Syzbot reported a kernel BUG triggered within nilfs_copy_dirty_pages(),
which copies dirty DAT file folios/pages to its shadow page cache.  The
BUG occurs when a retrieved dirty folio/page unexpectedly loses its
'dirty' status.

This issue arises because, since the commit referenced below, the 'dirty'
flag of a folio/page can be cleared asynchronously after the filesystem
detects metadata corruption and transitions to read-only mode.

Resolve the issue by returning an -EROFS error if the filesystem has
transitioned to read-only mode.  Also change the behavior to issue a
kernel warning only once instead of triggering a kernel BUG when this
unexpected 'dirty' state is detected while the filesystem is not in
read-only mode.

Reported-by: syzbot+8baf9a79a3ffc6271cb6@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=8baf9a79a3ffc6271cb6
Fixes: 8c26c4e2694a ("nilfs2: fix issue with flush kernel thread after remount in RO mode because of driver's internal error or metadata corruption")
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
---
Viacheslav, please apply this for the next cycle.

This fixes an issue reported by syzbot where a kernel BUG could be
triggered depending on timing after filesystem corruption is detected.

Thanks,
Ryusuke Konishi

 fs/nilfs2/page.c | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/fs/nilfs2/page.c b/fs/nilfs2/page.c
index a9d8aa65416f..1d00bce21c37 100644
--- a/fs/nilfs2/page.c
+++ b/fs/nilfs2/page.c
@@ -243,6 +243,7 @@ static void nilfs_copy_folio(struct folio *dst, struct folio *src,
 int nilfs_copy_dirty_pages(struct address_space *dmap,
 			   struct address_space *smap)
 {
+	struct inode *smap_inode = smap->host;
 	struct folio_batch fbatch;
 	unsigned int i;
 	pgoff_t index = 0;
@@ -258,8 +259,19 @@ int nilfs_copy_dirty_pages(struct address_space *dmap,
 		struct folio *folio = fbatch.folios[i], *dfolio;
 
 		folio_lock(folio);
-		if (unlikely(!folio_test_dirty(folio)))
-			NILFS_FOLIO_BUG(folio, "inconsistent dirty state");
+		if (unlikely(!folio_test_dirty(folio))) {
+			if (WARN_ONCE(!sb_rdonly(smap_inode->i_sb),
+					"inconsistent dirty state\n"))
+				goto unlock_folio;
+
+			/*
+			 * If the filesystem has been forced to read-only
+			 * due to metadata corruption.
+			 */
+			folio_unlock(folio);
+			err = -EROFS;
+			break;
+		}
 
 		dfolio = filemap_grab_folio(dmap, folio->index);
 		if (IS_ERR(dfolio)) {
@@ -277,6 +289,7 @@ int nilfs_copy_dirty_pages(struct address_space *dmap,
 
 		folio_unlock(dfolio);
 		folio_put(dfolio);
+unlock_folio:
 		folio_unlock(folio);
 	}
 	folio_batch_release(&fbatch);
-- 
2.43.0