[PATCH] ovl: only WARN_ON_ONCE() if dentry is NULL in ovl_encode_fh()

[Jiachen Zhang]

Some code paths cannot guarantee the inode have any dentry alias. So
WARN_ON() all !dentry may flood the kernel logs.

For example, when an overlayfs inode is watched by inotifywait (1), and
someone is trying to read the /proc/$(pidof inotifywait)/fdinfo/INOTIFY_FD,
at that time if the dentry has been reclaimed by kernel (such as
echo 2 > /proc/sys/vm/drop_caches), there will be a WARN_ON(). The
printed call stack would be like:

    ? show_mark_fhandle+0xf0/0xf0
    show_mark_fhandle+0x4a/0xf0
    ? show_mark_fhandle+0xf0/0xf0
    ? seq_vprintf+0x30/0x50
    ? seq_printf+0x53/0x70
    ? show_mark_fhandle+0xf0/0xf0
    inotify_fdinfo+0x70/0x90
    show_fdinfo.isra.4+0x53/0x70
    seq_show+0x130/0x170
    seq_read+0x153/0x440
    vfs_read+0x94/0x150
    ksys_read+0x5f/0xe0
    do_syscall_64+0x59/0x1e0
    entry_SYSCALL_64_after_hwframe+0x44/0xa9

So let's replace WARN_ON() with WARN_ON_ONCE() to avoid kernel log
flooding.

Reported-by: Hongbo Yin <yinhongbo@xxxxxxxxxxxxx>
Signed-off-by: Jiachen Zhang <zhangjiachen.jaycee@xxxxxxxxxxxxx>
Signed-off-by: Tianci Zhang <zhangtianci.1997@xxxxxxxxxxxxx>
---
 fs/overlayfs/export.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/overlayfs/export.c b/fs/overlayfs/export.c
index 2eada97bbd23..3a8650442aec 100644
--- a/fs/overlayfs/export.c
+++ b/fs/overlayfs/export.c
@@ -259,7 +259,7 @@ static int ovl_encode_fh(struct inode *inode, u32 *fid, int *max_len,
 		return FILEID_INVALID;
 
 	dentry = d_find_any_alias(inode);
-	if (WARN_ON(!dentry))
+	if (WARN_ON_ONCE(!dentry))
 		return FILEID_INVALID;
 
 	bytes = ovl_dentry_to_fid(ofs, dentry, fid, buflen);
-- 
2.20.1