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From:   David Howells <dhowells@redhat.com>
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References: <Yqe6EjGTpkvJUU28@ZenIV> <YqaAcKsd6uGfIQzM@zeniv-ca.linux.org.uk> <CAHk-=wjmCzdNDCt6L8-N33WSRaYjnj0=yTc_JG8A_Pd7ZEtEJw@mail.gmail.com>
To:     Al Viro <viro@zeniv.linux.org.uk>
Cc:     dhowells@redhat.com,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        nvdimm@lists.linux.dev
Subject: Re: [RFC][PATCH] fix short copy handling in copy_mc_pipe_to_iter()
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Date:   Tue, 14 Jun 2022 07:36:19 +0100
Message-ID: <1586153.1655188579@warthog.procyon.org.uk>
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Al Viro <viro@zeniv.linux.org.uk> wrote:

> What's wrong with
>         p_occupancy = pipe_occupancy(head, tail);
>         if (p_occupancy >= pipe->max_usage)
>                 return 0;
> 	else
> 		return pipe->max_usage - p_occupancy;

Because "pipe->max_usage - p_occupancy" can be negative.

post_one_notification() is limited by pipe->ring_size, not pipe->max_usage.

The idea is to allow some slack in a watch pipe for the watch_queue code to
use that userspace can't.

David