Ocfs2 devel - Oct 2018 - [PATCH 17/26] vfs: enable remap callers that can handle short operations

From: Darrick J. Wong <darrick.wong at oracle.com>

Plumb in a remap flag that enables the filesystem remap handler to
shorten remapping requests for callers that can handle it.  Now
copy_file_range can report partial success (in case we run up against
alignment problems, resource limits, etc.).

We also enable CAN_SHORTEN for fideduperange to maintain existing
userspace-visible behavior where xfs/btrfs shorten the dedupe range to
avoid stale post-eof data exposure.

Signed-off-by: Darrick J. Wong <darrick.wong at oracle.com>
Reviewed-by: Amir Goldstein <amir73il at gmail.com>
---
 fs/read_write.c    |   28 ++++++++++++++++++++--------
 include/linux/fs.h |    7 +++++--
 mm/filemap.c       |   16 ++++++++++++----
 3 files changed, 37 insertions(+), 14 deletions(-)


diff --git a/fs/read_write.c b/fs/read_write.c
index f6ab5beb935a..ee9314b7bfc3 100644
--- a/fs/read_write.c
+++ b/fs/read_write.c
@@ -1593,7 +1593,8 @@ ssize_t vfs_copy_file_range(struct file *file_in, loff_t
pos_in,
 
 		cloned = file_in->f_op->remap_file_range(file_in, pos_in,
 				file_out, pos_out,
-				min_t(loff_t, MAX_RW_COUNT, len), 0);
+				min_t(loff_t, MAX_RW_COUNT, len),
+				REMAP_FILE_CAN_SHORTEN);
 		if (cloned > 0) {
 			ret = cloned;
 			goto done;
@@ -1721,6 +1722,8 @@ static int remap_verify_area(struct file *file, loff_t
pos, loff_t len,
  * can't meaningfully compare post-EOF contents.
  *
  * For clone we only link a partial EOF block above the destination file's
EOF.
+ *
+ * Shorten the request if possible.
  */
 static int generic_remap_check_len(struct inode *inode_in,
 				   struct inode *inode_out,
@@ -1729,16 +1732,24 @@ static int generic_remap_check_len(struct inode
*inode_in,
 				   unsigned int remap_flags)
 {
 	u64 blkmask = i_blocksize(inode_in) - 1;
+	loff_t new_len = *len;
 
 	if ((*len & blkmask) == 0)
 		return 0;
 
-	if (remap_flags & REMAP_FILE_DEDUP)
-		*len &= ~blkmask;
-	else if (pos_out + *len < i_size_read(inode_out))
-		return -EINVAL;
+	if ((remap_flags & REMAP_FILE_DEDUP) ||
+	    pos_out + *len < i_size_read(inode_out))
+		new_len &= ~blkmask;
 
-	return 0;
+	if (new_len == *len)
+		return 0;
+
+	if (remap_flags & REMAP_FILE_CAN_SHORTEN) {
+		*len = new_len;
+		return 0;
+	}
+
+	return (remap_flags & REMAP_FILE_DEDUP) ? -EBADE : -EINVAL;
 }
 
 /* Update inode timestamps and remove security privileges when remapping. */
@@ -2023,7 +2034,8 @@ loff_t vfs_dedupe_file_range_one(struct file *src_file,
loff_t src_pos,
 {
 	loff_t ret;
 
-	WARN_ON_ONCE(remap_flags & ~(REMAP_FILE_DEDUP));
+	WARN_ON_ONCE(remap_flags & ~(REMAP_FILE_DEDUP |
+				     REMAP_FILE_CAN_SHORTEN));
 
 	ret = mnt_want_write_file(dst_file);
 	if (ret)
@@ -2124,7 +2136,7 @@ int vfs_dedupe_file_range(struct file *file, struct
file_dedupe_range *same)
 
