On Wed, Oct 14, 2015 at 01:59:40PM -0400, Anna Schumaker wrote: > On 10/12/2015 07:17 PM, Darrick J. Wong wrote: > > On Sun, Oct 11, 2015 at 07:22:03AM -0700, Christoph Hellwig wrote: > >> On Wed, Sep 30, 2015 at 01:26:52PM -0400, Anna Schumaker wrote: > >>> This allows us to have an in-kernel copy mechanism that avoids frequent > >>> switches between kernel and user space. This is especially useful so > >>> NFSD can support server-side copies. > >>> > >>> I make pagecache copies configurable by adding three new (exclusive) > >>> flags: > >>> - COPY_FR_REFLINK tells vfs_copy_file_range() to only create a reflink. > >>> - COPY_FR_COPY does a full data copy, but may be filesystem accelerated. > >>> - COPY_FR_DEDUP creates a reflink, but only if the contents of both > >>> ranges are identical. > >> > >> All but FR_COPY really should be a separate system call. Clones (an > >> dedup as a special case of clones) are really a separate beast from file > >> copies. > >> > >> If I want to clone a file I either want it clone fully or fail, not copy > >> a certain amount. That means that a) we need to return an error not > >> short "write", and b) locking impementations are important - we need to > >> prevent other applications from racing with our clone even if it is > >> large, while to get these semantics for the possible short returning > >> file copy will require a proper userland locking protocol. Last but not > >> least file copies need to be interruptible while clones should be not. > >> All this is already important for local file systems and even more > >> important for NFS exporting. > >> > >> So I'd suggest to drop this patch and just let your syscall handle > >> actualy copies with all their horrors. We can go with Peng's patches > >> to generalize the btrfs ioctls for clones for now which is what everyone > >> already uses anyway, and then add a separate sys_file_clone later. > > So what I'm hearing is that I should drop the reflink and dedup flags and > change this system call only perform a full copy (with preserving of > sparseness), correct? I can make those changes, but only if everybody is in > agreement that it's the best way forward. Sounds fine to me; I'll work on promoting EXTENT_SAME to the VFS. > The only reason I haven't done anything to make this system call > interruptible is because I haven't been able to find any documentation or > examples for making system calls interruptible. How do I do this? I thought it was mostly a matter of sprinkling in "if (signal_pending(...)) return -ERESTARTSYS" type things whenever it's convenient to check. The splice code already seems to have this, though I'm no expert on what the splice code actually does. :) --D > > Anna > > > > > Hm. Peng's patches only generalize the CLONE and CLONE_RANGE ioctls from > > btrfs, however they don't port over the (vastly different) EXTENT_SAME ioctl. > > > > What does everyone think about generalizing EXTENT_SAME? The interface enables > > one to ask the kernel to dedupe multiple file ranges in a single call. That's > > more complex than what I was proposing with COPY_FR_DEDUP(E), but I'm assuming > > that the extra complexity buys us the ability to ... multi-dedupe at the same > > time, with locks held on the source file? > > > > I'm happy to generalize the existing EXTENT_SAME, but please yell if you really > > hate the interface. > > > > --D > > > >> -- > >> To unsubscribe from this list: send the line "unsubscribe linux-api" in > >> the body of a message to majordomo@xxxxxxxxxxxxxxx > >> More majordomo info at http://vger.kernel.org/majordomo-info.html > -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
