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From:   Jiachen Zhang <zhangjiachen.jaycee@bytedance.com>
Date:   Thu, 28 Jul 2022 19:46:00 +0800
Message-ID: <CAFQAk7jW9ZpSURW4+h9SZNNtqXqq65LxsXrtheaijk4HFzbqcg@mail.gmail.com>
Subject: [PATCH] ovl: only WARN_ON_ONCE() if dentry is NULL in ovl_encode_fh()
To:     Miklos Szeredi <miklos@szeredi.hu>
Cc:     overlayfs <linux-unionfs@vger.kernel.org>,
        linux-kernel@vger.kernel.org, Hongbo Yin <yinhongbo@bytedance.com>,
        Tianci Zhang <zhangtianci.1997@bytedance.com>
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On Thu, Jul 28, 2022 at 4:57 PM Miklos Szeredi <miklos@szeredi.hu> wrote:
>
> On Thu, 28 Jul 2022 at 08:39, Jiachen Zhang
> <zhangjiachen.jaycee@bytedance.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

OK. I will send the v2 patch later.

Thanks,
Jiachen