Re: [PATCH v5] NFSD: Add support for the COPY operation

["J. Bruce Fields" <bfields@fieldses.org>]

On Fri, Sep 09, 2016 at 03:53:36PM -0400, J. Bruce Fields wrote:
> On Wed, Sep 07, 2016 at 03:57:29PM -0400, Anna Schumaker wrote:
> > This patch adds server support for the NFS v4.2 COPY operation.  I did some
> > performance testing on my own, and found that a 4MB copy cap has performance
> > pretty close to copying with no cap at all.  Here are my results for testing
> > with various file sizes and copy caps:
> 
> Thanks, but we expect the tradeoffs here to vary a lot depending on the
> latency of the client<->server connection and the server<->server copy
> bandwidth.  So this isn't really interesting without knowing your
> testing setup.

... but I guess I'm inclined to try this anyway.  It's simple.  We can
do something more complicated later if this turns out to be a problem.

Maybe it'd help to leave a little more documentation here.

Applying for 4.8 with that change.

--b.

diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
index 271a23cf6470..8ca642fe9b21 100644
--- a/fs/nfsd/vfs.c
+++ b/fs/nfsd/vfs.c
@@ -516,7 +516,15 @@ __be32 nfsd4_clone_file_range(struct file *src, u64 src_pos, struct file *dst,
 ssize_t nfsd_copy_file_range(struct file *src, u64 src_pos, struct file *dst,
 			     u64 dst_pos, u64 count)
 {
-	/* Arbitrary 4 megabyte copy cap */
+
+	/*
+	 * Limit copy to 4MB to prevent indefinitely blocking an nfsd
+	 * thread and client rpc slot.  The choice of 4MB is somewhat
+	 * arbitrary.  We might instead base this on r/wsize, or make it
+	 * tunable, or use a time instead of a byte limit, or implement
+	 * asynchronous copy.  In theory a client could also recognize a
+	 * limit like this and pipeline multiple COPY requests.
+	 */
 	count = min_t(u64, count, 1 << 22);
 	return vfs_copy_file_range(src, src_pos, dst, dst_pos, count, 0);
 }