Syzbot reported that page_symlink(), called by nilfs_symlink(),
triggers memory reclamation involving the filesystem layer, which can
result in circular lock dependencies among the reader/writer semaphore
nilfs->ns_segctor_sem, s_writers percpu_rwsem (intwrite) and the
fs_reclaim pseudo lock.

This is because after commit 21fc61c73c39 ("don't put symlink bodies in
pagecache into highmem"), the gfp flags of the page cache for symbolic
links are overwritten to GFP_KERNEL via inode_nohighmem().

This is not a problem for symlinks read from the backing device,
because the __GFP_FS flag is dropped after inode_nohighmem() is called.
However, when a new symlink is created with nilfs_symlink(), the gfp
flags remain overwritten to GFP_KERNEL.  Then, memory allocation called
from page_symlink() etc. triggers memory reclamation including the FS
layer, which may call nilfs_evict_inode() or nilfs_dirty_inode().  And
these can cause a deadlock if they are called while
nilfs->ns_segctor_sem is held:

Fix this issue by dropping the __GFP_FS flag from the page cache GFP
flags of newly created symlinks in the same way that nilfs_new_inode()
and __nilfs_read_inode() do, as a workaround until we adopt nofs
allocation scope consistently or improve the locking constraints.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Reported-by: syzbot+9ef37ac20608f4836256@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=9ef37ac20608f4836256
Fixes: 21fc61c73c39 ("don't put symlink bodies in pagecache into highmem")
Tested-by: syzbot+9ef37ac20608f4836256@syzkaller.appspotmail.com
Cc: stable@vger.kernel.org
---
Andrew, please apply this patch as a bug fix.

This fixes a potential deadlock issue recently reported by syzbot.

Thanks,
Ryusuke Konishi

 fs/nilfs2/namei.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/nilfs2/namei.c b/fs/nilfs2/namei.c
index 4905063790c5..9b108052d9f7 100644
--- a/fs/nilfs2/namei.c
+++ b/fs/nilfs2/namei.c
@@ -157,6 +157,9 @@ static int nilfs_symlink(struct mnt_idmap *idmap, struct inode *dir,
 	/* slow symlink */
 	inode->i_op = &nilfs_symlink_inode_operations;
 	inode_nohighmem(inode);
+	mapping_set_gfp_mask(inode->i_mapping,
+			     mapping_gfp_constraint(inode->i_mapping,
+						    ~__GFP_FS));
 	inode->i_mapping->a_ops = &nilfs_aops;
 	err = page_symlink(inode, symname, l);
 	if (err)
-- 
2.43.0