 		deduped = vfs_dedupe_file_range_one(file, off, dst_file,
 						    info->dest_offset, len,
-						    0);
+						    REMAP_FILE_CAN_SHORTEN);
 		if (deduped == -EBADE)
 			info->status = FILE_DEDUPE_RANGE_DIFFERS;
 		else if (deduped < 0)
diff --git a/include/linux/fs.h b/include/linux/fs.h
index c0ae85a7bd9d..594fe4ba0b15 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1726,14 +1726,17 @@ struct block_device_operations;
  * If it is called with len == 0 that means "remap to end of source
file".
  *
  * REMAP_FILE_DEDUP: only remap if contents identical (i.e. deduplicate)
+ * REMAP_FILE_CAN_SHORTEN: caller can handle a shortened request
  */
 #define REMAP_FILE_DEDUP		(1 << 0)
+#define REMAP_FILE_CAN_SHORTEN		(1 << 1)
 
 /* All valid REMAP_FILE flags */
-#define REMAP_FILE_VALID_FLAGS		(REMAP_FILE_DEDUP)
+#define REMAP_FILE_VALID_FLAGS		(REMAP_FILE_DEDUP | \
+					 REMAP_FILE_CAN_SHORTEN)
 
 /* REMAP_FILE flags taken care of by the vfs. */
-#define REMAP_FILE_ADVISORY		(0)
+#define REMAP_FILE_ADVISORY		(REMAP_FILE_CAN_SHORTEN)
 
 struct iov_iter;
 
diff --git a/mm/filemap.c b/mm/filemap.c
index 1e93269efafe..898eb358f7d2 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -3051,8 +3051,12 @@ int generic_remap_checks(struct file *file_in, loff_t
pos_in,
 	if (pos_in + count == size_in) {
 		bcount = ALIGN(size_in, bs) - pos_in;
 	} else {
-		if (!IS_ALIGNED(count, bs))
-			return -EINVAL;
+		if (!IS_ALIGNED(count, bs)) {
+			if (remap_flags & REMAP_FILE_CAN_SHORTEN)
+				count = ALIGN_DOWN(count, bs);
+			else
+				return -EINVAL;
+		}
 
 		bcount = count;
 	}
@@ -3063,10 +3067,14 @@ int generic_remap_checks(struct file *file_in, loff_t
pos_in,
 	    pos_out < pos_in + bcount)
 		return -EINVAL;
 
-	/* For now we don't support changing the length. */
-	if (*req_count != count)
+	/*
+	 * We shortened the request but the caller can't deal with that, so
+	 * bounce the request back to userspace.
+	 */
+	if (*req_count != count && !(remap_flags &
REMAP_FILE_CAN_SHORTEN))
 		return -EINVAL;
 
+	*req_count = count;
 	return 0;
 }

>  /* Update inode timestamps and remove security privileges when remapping.
*/
> @@ -2023,7 +2034,8 @@ loff_t vfs_dedupe_file_range_one(struct file
*src_file, loff_t src_pos,
>  {
>  	loff_t ret;
>  
> -	WARN_ON_ONCE(remap_flags & ~(REMAP_FILE_DEDUP));
> +	WARN_ON_ONCE(remap_flags & ~(REMAP_FILE_DEDUP |
> +				     REMAP_FILE_CAN_SHORTEN));
I guess this is where you could actually use REMAP_FILE_VALID_FLAGS..
>  /* REMAP_FILE flags taken care of by the vfs. */
> -#define REMAP_FILE_ADVISORY		(0)
> +#define REMAP_FILE_ADVISORY		(REMAP_FILE_CAN_SHORTEN)
And btw, they are not 'taken care of by the VFS', they need to be
taken care of by the fs (possibly using helpers) to take affect,
but they can be safely ignored.
> +		if (!IS_ALIGNED(count, bs)) {
> +			if (remap_flags & REMAP_FILE_CAN_SHORTEN)
> +				count = ALIGN_DOWN(count, bs);
> +			else
> +				return -EINVAL;
			if (!(remap_flags & REMAP_FILE_CAN_SHORTEN))
				return -EINVAL;
			count = ALIGN_DOWN(count, bs);