[RFC][PATCH 3/5] fs: add support for copy file range to another fs

[Amir Goldstein <amir73il@gmail.com>]

Implement copy to other fs in vfs_copy_file_range() -
vfs must call the operation on the source filesystem and the source
filesystem (e.g. overlayfs) will typically call vfs helpers to write
to the other fs, so avoid taking freeze protection and fsnotify write
hooks in vfs_copy_file_range().

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---
 fs/read_write.c | 43 ++++++++++++++++++++++++++++---------------
 1 file changed, 28 insertions(+), 15 deletions(-)

diff --git a/fs/read_write.c b/fs/read_write.c
index 78596d9c89119..846db21ac05ca 100644
--- a/fs/read_write.c
+++ b/fs/read_write.c
@@ -1478,6 +1478,7 @@ enum {
 	FS_COPY_SAME_FS = 2,
 	FS_COPY_CROSS_FS = 3,
 	FS_COPY_FROM_OTHER_FS = 4,
+	FS_COPY_TO_OTHER_FS = 5,
 };
 
 /*
@@ -1498,6 +1499,8 @@ static int copy_file_fs_cmp(struct file *f_in, struct file *f_out,
 		return FS_COPY_SAME_FS;
 	else if (f_out->f_op->fop_flags & FOP_CROSS_FS_COPY)
 		return FS_COPY_FROM_OTHER_FS;
+	else if (f_in->f_op->fop_flags & FOP_CROSS_FS_COPY)
+		return FS_COPY_TO_OTHER_FS;
 	else if (file_inode(f_in)->i_sb == file_inode(f_out)->i_sb)
 		return FS_COPY_SAME_SB;
 	else
@@ -1534,8 +1537,9 @@ static int generic_copy_file_checks(struct file *file_in, loff_t pos_in,
 	 */
 	if (fscmp == FS_COPY_SPLICE) {
 		/* cross sb splice is allowed */
-	} else if (fscmp == FS_COPY_FROM_OTHER_FS) {
-		/* Copy from other fs is allowed */
+	} else if (fscmp == FS_COPY_FROM_OTHER_FS ||
+		   fscmp == FS_COPY_TO_OTHER_FS) {
+		/* Copy from/to other fs is allowed */
 	} else if (file_out->f_op->copy_file_range) {
 		if (fscmp != FS_COPY_SAME_FS)
 			return -EXDEV;
@@ -1587,6 +1591,8 @@ ssize_t vfs_copy_file_range(struct file *file_in, loff_t pos_in,
 	ssize_t ret;
 	bool splice = flags & COPY_FILE_SPLICE;
 	int fscmp = copy_file_fs_cmp(file_in, file_out, flags);
+	const struct file_operations *fop =
+		(fscmp == FS_COPY_TO_OTHER_FS) ? file_in->f_op : file_out->f_op;
 
 	if (flags & ~COPY_FILE_SPLICE)
 		return -EINVAL;
@@ -1611,30 +1617,33 @@ ssize_t vfs_copy_file_range(struct file *file_in, loff_t pos_in,
 	 * Make sure return value doesn't overflow in 32bit compat mode.  Also
 	 * limit the size for all cases except when calling ->copy_file_range().
 	 */
-	if (splice || !file_out->f_op->copy_file_range || in_compat_syscall())
+	if (splice || !fop->copy_file_range || in_compat_syscall())
 		len = min_t(size_t, MAX_RW_COUNT, len);
 
-	file_start_write(file_out);
+	if (fscmp != FS_COPY_TO_OTHER_FS)
+		file_start_write(file_out);
 
 	/*
 	 * Cloning is supported by more file systems, so we implement copy on
 	 * same sb using clone, but for filesystems where both clone and copy
 	 * are supported (e.g. nfs,cifs), we only call the copy method.
+	 * Cross-fs copy (FROM_OTHER_FS / TO_OTHER_FS) is handled by whichever
+	 * side declared FOP_CROSS_FS_COPY.
 	 */
 	switch (fscmp) {
 	case FS_COPY_SPLICE:
 		break;
 	case FS_COPY_SAME_FS:
+	case FS_COPY_TO_OTHER_FS:
 	case FS_COPY_FROM_OTHER_FS:
-		ret = file_out->f_op->copy_file_range(file_in, pos_in,
-						      file_out, pos_out,
-						      len, flags);
+		ret = fop->copy_file_range(file_in, pos_in, file_out, pos_out,
+					   len, flags);
 		break;
 	case FS_COPY_SAME_SB:
-		if (file_in->f_op->remap_file_range)
-			ret = file_in->f_op->remap_file_range(file_in, pos_in,
-							file_out, pos_out, len,
-							REMAP_FILE_CAN_SHORTEN);
+		if (fop->remap_file_range)
+			ret = fop->remap_file_range(file_in, pos_in,
+						    file_out, pos_out, len,
+						    REMAP_FILE_CAN_SHORTEN);
 		/* Fallback to splice for same sb copy for backward compat */
 		if (ret <= 0)
 			splice = true;
@@ -1645,7 +1654,8 @@ ssize_t vfs_copy_file_range(struct file *file_in, loff_t pos_in,
 		break;
 	}
 
-	file_end_write(file_out);
+	if (fscmp != FS_COPY_TO_OTHER_FS)
+		file_end_write(file_out);
 
 	if (!splice)
 		goto done;
@@ -1676,13 +1686,16 @@ ssize_t vfs_copy_file_range(struct file *file_in, loff_t pos_in,
 			fsnotify_access(file_in);
 			add_rchar(current, ret);
 		}
-		fsnotify_modify(file_out);
-		add_wchar(current, ret);
+		if (fscmp != FS_COPY_TO_OTHER_FS) {
+			fsnotify_modify(file_out);
+			add_wchar(current, ret);
+		}
 	}
 
 	if (fscmp != FS_COPY_FROM_OTHER_FS)
 		inc_syscr(current);
-	inc_syscw(current);
+	if (fscmp != FS_COPY_TO_OTHER_FS)
+		inc_syscw(current);
 
 	return ret;
 }
-- 
2.54.0