[PATCH] iomap: Ensure iop->uptodate matches PageUptodate

["Matthew Wilcox (Oracle)" <willy@infradead.org>]

If the filesystem has block size < page size and we end up calling
iomap_page_create() in iomap_page_mkwrite_actor(), the uptodate bits
would be zero, which causes us to skip writeback of blocks which are
!uptodate in iomap_writepage_map().  This can lead to user data loss.

Found using generic/127 with the THP patches.  I don't think this can be
reproduced on mainline using that test (the THP code causes iomap_pages
to be discarded more frequently), but inspection shows it can happen
with an appropriate series of operations.

Fixes: 9dc55f1389f9 ("iomap: add support for sub-pagesize buffered I/O without buffer heads")
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 fs/iomap/buffered-io.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index a2b3b5455219..f0c5027bf33f 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -53,7 +53,10 @@ iomap_page_create(struct inode *inode, struct page *page)
 	atomic_set(&iop->read_count, 0);
 	atomic_set(&iop->write_count, 0);
 	spin_lock_init(&iop->uptodate_lock);
-	bitmap_zero(iop->uptodate, PAGE_SIZE / SECTOR_SIZE);
+	if (PageUptodate(page))
+		bitmap_fill(iop->uptodate, PAGE_SIZE / SECTOR_SIZE);
+	else
+		bitmap_zero(iop->uptodate, PAGE_SIZE / SECTOR_SIZE);
 
 	/*
 	 * migrate_page_move_mapping() assumes that pages with private data have
@@ -72,6 +75,8 @@ iomap_page_release(struct page *page)
 		return;
 	WARN_ON_ONCE(atomic_read(&iop->read_count));
 	WARN_ON_ONCE(atomic_read(&iop->write_count));
+	WARN_ON_ONCE(bitmap_full(iop->uptodate, PAGE_SIZE / SECTOR_SIZE) !=
+			PageUptodate(page));
 	kfree(iop);
 }
 
-- 
2.27.0