Write IOs should be page aligned as fuse server might need to copy data to another buffer otherwise in order to fulfill network or device storage requirements. Simple reproducer is with libfuse, example/passthrough* and opening a file with O_DIRECT - without this change writing to that file failed with -EINVAL if the underlying file system was requiring alignment. Required server side changes: Server needs to seek to the next page, without splice that is just page size buffer alignment, with splice another splice syscall is needed to seek over the unaligned area. Signed-off-by: Bernd Schubert <bschubert@xxxxxxx> --- Changes since v2: - Added a no-op return in fuse_copy_align for buffers that are already aligned (cs->len == PAGE_SIZE && cs->offset == 0). Some server implementations actually do that to compensate for fuse client misalignment. And it could also happen by accident for non aligned server allocation. Added suggestions from Joannes review: - Removed two sanity checks in fuse_copy_align() to have it generic. - Moved from args->in_args[0].align to args->in_args[1].align to have it in the arg that actually needs the alignment (for FUSE_WRITE) and updated fuse_copy_args() to align that arg. - Slight update in the commit body (removed "Reads"). libfuse patch: https://github.com/libfuse/libfuse/pull/983
