On 10/1/18 9:48 AM, Qu Wenruo wrote: > > > On 2018/10/1 下午10:32, Joshi wrote: >> I was wondering about the cross-fs copy through copy_file_range. > > The term "cross-fs" looks pretty confusing. > > If you mean "cross-subvolume", then it should work without problem in btrfs. > > If you mean reflink across two different file systems (not matter the > same fs type or not). > Then it's impossible to work. I believe Joshi is talking about vfs_copy_file_range() not vfs_clone_file range(), although _copy_ does call _clone_ if it can. > Reflink (clone_file_range) works by inserting data pointers into the > filesystem other than really copying the data. > Thus if the source is outside of the fs, it's really impossible to work, > as the source pointer/data is completely out of control of the dest fs. Yes, I would expect there to be problems with his modified kernel for a filesystem that supports clone_file_range, because vfs_copy_file_range() will clone if possible, and this should fail across filesystems. In general, though, I don't know for sure why we don't fall back to do_splice_direct() across filesystems, although the filesystems that implement their own ->copy_file_range ops may have their own, further restrictions within their implementations. This call /is/ documented in the manpage as only being valid for files on the same filesystem, though: http://man7.org/linux/man-pages/man2/copy_file_range.2.html -Eric
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