[v6,5/4] copy_file_range.2: New page documenting copy_file_range()

Commit Message

Schumaker, Anna Oct. 16, 2015, 9:08 p.m. UTC

copy_file_range() is a new system call for copying ranges of data
completely in the kernel.  This gives filesystems an opportunity to
implement some kind of "copy acceleration", such as reflinks or
server-side-copy (in the case of NFS).

Signed-off-by: Anna Schumaker <[e-mail]>
Reviewed-by: Darrick J. Wong <[e-mail]>
---
v6:
- Updates for removing most flags
---
 man2/copy_file_range.2 | 204 +++++++++++++++++++++++++++++++++++++++++++++++++
 man2/splice.2          |   1 +
 2 files changed, 205 insertions(+)
 create mode 100644 man2/copy_file_range.2

Comments

Andreas Dilger Oct. 16, 2015, 9:21 p.m. UTC | #1

> On Oct 16, 2015, at 3:08 PM, Anna Schumaker <[e-mail]> wrote:
> 
> copy_file_range() is a new system call for copying ranges of data
> completely in the kernel.  This gives filesystems an opportunity to
> implement some kind of "copy acceleration", such as reflinks or
> server-side-copy (in the case of NFS).
> 
> Signed-off-by: Anna Schumaker <[e-mail]>
> Reviewed-by: Darrick J. Wong <[e-mail]>
> ---
> v6:
> - Updates for removing most flags
> ---
> man2/copy_file_range.2 | 204 +++++++++++++++++++++++++++++++++++++++++++++++++
> man2/splice.2          |   1 +
> 2 files changed, 205 insertions(+)
> create mode 100644 man2/copy_file_range.2
> 
> diff --git a/man2/copy_file_range.2 b/man2/copy_file_range.2
> new file mode 100644
> index 0000000..6c52c85
> --- /dev/null
> +++ b/man2/copy_file_range.2
> @@ -0,0 +1,204 @@
> +.\"This manpage is Copyright (C) 2015 Anna Schumaker <[e-mail]>
> +.\"
> +.\" %%%LICENSE_START(VERBATIM)
> +.\" Permission is granted to make and distribute verbatim copies of this
> +.\" manual provided the copyright notice and this permission notice are
> +.\" preserved on all copies.
> +.\"
> +.\" Permission is granted to copy and distribute modified versions of
> +.\" this manual under the conditions for verbatim copying, provided that
> +.\" the entire resulting derived work is distributed under the terms of
> +.\" a permission notice identical to this one.
> +.\"
> +.\" Since the Linux kernel and libraries are constantly changing, this
> +.\" manual page may be incorrect or out-of-date.  The author(s) assume.
> +.\" no responsibility for errors or omissions, or for damages resulting.
> +.\" from the use of the information contained herein.  The author(s) may.
> +.\" not have taken the same level of care in the production of this.
> +.\" manual, which is licensed free of charge, as they might when working.

Is there a reason why every. one. of. those. lines. ends. in. a. period?
I don't think that is needed for nroff, and other paragraphs would support
that conclusion.

