[BUG]
With current subpage RW support, the following script can hang the fs on
with 64K page size.

 # mkfs.btrfs -f -s 4k $dev
 # mount $dev -o nospace_cache $mnt
 # fsstress -w -n 50 -p 1 -s 1607749395 -d $mnt

The kernel will do an infinite loop in btrfs_punch_hole_lock_range().

[CAUSE]
In btrfs_punch_hole_lock_range() we:
- Truncate page cache range
- Lock extent io tree
- Wait any ordered extents in the range.

We exit the loop until we meet all the following conditions:
- No ordered extent in the lock range
- No page is in the lock range

The latter condition has a pitfall, it only works for sector size ==
PAGE_SIZE case.

While can't handle the following subpage case:

  0       32K     64K     96K     128K
  |       |///////||//////|       ||

lockstart=32K
lockend=96K - 1

In this case, although the range cross 2 pages,
truncate_pagecache_range() will invalidate no page at all, but only zero
the [32K, 96K) range of the two pages.

Thus filemap_range_has_page(32K, 96K-1) will always return true, thus we
will never meet the loop exit condition.

[FIX]
Fix the problem by doing page alignment for the lock range.

Function filemap_range_has_page() has already handled lend < lstart
case, we only need to round up @lockstart, and round_down @lockend for
truncate_pagecache_range().

This modification should not change any thing for sector size ==
PAGE_SIZE case, as in that case our range is already page aligned.

Signed-off-by: Qu Wenruo <redacted>
---
 fs/btrfs/file.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 4a89697ae3a7..6ef44afa939c 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c

@@ -2486,6 +2486,16 @@ static int btrfs_punch_hole_lock_range(struct inode *inode,
 				       const u64 lockend,
 				       struct extent_state **cached_state)
 {
+	/*
+	 * For subpage case, if the range is not at page boundary, we could
+	 * have pages at the leading/tailing part of the range.
+	 * This could lead to dead loop since filemap_range_has_page()
+	 * will always return true.
+	 * So here we need to do extra page alignment for
+	 * filemap_range_has_page().
+	 */
+	u64 page_lockstart = round_up(lockstart, PAGE_SIZE);
+	u64 page_lockend = round_down(lockend + 1, PAGE_SIZE) - 1;
 	while (1) {
 		struct btrfs_ordered_extent *ordered;
 		int ret;

@@ -2506,7 +2516,7 @@ static int btrfs_punch_hole_lock_range(struct inode *inode,
 		    (ordered->file_offset + ordered->num_bytes <= lockstart ||
 		     ordered->file_offset > lockend)) &&
 		     !filemap_range_has_page(inode->i_mapping,
-					     lockstart, lockend)) {
+					     page_lockstart, page_lockend)) {
 			if (ordered)
 				btrfs_put_ordered_extent(ordered);
 			break;

-- 
2.31.1