[PATCH] iomap: Fix the write_count in iomap_add_to_ioend().

[Anju T Sudhakar]

From: Ritesh Harjani <riteshh@xxxxxxxxxxxxx>

__bio_try_merge_page() may return same_page = 1 and merged = 0. 
This could happen when bio->bi_iter.bi_size + len > UINT_MAX. 
Handle this case in iomap_add_to_ioend() by incrementing write_count.
This scenario mostly happens where we have too much dirty data accumulated. 

w/o the patch we hit below kernel warning,
 
 WARNING: CPU: 18 PID: 5130 at fs/iomap/buffered-io.c:74 iomap_page_release+0x120/0x150
 CPU: 18 PID: 5130 Comm: fio Kdump: loaded Tainted: G        W         5.8.0-rc3 #6
 Call Trace:
  __remove_mapping+0x154/0x320 (unreliable)
  iomap_releasepage+0x80/0x180
  try_to_release_page+0x94/0xe0
  invalidate_inode_page+0xc8/0x110
  invalidate_mapping_pages+0x1dc/0x540
  generic_fadvise+0x3c8/0x450
  xfs_file_fadvise+0x2c/0xe0 [xfs]
  vfs_fadvise+0x3c/0x60
  ksys_fadvise64_64+0x68/0xe0
  sys_fadvise64+0x28/0x40
  system_call_exception+0xf8/0x1c0
  system_call_common+0xf0/0x278

Reported-by: Shivaprasad G Bhat <sbhat@xxxxxxxxxxxxx>
Signed-off-by: Ritesh Harjani <riteshh@xxxxxxxxxxxxx>
Signed-off-by: Anju T Sudhakar <anju@xxxxxxxxxxxxxxxxxx>
---
 fs/iomap/buffered-io.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index bcfc288dba3f..4e8062279e66 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -1332,10 +1332,12 @@ iomap_add_to_ioend(struct inode *inode, loff_t offset, struct page *page,
 
 	merged = __bio_try_merge_page(wpc->ioend->io_bio, page, len, poff,
 			&same_page);
-	if (iop && !same_page)
+	if (iop && merged && !same_page)
 		atomic_inc(&iop->write_count);
 
 	if (!merged) {
+		if (iop)
+			atomic_inc(&iop->write_count);
 		if (bio_full(wpc->ioend->io_bio, len)) {
 			wpc->ioend->io_bio =
 				iomap_chain_bio(wpc->ioend->io_bio);
-- 
2.25.4