUFS``. This is to
> +  avoid confusing userspace with truncated data that cannot be properly
> +  decoded.
> +* Reads in the middle of an encoded extent can be returned by setting
> +  ``encoded_iov->unencoded_offset`` to non-zero.
> +* Truncated unencoded data (e.g., because the file does not end on a block
> +  boundary) may be returned by setting ``encoded_iov->len`` to a value smaller
> +  value than ``encoded_iov->unencoded_len - encoded_iov->unencoded_offset``.
> +
> +Writes
> +======
> +
> +Encoded ``write_iter`` should (in addition to the usual accounting/checks done
> +by ``write_iter``):
> +
> +1. Call ``copy_encoded_iov_from_iter()`` to get and validate the
> +   ``encoded_iov``.
> +2. Call ``generic_encoded_write_checks()`` instead of
> +   ``generic_write_checks()``.
> +3. Check that the provided encoding in ``encoded_iov`` is supported.
> +4. Advance ``kiocb->ki_pos`` by ``encoded_iov->len``.
> +5. Return the size of the encoded data written.
> +
> +Again, there are a few details:
> +
> +* Encoded ``write_iter`` doesn't need to support writing unencoded data.
> +* ``write_iter`` should either write all of the encoded data or none of it; it
> +  must not do partial writes.
> +* ``write_iter`` doesn't need to validate the encoded data; a subsequent read
> +  may return, e.g., ``-EIO`` if the data is not valid.
> +* The user may lie about the unencoded size of the data; a subsequent read
> +  should truncate or zero-extend the unencoded data rather than returning an
> +  error.
> +* Be careful of page cache coherency.
> diff --git a/Documentation/filesystems/index.rst b/Documentation/filesystems/index.rst
> index 98f59a864242..6d9e3ff0a455 100644
> --- a/Documentation/filesystems/index.rst
> +++ b/Documentation/filesystems/index.rst
> @@ -53,6 +53,7 @@ filesystem implementations.
>     journalling
>     fscrypt
>     fsverity
> +   encoded_io
>
>  Filesystems
>  ===========
> diff --git a/fs/read_write.c b/fs/read_write.c
> index 75f764b43418..e2ad418d2987 100644
> --- a/fs/read_write.c
> +++ b/fs/read_write.c
> @@ -1625,24 +1625,15 @@ int generic_write_check_limits(struct file *file, loff_t pos, loff_t *count)
>         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.
> - */
> -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 */
> @@ -1652,8 +1643,22 @@ 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 appropriate error code that caller should return or
> + * zero in case that write should be allowed.
> + */
> +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;
>
> @@ -1684,3 +1689,139 @@ int generic_file_rw_checks(struct file *file_in, struct file *file_out)
>
>         return 0;
>  }
> +
> +/**
> + * generic_encoded_write_checks() - check an encoded write
> + * @iocb: I/O context.
> + * @encoded: Encoding metadata.
> + *
> + * This should be called by RWF_ENCODED write implementations rather than
> + * generic_write_checks(). Unlike generic_write_checks(), it returns -EFBIG
> + * instead of adjusting the size of the write.
> + *
> + * Return: 0 on success, -errno on error.
> + */
> +int generic_encoded_write_checks(struct kiocb *iocb,
> +                                const struct encoded_iov *encoded)
> +{
> +       loff_t count = encoded->len;
> +       int ret;
> +
> +       if (!(iocb->ki_filp->f_flags & O_ALLOW_ENCODED))
> +               return -EPERM;
> +
> +       ret = generic_write_checks_common(iocb, &count);
> +       if (ret)
> +               return ret;
> +
> +       if (count != encoded->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);
> +
> +/**
> + * copy_encoded_iov_from_iter() - copy a &struct encoded_iov from userspace
> + * @encoded: Returned encoding metadata.
> + * @from: Source iterator.
> + *
> + * This copies in the &struct encoded_iov and does some basic sanity checks.