> +.\" professionally.
> +.\"
> +.\" Formatted or processed versions of this manual, if unaccompanied by
> +.\" the source, must acknowledge the copyright and authors of this work.
> +.\" %%%LICENSE_END
> +.\"
> +.TH COPY 2 2015-10-16 "Linux" "Linux Programmer's Manual"
> +.SH NAME
> +copy_file_range \- Copy a range of data from one file to another
> +.SH SYNOPSIS
> +.nf
> +.B #include <linux/fs.h>
> +.B #include <sys/syscall.h>
> +.B #include <unistd.h>
> +
> +.BI "ssize_t copy_file_range(int " fd_in ", loff_t *" off_in ", int " fd_out ",
> +.BI "                        loff_t *" off_out ", size_t " len \
> +", unsigned int " flags );
> +.fi
> +.SH DESCRIPTION
> +The
> +.BR copy_file_range ()
> +system call performs an in-kernel copy between two file descriptors
> +without the additional cost of transferring data from the kernel to userspace
> +and then back into the kernel.
> +It copies up to
> +.I len
> +bytes of data from file descriptor
> +.I fd_in
> +to file descriptor
> +.IR fd_out ,
> +overwriting any data that exists within the requested range of the target file.
> +
> +The following semantics apply for
> +.IR off_in ,
> +and similar statements apply to
> +.IR off_out :
> +.IP * 3
> +If
> +.I off_in
> +is NULL, then bytes are read from
> +.I fd_in
> +starting from the current file offset, and the offset is
> +adjusted by the number of bytes copied.
> +.IP *
> +If
> +.I off_in
> +is not NULL, then
> +.I off_in
> +must point to a buffer that specifies the starting
> +offset where bytes from
> +.I fd_in
> +will be read.  The current file offset of
> +.I fd_in
> +is not changed, but
> +.I off_in
> +is adjusted appropriately.
> +.PP
> +
> +The
> +.I flags
> +argument can have the following flag set:
> +.TP 1.9i
> +.B COPY_FR_REFLINK
> +Create a lightweight "reflink", where data is not copied until
> +one of the files is modified.

This is a circular definition.  Something like:

     Create a lightweight reference to the data blocks in the original
     file, where data is not copied until one of the files is modified.

although I'm not sure if "lightweight" is really valuable there.

> +.PP
> +The default behavior
> +.RI ( flags
> +== 0) is to perform a full data copy of the requested range.
> +.SH RETURN VALUE
> +Upon successful completion,
> +.BR copy_file_range ()
> +will return the number of bytes copied between files.
> +This could be less than the length originally requested.

This is a bit vague.  When COPY_FR_REFLINK is used, no data is "copied",
per se, but I doubt that "0" would be returned in that case either.  It
probably makes sense to write something like:

   ... return the number of bytes accessible in the target file.

or maybe (s/accessible/transferred/) or

   ... return the number of bytes added to the target file.

or similar.

> +
> +On error,
> +.BR copy_file_range ()
> +returns \-1 and
> +.I errno
> +is set to indicate the error.
> +.SH ERRORS
> +.TP
> +.B EBADF
> +One or more file descriptors are not valid; or
> +.I fd_in
> +is not open for reading; or
> +.I fd_out
> +is not open for writing.
> +.TP
> +.B EINVAL
> +Requested range extends beyond the end of the source file; or the
> +.I flags
> +argument is set to an invalid value.

Is it possible to return EINTR as well?

