On Mon, Sep 12, 2016 at 1:11 AM, Dave Chinner <david@xxxxxxxxxxxxx> wrote: > On Sat, Sep 10, 2016 at 09:54:59PM +0300, Amir Goldstein wrote: >> On Sat, Sep 10, 2016 at 2:52 AM, Dave Chinner <david@xxxxxxxxxxxxx> wrote: >> >> > You can test whether this is supported at mount time, so you do a >> > simply flag test at copyup to determine if a clone should be >> > attempted or not. >> > >> >> I am not sure that would not be over optimization. >> I am already checking for the obvious reason for clone to fail in copyup - >> different i_sb. > > Again, please don't do that. Call vfs_clone_file_range() as it > checks a whole lot more stuff that can cause a clone to fail. And it > makes sure that the write references to the mnt are taken so that > things like freeze and remount-ro behave correctly while a clone is > in progress. > OK >> After all, if I just call clone_file_range() once and it fails, then we are back >> to copying and that is going to make the cost of calling clone insignificant. > > Apart from the fact that the ->clone_file_range() calls assume that > all the validity checks have already been done by the caller, which > you are not doing. > >> > If cloning fails or is not supported, then try vfs_copy_file_range() >> > to do an optimised iterative partial range file copy. Finally, try >> > a slow, iterative partial range file copies using >> > do_splice_direct(). This part can be wholly handled by >> > vfs_copy_file_range() - this 'not supported' fallback doesn't need >> > to be implemented every time someone wants to copy data between two >> > files... >> >> You do realize that vfs_copy_file_range() itself does the 'not >> supported' fallback >> and if I do call it iteratively, then there will be a lot more 'not >> supported' attempts >> then there are in my current patch. > > No shit, Sherlock. But you're concentrating on the wrong thing - > the overhead of checking if .clone_file_range/.copy_file_range is > implemented and can be executed is effectively zero compared to > copying any amount of data. > > IOWs, Amir, you're trying to *optimise the wrong thing*. It's the > data copy that is costly and needs to be optimised, not the > iteration or the checks done to determine what type of clone/copy > can be executed. Shortcuts around generic helpers like you are > proposing are more costly in the long run because code like this is > much more likely to contain/mask bugs that only appear months or > years later when something else is changed. Case in point: the mnt > write references that need to be taken before calling > clone/copy_file_range().... > > Please, just use vfs_clone_file_range() and vfs_copy_file_range() > and only fall back to a slower method if the error returned is > -EOPNOTSUPP. For any other error, the copy should fail, not be > ignored. > Obviously, you meant to check for -EOPNOTSUPP or -EXDEV >> But regardless, as I wrote to Christoph, changing the >> vfs_copy_file_range() helper >> and changing users of do_splice to use it like you suggested sounds >> like it may be the right thing to do, but without consensus, I am a bit hesitant >> to make those changes. I am definitely willing to draft the patch and test it >> if I get more ACKs on the idea. > > Send a patch - that's the only way you'll get anyone to comment > on it. > Will do. Thanks, Amir. -- To unsubscribe from this list: send the line "unsubscribe linux-unionfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
