Re: [PATCH v1 9/8] copy_file_range.2: New page documenting copy_file_range()

[Anna Schumaker <Anna.Schumaker-HgOvQuBEEgTQT0dZR+AlfA@public.gmane.org>]

On 09/04/2015 06:31 PM, Andreas Dilger wrote:
> On Sep 4, 2015, at 3:38 PM, Darrick J. Wong <darrick.wong-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org> wrote:
>>
>> On Fri, Sep 04, 2015 at 04:17:03PM -0400, 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-ZwjVKphTwtPQT0dZR+AlfA@public.gmane.org>
>>> ---
>>> man2/copy_file_range.2 | 168 +++++++++++++++++++++++++++++++++++++++++++++++++
>>> 1 file changed, 168 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..4a4cb73
>>> --- /dev/null
>>> +++ b/man2/copy_file_range.2
>>> @@ -0,0 +1,168 @@
>>> +.\"This manpage is Copyright (C) 2015 Anna Schumaker <Anna.Schumaker-ZwjVKphTwtPQT0dZR+AlfA@public.gmane.org>
>>> +.TH COPY 2 2015-8-31 "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/copy.h>
>>> +.B #include <sys/syscall.h>
>>> +.B #include <unistd.h>
>>> +
>>> +.BI "ssize_t syscall(__NR_copy_file_range, int " fd_in ", loff_t * " off_in ",
>>> +.BI "                int " fd_out ", loff_t * " off_out ", size_t " len ",
>>> +.BI "                unsigned int " flags );
>>> +.fi
>>> +.SH DESCRIPTION
>>> +The
>>> +.BR copy_file_range ()
>>> +system call performs an in-kernel copy between two file descriptors
>>> +without all that tedious mucking about in userspace.
>>
>> ;)
>>
>>> +It copies up to
>>> +.I len
>>> +bytes of data from file descriptor
>>> +.I fd_in
>>> +to file descriptor
>>> +.I fd_out
>>> +at
>>> +.IR off_out .
>>> +The file descriptors must not refer to the same file.
>>
>> Why?  btrfs (and XFS) reflink can handle the case of a file sharing blocks
>> with itself.
>>
>>> +
>>> +The following semantics apply for
>>> +.IR fd_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 current
>>> +file offset is adjusted appropriately.
>>> +.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 default behavior of
>>> +.BR copy_file_range ()
>>> +is filesystem specific, and might result in creating a
>>> +copy-on-write reflink.
>>> +In the event that a given filesystem does not implement
>>> +any form of copy acceleration, the kernel will perform
>>> +a deep copy of the requested range by reading bytes from
>>
>> I wonder if it's wise to allow deep copies -- what happens if
>> len == 1T? Will this syscall just block for a really long time?
> 
> It should be interruptible, and return the length of the number of
> bytes copied so far, just like read() and write().  That allows
> the caller to continue where it left off, or abort and delete the
> target file, or whatever it wants to do.

We already return the number of bytes copied so far, so I'll look into making it interruptable!

Thanks,
Anna

> 
> Cheers, Andreas
> 
>>> +.I fd_in
>>> +and writing them to
>>> +.IR fd_out .
>>
>> "...if COPY_REFLINK is not set in flags."
>>
>>> +
>>> +Currently, Linux only supports the following flag:
>>> +.TP 1.9i
>>> +.B COPY_REFLINK
>>> +Only perform the copy if the filesystem can do it as a reflink.
>>> +Do not fall back on performing a deep copy.
>>> +.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 do not have proper read-write mode.
>>
>> "or fd_out is not opened for writing"?
>>
>>> +.TP
>>> +.B EINVAL
>>> +Requested range extends beyond the end of the file;
>>> +.I flags
>>> +argument is set to an invalid value.
>>> +.TP
>>> +.B EOPNOTSUPP
>>> +.B COPY_REFLINK
>>> +was specified in
>>> +.IR flags ,
>>> +but the target filesystem does not support reflinks.
>>> +.TP
>>> +.B EXDEV
>>> +Target filesystem doesn't support cross-filesystem copies.
>>> +.SH VERSIONS
>>
>> Perhaps this ought to list a few more errors (EIO, ENOSPC, ENOSYS, EPERM...)
>> that can be returned?  (I was looking at the fallocate manpage.)
>>
>> --D
>>
>>> +The
>>> +.BR copy_file_range ()
>>> +system call first appeared in Linux 4.3.
>>> +.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/copy.h>
>>> +#include <stdio.h>
>>> +#include <stdlib.h>
>>> +#include <sys/stat.h>
>>> +#include <sys/syscall.h>
>>> +#include <unistd.h>
>>> +
>>> +
>>> +int main(int argc, char **argv)
>>> +{
>>> +    int fd_in, fd_out;
>>> +    struct stat stat;
>>> +    loff_t len, ret;
>>> +
>>> +    if (argc != 3) {
>>> +        fprintf(stderr, "Usage: %s <pathname> <pathname>\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_WRONLY | O_CREAT, 0644);
>>> +    if (fd_out == -1) {
>>> +        perror("open (argv[2])");
>>> +        exit(EXIT_FAILURE);
>>> +    }
>>> +
>>> +    do {
>>> +        ret = syscall(__NR_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)
>>> -- 
>>> 2.5.1
>>>
>>> --
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> 
> 
> Cheers, Andreas
> 
> 
> 
> 
> 

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