Cheers, Andreas

> +.TP
> +.B EIO
> +A low level I/O error occurred while copying.
> +.TP
> +.B ENOMEM
> +Out of memory.
> +.TP
> +.B ENOSPC
> +There is not enough space on the target filesystem to complete the copy.
> +.TP
> +.B EOPNOTSUPP
> +.B COPY_REFLINK
> +was specified in
> +.IR flags ,
> +but the target filesystem does not support reflinks.
> +.TP
> +.B EXDEV
> +.IR file_in " and " file_out
> +are not on the same mounted filesystem.
> +.SH VERSIONS
> +The
> +.BR copy_file_range ()
> +system call first appeared in Linux 4.4.
> +.SH CONFORMING TO
> +The
> +.BR copy_file_range ()
> +system call is a nonstandard Linux extension.
> +.SH EXAMPLE
> +.nf
> +#define _GNU_SOURCE
> +#include <fcntl.h>
> +#include <linux/fs.h>
> +#include <stdio.h>
> +#include <stdlib.h>
> +#include <sys/stat.h>
> +#include <sys/syscall.h>
> +#include <unistd.h>
> +
> +loff_t copy_file_range(int fd_in, loff_t *off_in, int fd_out,
> +                       loff_t *off_out, size_t len, unsigned int flags)
> +{
> +    return syscall(__NR_copy_file_range, fd_in, off_in, fd_out,
> +                   off_out, len, flags);
> +}
> +
> +int main(int argc, char **argv)
> +{
> +    int fd_in, fd_out;
> +    struct stat stat;
> +    loff_t len, ret;
> +    char buf[2];
> +
> +    if (argc != 3) {
> +        fprintf(stderr, "Usage: %s <source> <destination>\\n", argv[0]);
> +        exit(EXIT_FAILURE);
> +    }
> +
> +    fd_in = open(argv[1], O_RDONLY);
> +    if (fd_in == \-1) {
> +        perror("open (argv[1])");
> +        exit(EXIT_FAILURE);
> +    }
> +
> +    if (fstat(fd_in, &stat) == \-1) {
> +        perror("fstat");
> +        exit(EXIT_FAILURE);
> +    }
> +    len = stat.st_size;
> +
> +    fd_out = open(argv[2], O_CREAT|O_WRONLY|O_TRUNC, 0644);
> +    if (fd_out == \-1) {
> +        perror("open (argv[2])");
> +        exit(EXIT_FAILURE);
> +    }
> +
> +    do {
> +        ret = copy_file_range(fd_in, NULL, fd_out, NULL, len, 0);
> +        if (ret == \-1) {
> +            perror("copy_file_range");
> +            exit(EXIT_FAILURE);
> +        }
> +
> +        len \-= ret;
> +    } while (len > 0);
> +
> +    close(fd_in);
> +    close(fd_out);
> +    exit(EXIT_SUCCESS);
> +}
> +.fi
> +.SH SEE ALSO
> +.BR splice (2)
> diff --git a/man2/splice.2 b/man2/splice.2
> index b9b4f42..5c162e0 100644
> --- a/man2/splice.2
> +++ b/man2/splice.2
> @@ -238,6 +238,7 @@ only pointers are copied, not the pages of the buffer.
> See
> .BR tee (2).
> .SH SEE ALSO
> +.BR copy_file_range (2),
> .BR sendfile (2),
> .BR tee (2),
> .BR vmsplice (2)
> --
> 2.6.1
> 
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Cheers, Andreas

Pádraig Brady Oct. 16, 2015, 9:42 p.m. UTC | #2

It's probably worth mentioning the sparse expansion caveat
in the docs, as it's important and not obvious.

I.E. copy_file_range() could be called from, but would
still benefit from a user space app using a SEEK_{HOLE,DATA} loop.

thanks,
Pádraig.
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Christoph Hellwig Oct. 18, 2015, 6:30 p.m. UTC | #3

Just commenting on the man page here as the comment is about sematics.
All the infrastructure in the patch looks reasonable to me, but this
is something we need to get right.

> +.B COPY_FR_REFLINK
> +Create a lightweight "reflink", where data is not copied until
> +one of the files is modified.
> +.PP
> +The default behavior
> +.RI ( flags
> +== 0) is to perform a full data copy of the requested range.
> +.SH RETURN VALUE
> +Upon successful completion,
> +.BR copy_file_range ()
> +will return the number of bytes copied between files.
> +This could be less than the length originally requested.

As mentioned in the previous discussion I fundamentally disagree with
the way your word the flags here.

flags = 0 gives you the data from source at dest, period.  How it's
implemented is up to the file system as a user cannot observe how data
actually is stored underneath.

Additionaly I think the 'clone' option with it's stronger guarantees
should be a separate system call.  So for now just have no supported
flag and leave it up to the file system and storage device how to
implement it.

For the future a COPY_FALLOC flag taht guaranatees you do not get ENOSPC
on the copied range will be very useful, but given the complexity I
think it's not something we should add now.
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J. Bruce Fields Oct. 19, 2015, 8:45 p.m. UTC | #4

On Sun, Oct 18, 2015 at 11:30:13AM -0700, Christoph Hellwig wrote:
> Just commenting on the man page here as the comment is about sematics.
> All the infrastructure in the patch looks reasonable to me, but this
> is something we need to get right.
> 
> > +.B COPY_FR_REFLINK
> > +Create a lightweight "reflink", where data is not copied until
> > +one of the files is modified.
> > +.PP
> > +The default behavior
> > +.RI ( flags
> > +== 0) is to perform a full data copy of the requested range.
> > +.SH RETURN VALUE
> > +Upon successful completion,
> > +.BR copy_file_range ()
> > +will return the number of bytes copied between files.
> > +This could be less than the length originally requested.
> 
> As mentioned in the previous discussion I fundamentally disagree with
> the way your word the flags here.
> 
> flags = 0 gives you the data from source at dest, period.  How it's
> implemented is up to the file system as a user cannot observe how data
> actually is stored underneath.
> 
> Additionaly I think the 'clone' option with it's stronger guarantees
> should be a separate system call.  So for now just have no supported
> flag and leave it up to the file system and storage device how to
> implement it.

