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Date:   Thu, 21 Feb 2019 08:30:35 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Fam Zheng <zhengfeiran@bytedance.com>
Cc:     linux-fsdevel@vger.kernel.org, Fam Zheng <fam@euphon.net>,
        =?utf-8?B?5q6154aK5pil?= <duanxiongchun@bytedance.com>,
        Shine Liu <liuxian.1@bytedance.com>
Subject: Re: d_add and dentry leak
Message-ID: <20190221083034.GA2217@ZenIV.linux.org.uk>
References: <88C84EC4-6B70-455E-9203-A6DB398D3FA4@bytedance.com>
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On Thu, Feb 21, 2019 at 03:45:06PM +0800, Fam Zheng wrote:
> We’ve noticed an issue that frequent open,close,open,close... of /dev/ptmx eventually causes soft lockup.
> 
> Here is a summary of what happened.
> 
> Upon user opening /dev/ptmx, devpts_pty_new calls d_alloc_name and d_add, the new dentry is inserted into dcache_hashtable.
> 
> Later when closing, devpts_pty_kill calls d_delete and dput,

... since it looks like a normal negative dentry, to be eventually
evicted upon memory pressure.  That's not the worst problem in
there, actually - consider
touch /tmp/a
mount --bind /tmp/a /dev/pts/<number>
followed by devpts_pty_kill() taking that sucker out...