cp failing sporadically due to EAGAIN being returned from copy_file_range

[Tobi29]

System information

Type	Version/Name
Distribution Name	Arch Linux
Distribution Version	Rolling release
Kernel Version	6.4.9-zen1-1-zen
Architecture	x86_64
OpenZFS Version	2.2.99-61_g8ce2eba9e6

Describe the problem you're observing

Copying files using e.g. cp occasionally fails, giving an error like cp: error copying 'file_a' to 'file_b': Resource temporarily unavailable.
This is caused by copy_file_range in zfs returning EAGAIN, something that seems to have been added in f21662d.
The man page for copy_file_range does not mention EAGAIN as a possible error and I cannot say how many other programs, beyond cp, also handle EAGAIN as a hard error in this case.
Switching to cp --reflink=never fixes the problem.

The end result of this is that many automation scripts, e.g. makepkg on Arch Linux, are unusable because of files, because they trigger this error very frequently.

Describe how to reproduce the problem

This example script triggers the issue almost every time:

#!/bin/bash

dd if=/dev/urandom of=file_a bs=4MiB count=1
cp file_a file_b
dd if=/dev/urandom of=file_a bs=4MiB count=1
cp file_a file_b

# just cleanup
rm file_a file_b

It is required to run these commands in very quick succession.

Include any warning/errors/backtraces from the system logs

The mentioned script would output:

1+0 records in
1+0 records out
4194304 bytes (4.2 MB, 4.0 MiB) copied, 0.00903691 s, 464 MB/s
1+0 records in
1+0 records out
4194304 bytes (4.2 MB, 4.0 MiB) copied, 0.00990963 s, 423 MB/s
cp: error copying 'file_a' to 'file_b': Resource temporarily unavailable

Nothing can be seen in dmesg.

[rincebrain]

@oromenahar

[oromenahar]

Got an update on this, will be merged pretty fast.
Thanks for this issue.