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List: linux-api
Subject: Re: [PATCH v4 3/9] fs: add RWF_ENCODED for reading/writing compressed data
From: Omar Sandoval <osandov () osandov ! com>
Date: 2020-02-29 18:10:53
Message-ID: 20200229181053.GB157744 () vader
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On Sat, Feb 29, 2020 at 12:40:58PM +0200, Amir Goldstein wrote:
> On Sat, Feb 29, 2020 at 1:14 AM 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.
> >
> > Signed-off-by: Omar Sandoval <osandov@fb.com>
> > ---
> > Documentation/filesystems/encoded_io.rst | 74 ++++++++++
> > Documentation/filesystems/index.rst | 1 +
> > include/linux/fs.h | 16 +++
> > include/uapi/linux/fs.h | 33 ++++-
> > mm/filemap.c | 166 +++++++++++++++++++++--
> > 5 files changed, 276 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::
> > +
> > + 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 386eaad008b2..e074a3f1f856 100644
> > --- a/Documentation/filesystems/index.rst
> > +++ b/Documentation/filesystems/index.rst
> > @@ -37,6 +37,7 @@ filesystem implementations.
> > journalling
> > fscrypt
> > fsverity
> > + encoded_io
> >
> > Filesystems
> > ===========
> > diff --git a/include/linux/fs.h b/include/linux/fs.h
> > index 3cd4fe6b845e..aa7efd3430d1 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;
> > @@ -3109,6 +3113,13 @@ 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 *,
> > + 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_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);
> > @@ -3434,6 +3445,11 @@ 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..f8c6c1e08def 100644
> > --- a/include/uapi/linux/fs.h
> > +++ b/include/uapi/linux/fs.h
> > @@ -278,6 +278,34 @@ 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_ZLIB,
> > +#define ENCODED_IOV_COMPRESSION_ZLIB ENCODED_IOV_COMPRESSION_ZLIB
> > + ENCODED_IOV_COMPRESSION_LZO,
> > +#define ENCODED_IOV_COMPRESSION_LZO ENCODED_IOV_COMPRESSION_LZO
> > + ENCODED_IOV_COMPRESSION_ZSTD,
> > +#define ENCODED_IOV_COMPRESSION_ZSTD ENCODED_IOV_COMPRESSION_ZSTD
> > + ENCODED_IOV_COMPRESSION_TYPES = ENCODED_IOV_COMPRESSION_ZSTD,
> > +};
> > +
> > +enum {
> > + ENCODED_IOV_ENCRYPTION_NONE,
> > +#define ENCODED_IOV_ENCRYPTION_NONE ENCODED_IOV_ENCRYPTION_NONE
> > + ENCODED_IOV_ENCRYPTION_TYPES = ENCODED_IOV_ENCRYPTION_NONE,
> > +};
> > +
>
> What are those defines???
They're so you can check whether an enum value is defined in the UAPI
headers via ifdef. E.g., if we were to add
ENCODED_IOV_COMPRESSION_SOME_NEW_ALGORITHM, applications could use:
#ifndef ENCODED_IOV_COMPRESSION_SOME_NEW_ALGORITHM
#define ENCODED_IOV_COMPRESSION_SOME_NEW_ALGORITHM 4
#endif
In my experience, this makes dealing with lagging UAPI headers less
annoying. This is done elsewhere in UAPI headers (e.g.,
include/uapi/linux/in.h).
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