diff options
Diffstat (limited to 'src/internal/poll/copy_file_range_linux.go')
| -rw-r--r-- | src/internal/poll/copy_file_range_linux.go | 128 |
1 files changed, 128 insertions, 0 deletions
diff --git a/src/internal/poll/copy_file_range_linux.go b/src/internal/poll/copy_file_range_linux.go new file mode 100644 index 0000000..ba33f51 --- /dev/null +++ b/src/internal/poll/copy_file_range_linux.go @@ -0,0 +1,128 @@ +// Copyright 2020 The Go Authors. All rights reserved. +// Use of this source code is governed by a BSD-style +// license that can be found in the LICENSE file. + +package poll + +import ( + "internal/syscall/unix" + "sync" + "syscall" +) + +var ( + kernelVersion53Once sync.Once + kernelVersion53 bool +) + +const maxCopyFileRangeRound = 1 << 30 + +// CopyFileRange copies at most remain bytes of data from src to dst, using +// the copy_file_range system call. dst and src must refer to regular files. +func CopyFileRange(dst, src *FD, remain int64) (written int64, handled bool, err error) { + kernelVersion53Once.Do(func() { + major, minor := unix.KernelVersion() + // copy_file_range(2) is broken in various ways on kernels older than 5.3, + // see issue #42400 and + // https://man7.org/linux/man-pages/man2/copy_file_range.2.html#VERSIONS + if major > 5 || (major == 5 && minor >= 3) { + kernelVersion53 = true + } + }) + + if !kernelVersion53 { + return 0, false, nil + } + + for remain > 0 { + max := remain + if max > maxCopyFileRangeRound { + max = maxCopyFileRangeRound + } + n, err := copyFileRange(dst, src, int(max)) + switch err { + case syscall.ENOSYS: + // copy_file_range(2) was introduced in Linux 4.5. + // Go supports Linux >= 2.6.33, so the system call + // may not be present. + // + // If we see ENOSYS, we have certainly not transferred + // any data, so we can tell the caller that we + // couldn't handle the transfer and let them fall + // back to more generic code. + return 0, false, nil + case syscall.EXDEV, syscall.EINVAL, syscall.EIO, syscall.EOPNOTSUPP, syscall.EPERM: + // Prior to Linux 5.3, it was not possible to + // copy_file_range across file systems. Similarly to + // the ENOSYS case above, if we see EXDEV, we have + // not transferred any data, and we can let the caller + // fall back to generic code. + // + // As for EINVAL, that is what we see if, for example, + // dst or src refer to a pipe rather than a regular + // file. This is another case where no data has been + // transferred, so we consider it unhandled. + // + // If src and dst are on CIFS, we can see EIO. + // See issue #42334. + // + // If the file is on NFS, we can see EOPNOTSUPP. + // See issue #40731. + // + // If the process is running inside a Docker container, + // we might see EPERM instead of ENOSYS. See issue + // #40893. Since EPERM might also be a legitimate error, + // don't mark copy_file_range(2) as unsupported. + return 0, false, nil + case nil: + if n == 0 { + // If we did not read any bytes at all, + // then this file may be in a file system + // where copy_file_range silently fails. + // https://lore.kernel.org/linux-fsdevel/20210126233840.GG4626@dread.disaster.area/T/#m05753578c7f7882f6e9ffe01f981bc223edef2b0 + if written == 0 { + return 0, false, nil + } + // Otherwise src is at EOF, which means + // we are done. + return written, true, nil + } + remain -= n + written += n + default: + return written, true, err + } + } + return written, true, nil +} + +// copyFileRange performs one round of copy_file_range(2). +func copyFileRange(dst, src *FD, max int) (written int64, err error) { + // The signature of copy_file_range(2) is: + // + // ssize_t copy_file_range(int fd_in, loff_t *off_in, + // int fd_out, loff_t *off_out, + // size_t len, unsigned int flags); + // + // Note that in the call to unix.CopyFileRange below, we use nil + // values for off_in and off_out. For the system call, this means + // "use and update the file offsets". That is why we must acquire + // locks for both file descriptors (and why this whole machinery is + // in the internal/poll package to begin with). + if err := dst.writeLock(); err != nil { + return 0, err + } + defer dst.writeUnlock() + if err := src.readLock(); err != nil { + return 0, err + } + defer src.readUnlock() + var n int + for { + n, err = unix.CopyFileRange(src.Sysfd, nil, dst.Sysfd, nil, max, 0) + if err != syscall.EINTR { + break + } + } + return int64(n), err +} |