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Date:   Tue, 7 May 2019 20:16:13 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     yangerkun <yangerkun@huawei.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        yi.zhang@huawei.com, houtao1@huawei.com, miaoxie@huawei.com
Subject: Re: system panic while dentry reference count overflow
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On Tue, May 07, 2019 at 08:26:06AM -0700, Linus Torvalds wrote:
> On Mon, May 6, 2019 at 9:15 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
> >
> > Umm...  Where would you put the cutoff for try_dget()?  1G?  Because
> > 2G-<something relatively small> is risky - have it reached, then
> > get the rest of the way to 2G by normal dget() and you've got trouble.
> 
> I'd make the limit be 2G exactly like the page count. Negative counts
> are fine - they work exactly like large integers. It's only 0 that is
> special.

Negative ->d_lockref.count are used for "lockref is dead"...

>  - add the "limit negative dentries" patches that were already written
> for other reasons by Waiman Long.

Irrelevant here, IMO - negative or not (and evictable or pinned, for that
matter) it's easier solved by having d_alloc() fail on parent's ->d_count
overflow.  And I'm pretty sure we can hit that crap without creating any
negative dentries.