So, continue to include a "flags" field but just error out if it's
anything but zero for now?

Sounds fine by me.  We can always implement the other stuff later.

--b.

> 
> For the future a COPY_FALLOC flag taht guaranatees you do not get ENOSPC
> on the copied range will be very useful, but given the complexity I
> think it's not something we should add now.
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Christoph Hellwig Oct. 19, 2015, 9:03 p.m. UTC | #5

On Mon, Oct 19, 2015 at 04:45:03PM -0400, J. Bruce Fields wrote:
> So, continue to include a "flags" field but just error out if it's
> anything but zero for now?

Exactly!
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Patch

diff --git a/man2/copy_file_range.2 b/man2/copy_file_range.2
new file mode 100644
index 0000000..6c52c85
--- /dev/null
+++ b/man2/copy_file_range.2
@@ -0,0 +1,204 @@ 
+.\"This manpage is Copyright (C) 2015 Anna Schumaker <[e-mail]>
+.\"
+.\" %%%LICENSE_START(VERBATIM)
+.\" Permission is granted to make and distribute verbatim copies of this
+.\" manual provided the copyright notice and this permission notice are
+.\" preserved on all copies.
+.\"
+.\" Permission is granted to copy and distribute modified versions of
+.\" this manual under the conditions for verbatim copying, provided that
+.\" the entire resulting derived work is distributed under the terms of
+.\" a permission notice identical to this one.
+.\"
+.\" Since the Linux kernel and libraries are constantly changing, this
+.\" manual page may be incorrect or out-of-date.  The author(s) assume.
+.\" no responsibility for errors or omissions, or for damages resulting.
+.\" from the use of the information contained herein.  The author(s) may.
+.\" not have taken the same level of care in the production of this.
+.\" manual, which is licensed free of charge, as they might when working.
+.\" professionally.
+.\"
+.\" Formatted or processed versions of this manual, if unaccompanied by
+.\" the source, must acknowledge the copyright and authors of this work.
+.\" %%%LICENSE_END
+.\"
+.TH COPY 2 2015-10-16 "Linux" "Linux Programmer's Manual"
+.SH NAME
+copy_file_range \- Copy a range of data from one file to another
+.SH SYNOPSIS
+.nf
+.B #include <linux/fs.h>
+.B #include <sys/syscall.h>
+.B #include <unistd.h>
+
+.BI "ssize_t copy_file_range(int " fd_in ", loff_t *" off_in ", int " fd_out ",
+.BI "                        loff_t *" off_out ", size_t " len \
+", unsigned int " flags );
+.fi
+.SH DESCRIPTION
+The
+.BR copy_file_range ()
+system call performs an in-kernel copy between two file descriptors
+without the additional cost of transferring data from the kernel to userspace
+and then back into the kernel.
+It copies up to
+.I len
+bytes of data from file descriptor
+.I fd_in
+to file descriptor
+.IR fd_out ,
+overwriting any data that exists within the requested range of the target file.
+
+The following semantics apply for
+.IR off_in ,
+and similar statements apply to
+.IR off_out :
+.IP * 3
+If
+.I off_in
+is NULL, then bytes are read from
+.I fd_in
+starting from the current file offset, and the offset is
+adjusted by the number of bytes copied.
+.IP *
+If
+.I off_in
+is not NULL, then
+.I off_in
+must point to a buffer that specifies the starting
+offset where bytes from
+.I fd_in
