add new dependency: fiemap

[oech3]

I want to use fiemap for

• a todo for cp

  coreutils/src/uu/cp/src/platform/linux.rs

  Lines 172 to 174 in 7b0d6c8

  	// TODO Perhaps we can employ the "fiemap ioctl" API to get the
  	// file extent mappings:
  	// https://www.kernel.org/doc/html/latest/filesystems/fiemap.html

• improving performance for cat, head -c and tail. copy_file_range is slower than splice on the FS without reflink support. uutils/cat doesn't use copy_file_range by the reason while GNU cat supports reflink via copy_file_range. fiemap allows to check that copy_file_range worked as reflink.

https://github.com/lilydjwg/fiemap-rs

There is no other safe-wrapper on rustix currently.

[sylvestre]

i won't accept a PR for this for now.

1. not even published on crates.io, even if started a year ago
2. only 3 contributors
3. no real user for now
4. no CI
5. no real activity since September
   etc

[HackingRepo]

i see you did a great job, that's how a supply chain security works and good, great job

[oech3]

How about having our own unsafe code for this?

@HackingRepo sarcasm is not helpful.

[sylvestre]

sure, we could add the equivalent functions in uucore

[pixelb]

We used fiemap in GNU cp for years, but moved away from it due to issues with requiring io sync'ing and advice from kernel folks to avoid this somewhat low level interface.

It's less required these days with copy offload interfaces like copy_file_range, and other read reduction interfaces like SEEK_HOLE

[oech3]

I want to use copy_file_range restricted to reflink because internal splice fallback is slower than splice and ficlone_range needs alighnment. Is there any reasonable way to check that 1st block is shared after copy_file_range?

[pixelb]

Oh right, I don't know of another file system agnostic way to determine that. Anyway just be wary of fiemap and unsync'd files