> + * This should always be used rather than a plain copy_from_iter(), as it does
> + * the proper handling for backward- and forward-compatibility.
> + *
> + * Return: 0 on success, -EFAULT if access to userspace failed, -E2BIG if the
> + *         copied structure contained non-zero fields that this kernel doesn't
> + *         support, -EINVAL if the copied structure was invalid.
> + */
> +int copy_encoded_iov_from_iter(struct encoded_iov *encoded,
> +                              struct iov_iter *from)
> +{
> +       size_t usize;
> +       int ret;
> +
> +       usize = iov_iter_single_seg_count(from);
> +       if (usize > PAGE_SIZE)
> +               return -E2BIG;
> +       if (usize < ENCODED_IOV_SIZE_VER0)
> +               return -EINVAL;
> +       ret = copy_struct_from_iter(encoded, sizeof(*encoded), from, usize);
> +       if (ret)
> +               return ret;
> +
> +       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;
> +       if (encoded->len > encoded->unencoded_len - encoded->unencoded_offset)
> +               return -EINVAL;
> +       return 0;
> +}
> +EXPORT_SYMBOL(copy_encoded_iov_from_iter);
> +
> +/**
> + * generic_encoded_read_checks() - sanity check an RWF_ENCODED read
> + * @iocb: I/O context.
> + * @iter: Destination iterator for read.
> + *
> + * This should always be called by RWF_ENCODED read implementations before
> + * returning any data.
> + *
> + * Return: Number of bytes available to return encoded data in @iter on success,
> + *         -EPERM if the file was not opened with O_ALLOW_ENCODED, -EINVAL if
> + *         the size of the &struct encoded_iov iovec is invalid.
> + */
> +ssize_t generic_encoded_read_checks(struct kiocb *iocb, struct iov_iter *iter)
> +{
> +       size_t usize;
> +
> +       if (!(iocb->ki_filp->f_flags & O_ALLOW_ENCODED))
> +               return -EPERM;
> +       usize = iov_iter_single_seg_count(iter);
> +       if (usize > PAGE_SIZE || usize < ENCODED_IOV_SIZE_VER0)
> +               return -EINVAL;
> +       return iov_iter_count(iter) - usize;
> +}
> +EXPORT_SYMBOL(generic_encoded_read_checks);
> +
> +/**
> + * copy_encoded_iov_to_iter() - copy a &struct encoded_iov to userspace
> + * @encoded: Encoding metadata to return.
> + * @to: Destination iterator.
> + *
> + * This should always be used by RWF_ENCODED read implementations rather than a
> + * plain copy_to_iter(), as it does the proper handling for backward- and
> + * forward-compatibility. The iterator must be sanity-checked with
> + * generic_encoded_read_checks() before this is called.
> + *
> + * Return: 0 on success, -EFAULT if access to userspace failed, -E2BIG if there
> + *         were non-zero fields in @encoded that the user buffer could not
> + *         accommodate.
> + */
> +int copy_encoded_iov_to_iter(const struct encoded_iov *encoded,
> +                            struct iov_iter *to)
> +{
> +       size_t ksize = sizeof(*encoded);
> +       size_t usize = iov_iter_single_seg_count(to);
> +       size_t size = min(ksize, usize);
> +
> +       /* We already sanity-checked usize in generic_encoded_read_checks(). */
> +
> +       if (usize < ksize &&
> +           memchr_inv((char *)encoded + usize, 0, ksize - usize))
> +               return -E2BIG;
> +       if (copy_to_iter(encoded, size, to) != size ||
> +           (usize > ksize &&
> +            iov_iter_zero(usize - ksize, to) != usize - ksize))
> +               return -EFAULT;
> +       return 0;
> +}
> +EXPORT_SYMBOL(copy_encoded_iov_to_iter);
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 8667d0cdc71e..67810bf6fb1c 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -178,6 +178,9 @@ typedef int (dio_iodone_t)(struct kiocb *iocb, loff_t offset,
>  /* File supports async buffered reads */