+will be read.  The current file offset of
+.I fd_in
+is not changed, but
+.I off_in
+is adjusted appropriately.
+.PP
+
+The
+.I flags
+argument can have the following flag set:
+.TP 1.9i
+.B COPY_FR_REFLINK
+Create a lightweight "reflink", where data is not copied until
+one of the files is modified.
+.PP
+The default behavior
+.RI ( flags
+== 0) is to perform a full data copy of the requested range.
+.SH RETURN VALUE
+Upon successful completion,
+.BR copy_file_range ()
+will return the number of bytes copied between files.
+This could be less than the length originally requested.
+
+On error,
+.BR copy_file_range ()
+returns \-1 and
+.I errno
+is set to indicate the error.
+.SH ERRORS
+.TP
+.B EBADF
+One or more file descriptors are not valid; or
+.I fd_in
+is not open for reading; or
+.I fd_out
+is not open for writing.
+.TP
+.B EINVAL
+Requested range extends beyond the end of the source file; or the
+.I flags
+argument is set to an invalid value.
+.TP
+.B EIO
+A low level I/O error occurred while copying.
+.TP
+.B ENOMEM
+Out of memory.
+.TP
+.B ENOSPC
+There is not enough space on the target filesystem to complete the copy.
+.TP
+.B EOPNOTSUPP
+.B COPY_REFLINK
+was specified in
+.IR flags ,
+but the target filesystem does not support reflinks.
+.TP
+.B EXDEV
+.IR file_in " and " file_out
+are not on the same mounted filesystem.
+.SH VERSIONS
+The
+.BR copy_file_range ()
+system call first appeared in Linux 4.4.
+.SH CONFORMING TO
+The
+.BR copy_file_range ()
+system call is a nonstandard Linux extension.
+.SH EXAMPLE
+.nf
+#define _GNU_SOURCE
+#include <fcntl.h>
+#include <linux/fs.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/stat.h>
+#include <sys/syscall.h>
+#include <unistd.h>
+
+loff_t copy_file_range(int fd_in, loff_t *off_in, int fd_out,
+                       loff_t *off_out, size_t len, unsigned int flags)
+{
+    return syscall(__NR_copy_file_range, fd_in, off_in, fd_out,
+                   off_out, len, flags);
+}
+
+int main(int argc, char **argv)
+{
+    int fd_in, fd_out;
+    struct stat stat;
+    loff_t len, ret;
+    char buf[2];
+
+    if (argc != 3) {
+        fprintf(stderr, "Usage: %s <source> <destination>\\n", argv[0]);
+        exit(EXIT_FAILURE);
+    }
+
+    fd_in = open(argv[1], O_RDONLY);
+    if (fd_in == \-1) {
+        perror("open (argv[1])");
+        exit(EXIT_FAILURE);
+    }
+
+    if (fstat(fd_in, &stat) == \-1) {
+        perror("fstat");
+        exit(EXIT_FAILURE);
+    }
+    len = stat.st_size;
+
+    fd_out = open(argv[2], O_CREAT|O_WRONLY|O_TRUNC, 0644);
+    if (fd_out == \-1) {
+        perror("open (argv[2])");
+        exit(EXIT_FAILURE);
+    }
+
+    do {
+        ret = copy_file_range(fd_in, NULL, fd_out, NULL, len, 0);
+        if (ret == \-1) {
+            perror("copy_file_range");
+            exit(EXIT_FAILURE);
+        }
+
+        len \-= ret;
+    } while (len > 0);
+
+    close(fd_in);
+    close(fd_out);
+    exit(EXIT_SUCCESS);
+}
+.fi
+.SH SEE ALSO
+.BR splice (2)
diff --git a/man2/splice.2 b/man2/splice.2
index b9b4f42..5c162e0 100644
--- a/man2/splice.2
+++ b/man2/splice.2
@@ -238,6 +238,7 @@  only pointers are copied, not the pages of the buffer.
 See
 .BR tee (2).
 .SH SEE ALSO
+.BR copy_file_range (2),
 .BR sendfile (2),
 .BR tee (2),
 .BR vmsplice (2)