On 2015-09-08 16:39, Darrick J. Wong wrote:
I'd personally love to see that be tunable by a sysctl (kind of like how you can control the maximum number of AIO requests in flight), and for that matter we might want to be able to limit the number of in-progress copies going on.On Tue, Sep 08, 2015 at 11:04:03AM -0400, Anna Schumaker wrote:On 09/04/2015 05:38 PM, Darrick J. Wong 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@xxxxxxxxxx> --- 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@xxxxxxxxxx> +.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.I've never really thought about it... Zach had that in his initial submission, so mentioned it in the man page. Should I remove that bit?Yes, please! I could be wrong, but I think btrfs only started supporting files that share blocks with themselves relatively recently(?) I'm not sure why zab added this; was hoping he'd speak up. ;)+ +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 fromI wonder if it's wise to allow deep copies -- what happens if len == 1T? Will this syscall just block for a really long time?We use rw_verify_area(), (similar to read and write) so we won't allow a value of len that long. I can mention this in an updated version of this man page!Ok. I guess MAX_RW_COUNT limits us to about 4G at once, which for a splice copy is probably reasonable. The reason why I asked about len == 1T specifically is that I can (with somewhat long delays) reflink about 260 million extents at a time on XFS, which is about 1TB. Given that locks get held for the duration, it's probably not a bad thing to limit userspace to 4G at a time.
(But hey, it's fun to stress-test once in a while. :)) --D+.I fd_in +and writing them to +.IR fd_out ."...if COPY_REFLINK is not set in flags."Sure.+ +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"?I'll add that.+.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 VERSIONSPerhaps this ought to list a few more errors (EIO, ENOSPC, ENOSYS, EPERM...) that can be returned? (I was looking at the fallocate manpage.)Okay. I'll poke around for what else could be returned! Thanks, Anna--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 -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html-- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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