>  #define FMODE_BUF_RASYNC       ((__force fmode_t)0x40000000)
>
> +/* File supports encoded IO */
> +#define FMODE_ENCODED_IO       ((__force fmode_t)0x80000000)
> +
>  /*
>   * Attribute flags.  These should be or-ed together to figure out what
>   * has been changed!
> @@ -308,6 +311,7 @@ enum rw_hint {
>  #define IOCB_SYNC              (__force int) RWF_SYNC
>  #define IOCB_NOWAIT            (__force int) RWF_NOWAIT
>  #define IOCB_APPEND            (__force int) RWF_APPEND
> +#define IOCB_ENCODED           (__force int) RWF_ENCODED
>
>  /* non-RWF related bits - start at 16 */
>  #define IOCB_EVENTFD           (1 << 16)
> @@ -2964,6 +2968,13 @@ extern int generic_file_readonly_mmap(struct file *, struct vm_area_struct *);
>  extern ssize_t generic_write_checks(struct kiocb *, struct iov_iter *);
>  extern int generic_write_check_limits(struct file *file, loff_t pos,
>                 loff_t *count);
> +struct encoded_iov;
> +extern int generic_encoded_write_checks(struct kiocb *,
> +                                       const struct encoded_iov *);
> +extern int copy_encoded_iov_from_iter(struct encoded_iov *, struct iov_iter *);
> +extern ssize_t generic_encoded_read_checks(struct kiocb *, struct iov_iter *);
> +extern int copy_encoded_iov_to_iter(const struct encoded_iov *,
> +                                   struct iov_iter *);
>  extern int generic_file_rw_checks(struct file *file_in, struct file *file_out);
>  extern ssize_t generic_file_buffered_read(struct kiocb *iocb,
>                 struct iov_iter *to, ssize_t already_read);
> diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
> index f44eb0a04afd..95493420117a 100644
> --- a/include/uapi/linux/fs.h
> +++ b/include/uapi/linux/fs.h
> @@ -279,6 +279,42 @@ struct fsxattr {
>                                          SYNC_FILE_RANGE_WAIT_BEFORE | \
>                                          SYNC_FILE_RANGE_WAIT_AFTER)
>
> +enum {
> +       ENCODED_IOV_COMPRESSION_NONE,
> +#define ENCODED_IOV_COMPRESSION_NONE ENCODED_IOV_COMPRESSION_NONE
> +       ENCODED_IOV_COMPRESSION_BTRFS_ZLIB,
> +#define ENCODED_IOV_COMPRESSION_BTRFS_ZLIB ENCODED_IOV_COMPRESSION_BTRFS_ZLIB
> +       ENCODED_IOV_COMPRESSION_BTRFS_ZSTD,
> +#define ENCODED_IOV_COMPRESSION_BTRFS_ZSTD ENCODED_IOV_COMPRESSION_BTRFS_ZSTD
> +       ENCODED_IOV_COMPRESSION_BTRFS_LZO_4K,
> +#define ENCODED_IOV_COMPRESSION_BTRFS_LZO_4K ENCODED_IOV_COMPRESSION_BTRFS_LZO_4K
> +       ENCODED_IOV_COMPRESSION_BTRFS_LZO_8K,
> +#define ENCODED_IOV_COMPRESSION_BTRFS_LZO_8K ENCODED_IOV_COMPRESSION_BTRFS_LZO_8K
> +       ENCODED_IOV_COMPRESSION_BTRFS_LZO_16K,
> +#define ENCODED_IOV_COMPRESSION_BTRFS_LZO_16K ENCODED_IOV_COMPRESSION_BTRFS_LZO_16K
> +       ENCODED_IOV_COMPRESSION_BTRFS_LZO_32K,
> +#define ENCODED_IOV_COMPRESSION_BTRFS_LZO_32K ENCODED_IOV_COMPRESSION_BTRFS_LZO_32K
> +       ENCODED_IOV_COMPRESSION_BTRFS_LZO_64K,
> +#define ENCODED_IOV_COMPRESSION_BTRFS_LZO_64K ENCODED_IOV_COMPRESSION_BTRFS_LZO_64K
> +       ENCODED_IOV_COMPRESSION_TYPES = ENCODED_IOV_COMPRESSION_BTRFS_LZO_64K,
> +};
> +

I am not a fan of this trick.
There is no shortage of enums in uapi headers, but I think that if we want
to set values in stone, the values should be set explicitly and not
auto assigned
by compiler.

If anybody ever adds a line, say ENCODED_IOV_COMPRESSION_BTRFS_ZLIB_V2
in the middle of the enum list, it won't be obvious that it's a uapi breakage.

In principle, we could have partitioned the encoding types by domains
(e.g. btrfs),
and the btrfs specific encodings would have been a part of a btrfs
header, but it's
not that important.

However, please move all encoded_io stuff to a new uapi header and do
not include it
from fs.h to avoid having to compile most filesystems every time a new
btrfs private encoding
type is added.

Thanks,
Amir.