kerneltrap

Not necessarily, while reflink works only within the boundaries of the same filesystem there are other optimizations that can operate across filesystems (even networked ones). For example copy_file_range() [1] allows for copy offloading (depending on the underlying fs/storage), which was incidentally also added to coreutils v9.0 [2].

There are also optimizations on other layers, e.g. transparent compression (e.g. btrfs) and online block-level dedup (e.g. dm-vdo), that can aid in minimizing and/or avoiding actual block copies.

When all else fails, cp will anyway fallback to buffered IO, so it won't be bound by the rate of IO (as long as writeback can keep up with memory pressure).

Of course improvements on all layers are always welcome and there are certainly workloads that can benefit from an optimized raw throughput, low-latency and scalability. Demanding workloads are typically using dedicated IO stacks and bypass the kernel altogether (e.g SPDK), but the amazing efforts of Axboe and everyone else in the kernel community have been continuously bringing linux up to par in terms of IO performance.

[1] https://man7.org/linux/man-pages/man2/copy_file_range.2.html [2] https://git.savannah.gnu.org/cgit/coreutils.git/commit/?id=4...

The fastest `cp` is actually doing no data copies at all (relying on copy-on-write), on filesystems with reflink support. Incidentally, coreutils v9.0 cp switched to doing reflinks by default [1], so there's already a faster cp.

[1] https://git.savannah.gnu.org/cgit/coreutils.git/commit/?id=2...

Your first point is the correct one.

Basically the offset passed to bdev_write_page() was wrong, and has been like that since it was introduced to swap code back in v3.16.

In practice this means that the same filesystem corruption could happen with swapfiles sitting on top of filesystems which are created on top of block devices that bdev_write_page() supports (in specific: brd, zram, btt, pmem). This appears to be a non-common setup that apparently was not encountered. Nevertheless the fix is being backported to all stable kernels to cover those use-cases.

edit: bdev_write_page() was never broken, just the offset handed to it was wrong.