[PATCH v5 3/9] fs: add RWF_ENCODED for reading/writing compressed data

Omar Sandoval
2020-08-21T07:39:24Z

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 4c536e66dc4c..9adbbc3c7441 100644
--- a/Documentation/filesystems/index.rst
+++ b/Documentation/filesystems/index.rst
@@ -55,6 +55,7 @@ filesystem implementations.
journalling
fscrypt
fsverity
+ encoded_io

Filesystems
===========
diff --git a/include/linux/fs.h b/include/linux/fs.h
index e019ea2f1347..a3fe6fa9687c 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)
+
/*
* Flag for rw_copy_check_uvector and compat_rw_copy_check_uvector
* that indicates that they should check the contents of the iovec are
@@ -321,6 +324,7 @@ enum rw_hint {
/* iocb->ki_waitq is valid */
#define IOCB_WAITQ (1 << 8)
#define IOCB_NOIO (1 << 9)
+#define IOCB_ENCODED (1 << 10)

struct kiocb {
struct file *ki_filp;
@@ -3006,6 +3010,13 @@ 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);
@@ -3324,6 +3335,11 @@ static inline int kiocb_set_rw_flags(struct kiocb *ki, rwf_t flags)
return -EOPNOTSUPP;
kiocb_flags |= IOCB_NOWAIT | IOCB_NOIO;
}
+ if (flags & RWF_ENCODED) {
+ if (!(ki->ki_filp->f_mode & FMODE_ENCODED_IO))
+ return -EOPNOTSUPP;
+ kiocb_flags |= IOCB_ENCODED;
+ }
if (flags & RWF_HIPRI)
kiocb_flags |= IOCB_HIPRI;
if (flags & RWF_DSYNC)
diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
index f44eb0a04afd..972390d3089d 100644
--- a/include/uapi/linux/fs.h
+++ b/include/uapi/linux/fs.h
@@ -279,6 +279,34 @@ 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,
+};
+
+struct encoded_iov {
+ __aligned_u64 len;
+ __aligned_u64 unencoded_len;
+ __aligned_u64 unencoded_offset;
+ __u32 compression;
+ __u32 encryption;
+};
+
+#define ENCODED_IOV_SIZE_VER0 32
+
/*
* Flags for preadv2/pwritev2:
*/
@@ -300,8 +328,11 @@ typedef int __bitwise __kernel_rwf_t;
/* per-IO O_APPEND */
#define RWF_APPEND ((__force __kernel_rwf_t)0x00000010)

+/* encoded (e.g., compressed and/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 1aaea26556cc..f28b42888144 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -3022,24 +3022,15 @@ 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 */
@@ -3049,8 +3040,21 @@ 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;

@@ -3059,6 +3063,142 @@ inline ssize_t generic_write_checks(struct kiocb *iocb, struct iov_iter *from)
}
EXPORT_SYMBOL(generic_write_checks);

+/**
+ * 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);
+
/*
* Performs necessary checks before doing a clone.
*
--
2.28.0