diff --git a/Doc/library/os.rst b/Doc/library/os.rst
index d5e9bbcffc31cf..87e9eb6b69c84b 100644
--- a/Doc/library/os.rst
+++ b/Doc/library/os.rst
@@ -798,18 +798,32 @@ as internal buffering of data.
Copy *count* bytes from file descriptor *src*, starting from offset
*offset_src*, to file descriptor *dst*, starting from offset *offset_dst*.
If *offset_src* is None, then *src* is read from the current position;
- respectively for *offset_dst*. The files pointed by *src* and *dst*
+ respectively for *offset_dst*.
+
+ In Linux kernel older than 5.3, the files pointed by *src* and *dst*
must reside in the same filesystem, otherwise an :exc:`OSError` is
raised with :attr:`~OSError.errno` set to :data:`errno.EXDEV`.
This copy is done without the additional cost of transferring data
from the kernel to user space and then back into the kernel. Additionally,
- some filesystems could implement extra optimizations. The copy is done as if
- both files are opened as binary.
+ some filesystems could implement extra optimizations, such as the use of
+ reflinks (i.e., two or more inodes that share pointers to the same
+ copy-on-write disk blocks; supported file systems include btrfs and XFS)
+ and server-side copy (in the case of NFS).
+
+ The function copies bytes between two file descriptors. Text options, like
+ the encoding and the line ending, are ignored.
The return value is the amount of bytes copied. This could be less than the
amount requested.
+ .. note::
+
+ On Linux, :func:`os.copy_file_range` should not be used for copying a
+ range of a pseudo file from a special filesystem like procfs and sysfs.
+ It will always copy no bytes and return 0 as if the file was empty
+ because of a known Linux kernel issue.
+
.. availability:: Linux kernel >= 4.5 or glibc >= 2.27.
.. versionadded:: 3.8
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