================================================
WARNING: lock held when returning to user space!
4.18.5 #1 Not tainted
------------------------------------------------
worker/6123 is leaving the kernel with locks still held!
1 lock held by worker/6123:
 #0: 000000009eab4f1b (sb_internal#2){.+.+}, at: xfs_trans_alloc+0x17c/0x220

And eventually the filesystem deadlocks because it runs out of log
space that is reserved by the leaked transaction and never gets
released.

The logic flow in xfs_reflink_allocate_cow() is a convoluted mess of
gotos - it's no surprise that it has bug where the flow through
several goto jumps then fails to clean up context from a non-obvious
logic path. CLean up the logic flow and make sure every path does
the right thing.

Reported-by: Alexander Y. Fomichev <git.user@gmail.com>
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=200981
Signed-off-by: Dave Chinner <dchinner@redhat.com>
---
 fs/xfs/xfs_reflink.c | 98 +++++++++++++++++++++++++++++---------------
 1 file changed, 66 insertions(+), 32 deletions(-)

diff --git a/fs/xfs/xfs_reflink.c b/fs/xfs/xfs_reflink.c
index 38f405415b88..b39f5afa57aa 100644
--- a/fs/xfs/xfs_reflink.c
+++ b/fs/xfs/xfs_reflink.c
@@ -352,6 +352,47 @@ xfs_reflink_convert_cow(
 	return error;
 }
 
+/*
+ * Find the extent that maps the given range in the COW fork. Even if the extent
+ * is not shared we might have a preallocation for it in the COW fork. If so we
+ * use it that rather than trigger a new allocation.
+ */
+static int
+find_trim_cow_extent(
+	struct xfs_inode	*ip,
+	struct xfs_bmbt_irec	*imap,
+	bool			*shared,
+	bool			*found)
+{
+	xfs_fileoff_t		offset_fsb = imap->br_startoff;
+	xfs_filblks_t		count_fsb = imap->br_blockcount;
+	struct xfs_iext_cursor	icur;
+	struct xfs_bmbt_irec	got;
+	bool			trimmed;
+
+	*found = false;
+
+	/*
+	 * if we don't find an overlapping extent, trim the range we need to
+	 * allocate to fit the hole we found.
+	 */
+	if (!xfs_iext_lookup_extent(ip, ip->i_cowfp, offset_fsb, &icur, &got) ||
+	    got.br_startoff > offset_fsb)
+		return xfs_reflink_trim_around_shared(ip, imap, shared, &trimmed);
+
+	*shared = true;
+	if (isnullstartblock(got.br_startblock)) {
+		xfs_trim_extent(imap, got.br_startoff, got.br_blockcount);
+		return 0;
+	}
+
+	/* real extent found - no need to allocate */
+	xfs_trim_extent(&got, offset_fsb, count_fsb);
+	*imap = got;
+	*found = true;
+	return 0;
+}
+
 /* Allocate all CoW reservations covering a range of blocks in a file. */
 int
 xfs_reflink_allocate_cow(
@@ -363,41 +404,37 @@ xfs_reflink_allocate_cow(
 	struct xfs_mount	*mp = ip->i_mount;
 	xfs_fileoff_t		offset_fsb = imap->br_startoff;
 	xfs_filblks_t		count_fsb = imap->br_blockcount;
-	struct xfs_bmbt_irec	got;
 	struct xfs_trans	*tp = NULL;
 	int			nimaps, error = 0;
-	bool			trimmed;
+	bool			found;
 	xfs_filblks_t		resaligned;
 	xfs_extlen_t		resblks = 0;
-	struct xfs_iext_cursor	icur;
 
-retry:
-	ASSERT(xfs_is_reflink_inode(ip));
 	ASSERT(xfs_isilocked(ip, XFS_ILOCK_EXCL));
 
 	/*
-	 * Even if the extent is not shared we might have a preallocation for
-	 * it in the COW fork.  If so use it.
+	 * do-while to run a single lookup retry after transaction allocation.
+	 * All exits from the loop need to check if the transaction needs
+	 * cancelling. Dropping the ILOCK to allocate the transaction allows
+	 * races with other COW fork allocations, so we need to start the extent
+	 * lookup from scratch once we have the ILOCK again.
 	 */
-	if (xfs_iext_lookup_extent(ip, ip->i_cowfp, offset_fsb, &icur, &got) &&
-	    got.br_startoff <= offset_fsb) {
-		*shared = true;
+	do {
+		ASSERT(xfs_is_reflink_inode(ip));
 
-		/* If we have a real allocation in the COW fork we're done. */
-		if (!isnullstartblock(got.br_startblock)) {
-			xfs_trim_extent(&got, offset_fsb, count_fsb);
-			*imap = got;
+		error = find_trim_cow_extent(ip, imap, shared, &found);
+		if (error || !*shared)
+			goto out_trans_cancel;
+		if (found) {
+			if (tp)
+				xfs_trans_cancel(tp);
 			goto convert;
 		}
 
-		xfs_trim_extent(imap, got.br_startoff, got.br_blockcount);
-	} else {
-		error = xfs_reflink_trim_around_shared(ip, imap, shared, &trimmed);
-		if (error || !*shared)
-			goto out;
-	}
+		/* if we have a transaction, it's time to allocate */
+		if (tp)
+			break;
 
-	if (!tp) {
 		resaligned = xfs_aligned_fsb_count(imap->br_startoff,
 			imap->br_blockcount, xfs_get_cowextsz_hint(ip));
 		resblks = XFS_DIOSTRAT_SPACE_RES(mp, resaligned);
@@ -412,29 +449,25 @@ xfs_reflink_allocate_cow(
 
 		error = xfs_qm_dqattach_locked(ip, false);
 		if (error)
-			goto out;
-		goto retry;
-	}
+			goto out_trans_cancel;
+	} while (tp);
 
 	error = xfs_trans_reserve_quota_nblks(tp, ip, resblks, 0,
 			XFS_QMOPT_RES_REGBLKS);
 	if (error)
-		goto out;
+		goto out_trans_cancel;
 
 	xfs_trans_ijoin(tp, ip, 0);
 
-	nimaps = 1;
-
 	/* Allocate the entire reservation as unwritten blocks. */
+	nimaps = 1;
 	error = xfs_bmapi_write(tp, ip, imap->br_startoff, imap->br_blockcount,
 			XFS_BMAPI_COWFORK | XFS_BMAPI_PREALLOC,
 			resblks, imap, &nimaps);
 	if (error)
-		goto out_trans_cancel;
+		goto out_unreserve;
 
 	xfs_inode_set_cowblocks_tag(ip);
-
-	/* Finish up. */
 	error = xfs_trans_commit(tp);
 	if (error)
 		return error;
@@ -447,10 +480,11 @@ xfs_reflink_allocate_cow(
 		return -ENOSPC;
 convert:
 	return xfs_reflink_convert_cow_extent(ip, imap, offset_fsb, count_fsb);
-out_trans_cancel:
+
+out_unreserve:
 	xfs_trans_unreserve_quota_nblks(tp, ip, (long)resblks, 0,
 			XFS_QMOPT_RES_REGBLKS);
-out:
+out_trans_cancel:
 	if (tp)
 		xfs_trans_cancel(tp);
 	return error;
