linux-kernel - Re: [PATCH] ovl: only WARN_ON_ONCE() if dentry is NULL in ovl_encode

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Date:   Thu, 28 Jul 2022 10:57:25 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Jiachen Zhang <zhangjiachen.jaycee@...edance.com>
Cc:     overlayfs <linux-unionfs@...r.kernel.org>,
        linux-kernel@...r.kernel.org, Hongbo Yin <yinhongbo@...edance.com>,
        Tianci Zhang <zhangtianci.1997@...edance.com>
Subject: Re: [PATCH] ovl: only WARN_ON_ONCE() if dentry is NULL in ovl_encode_fh()

On Thu, 28 Jul 2022 at 08:39, Jiachen Zhang
<zhangjiachen.jaycee@...edance.com> wrote:
>
> 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.

Better just drop the WARN_ON() completely in that case, since it's a
normally occurring condition.

Thanks,
Miklos