Re: [PATCH v8 5/4] copy_file_range.2: New page documenting copy_file_range()

[Anna Schumaker <Anna.Schumaker@netapp.com>]

On 01/26/2016 04:49 AM, Michael Kerrisk (man-pages) wrote:
> Hi Anna,
> 
> Thanks for writing this page! I've merged it, and pushed to Git.
> I made a few tweaks, which I think are all straightforward, 
> but you might like to review my comments below.
> 
> On 11/06/2015 10:18 PM, Anna Schumaker 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 <Anna.Schumaker@Netapp.com>
>> Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
>> Reviewed-by: Christoph Hellwig <hch@lst.de>
>> ---
>> v8:
>> - Document that files can not be open with O_APPEND.
>> ---
>>  man2/copy_file_range.2 | 201 +++++++++++++++++++++++++++++++++++++++++++++++++
>>  man2/splice.2          |   1 +
>>  2 files changed, 202 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..d9f76d1
>> --- /dev/null
>> +++ b/man2/copy_file_range.2
>> @@ -0,0 +1,201 @@
>> +.\"This manpage is Copyright (C) 2015 Anna Schumaker <Anna.Schumaker@Netapp.com>
>> +.\"
>> +.\" %%%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 +.\" manual page may be incorrect or out-of-date.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 2015-11-06 "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 <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 must be set to 0.
>> +.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; or
>> +.I fd_out
>> +is open for appending.
>> +.TP
>> +.B EINVAL
>> +Requested range extends beyond the end of the source file; or the
>> +.I flags
>> +argument is not 0.
>> +.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 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 NOTES
>> +If
>> +.I file_in
>> +is a sparse file, then
>> +.BR copy_file_range ()
>> +may expand any holes existing in the requested range.
>> +Users may benefit from calling
>> +.BR copy_file_range ()
>> +in a loop, and using
>> +.BR lseek (2)
>> +to find the locations of data segments.
> 
> Here, I explicitly added mention of SEEK_HOLE and SEEK_DATA. okay?

Yeah, that makes sense.

> 
>> +.SH EXAMPLE
>> +.nf
>> +#define _GNU_SOURCE
>> +#include <fcntl.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];
> 
> 'buf' is unused, so I removed it. I assume it was accidental cruft; this
> is just a heads-up, in case some you meant have some code in the program
> that would use that variable.

I don't even remember what I used 'buf' for, so that makes sense too.

Thanks for committing it!
Anna

> 
>> +
>> +    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)
> 
> Thanks, 
> 
> Michael
> 
>