On Sun, Apr 14, 2019 at 4:04 AM Darrick J. Wong <darrick.wong@xxxxxxxxxx> wrote: > > On Sat, Apr 13, 2019 at 03:54:39PM -0500, Shawn Landden wrote: > > /me pulls out his close-reading glasses and the copy_file_range manpage... > > > If flags includes COPY_FILE_RANGE_FILESIZE then the length > > copied is the length of the file. off_in and off_out are > > ignored. len must be 0 or the file size. > > They're ignored? As in the copy operation reads the number of bytes in > the file referenced by fd_in from fd_in at its current position and is > writes that out to fd_out at its current position? I don't see why I > would want such an operation... > > ...but I can see how people could make use of a CFR_ENTIRE_FILE that > would check that both file descriptors are actually regular files, and > if so copy the entire contents of the fd_in file into the same position > in the fd_out file, and then set the fd_out file's length to match. If > @off_in or @off_out are non-NULL then they'll be updated to the new EOFs > if the copy completes succesfully and @len can be anything. > IDGI. In what way would that be helpful? Would the syscall fail if it cannot copy entire file (like clone_file_range) or return bytes copied? If latter, then user will have to call syscall again until getting 0 return value. User can already call copy_file_range with len=SSIZE_MAX and get almost the same thing. Unless the idea is to optimize for less syscalls for copying very large files?? In that case, MAX_RW_COUNT limit for this syscall would need to be relaxed. While on the subject, something that has been discussed in the past is that copy_file_range() and sendfile() of a large file are not killable, so that is that should be fixed, especially if the interface is going to be used to copy more data in-kernel. IOW, the motivation of the patch is not clear to me: > This implementation saves a call to stat() in the common case What is the real life workload where this micro optimization would have any affect? > It does not fix any race conditions, but that is possible in the future > with this interface. Then please present a plan or an implementation of how that interface can solve race conditions and if that is the only motivation for the interface than I do not see why we should merge the interface before the implementation. Please let me know if I am missing something. Thanks, Amir.
