Re: [PATCH 1/6] fs: Add flag to file_system_type to indicate content is generated

Greg KH [off-list ref] writes:

On Fri, Feb 12, 2021 at 12:41:48PM +0000, Luis Henriques wrote:

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Greg KH [off-list ref] writes:

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Our option now are:
- Restore the cross-fs restriction into generic_copy_file_range()

Yes.

Restoring this restriction will actually change the current cephfs CFR
behaviour.  Since that commit we have allowed doing remote copies between
different filesystems within the same ceph cluster.  See commit
6fd4e6348352 ("ceph: allow object copies across different filesystems in
the same cluster").

Although I'm not aware of any current users for this scenario, the
performance impact can actually be huge as it's the difference between
asking the OSDs for copying a file and doing a full read+write on the
client side.

Regression in performance is ok if it fixes a regression for things that
used to work just fine in the past :)

First rule, make it work.

Sure, I just wanted to point out that *maybe* there are other options than
simply reverting that commit :-)

Something like the patch below (completely untested!) should revert to the
old behaviour in filesystems that don't implement the CFR syscall.

Cheers,
-- 
Luis

diff --git a/fs/read_write.c b/fs/read_write.c
index 75f764b43418..bf5dccc43cc9 100644
--- a/fs/read_write.c
+++ b/fs/read_write.c

@@ -1406,8 +1406,11 @@ static ssize_t do_copy_file_range(struct file *file_in, loff_t pos_in,
 						       file_out, pos_out,
 						       len, flags);
 
-	return generic_copy_file_range(file_in, pos_in, file_out, pos_out, len,
-				       flags);
+	if (file_inode(file_in)->i_sb != file_inode(file_out)->i_sb)
+		return -EXDEV;
+	else
+		generic_copy_file_range(file_in, pos_in, file_out, pos_out, len,
+					flags);
 }
 
 /*

That would make much more sense to me.

Great.  I can send a proper patch with changelog, if this is the really
what we want.  But I would rather hear from others first.  I guess that at
least the NFS devs have something to say here.

Cheers,
-- 
Luis