Re: [PATCH v5 8/9] vfs: Add vfs_copy_file_range() support for pagecache copies

On 10/09/2015 07:15 AM, Pádraig Brady wrote:

On 08/10/15 02:40, Neil Brown wrote:

quoted

Anna Schumaker [off-list ref] writes:

quoted

@@ -1338,34 +1362,26 @@ ssize_t vfs_copy_file_range(struct file *file_in, loff_t pos_in,
 			    struct file *file_out, loff_t pos_out,
 			    size_t len, unsigned int flags)
 {
-	struct inode *inode_in;
-	struct inode *inode_out;
 	ssize_t ret;
 
-	if (flags)
+	/* Flags should only be used exclusively. */
+	if ((flags & COPY_FR_COPY) && (flags & ~COPY_FR_COPY))
+		return -EINVAL;
+	if ((flags & COPY_FR_REFLINK) && (flags & ~COPY_FR_REFLINK))
+		return -EINVAL;
+	if ((flags & COPY_FR_DEDUP) && (flags & ~COPY_FR_DEDUP))
 		return -EINVAL;
 

Do you also need:

   if (flags & ~(COPY_FR_COPY | COPY_FR_REFLINK | COPY_FR_DEDUP))
   	return -EINVAL;

so that future user-space can test if the kernel supports new flags?

Seems like a good idea, yes.

Also that got me thinking about COPY_FR_SPARSE.
What's the current behavior when copying a sparse range?
Is the hole propagated by default (good), or is it expanded?

I haven't tried it, but I think the hole would be expanded :(.  I'm having splice() handle the pagecache copy part, and (as far as I know) splice() doesn't know anything about sparse files.  I might be able to put in some kind of fallocate() / splice() loop to copy the range in multiple pieces.

I don't want to add COPY_FR_SPARSE_AUTO, because then the kernel will have to determine how best to interpret "auto".  I'm more inclined to add a single COPY_FR_SPARSE flag to enable creating sparse files, and then have the application tell us what to do for any given range.

Anna

Note cp(1) has --sparse={never,auto,always}. Auto is the default,
so it would be good I think if that was the default mode for copy_file_range().
With other sparse modes, we'd have to avoid copy_file_range() unless
there was control possible with COPY_FR_SPARSE_{AUTO,NONE,ALWAYS}.
Note currently cp --sparse=always will detect runs of zeros and also
avoid speculative preallocation by using fallocate (fd, FALLOC_FL_PUNCH_HOLE, ...)

thanks,
Pádraig.