netdev - Re: [PATCH v2] epoll: use refcount to reduce ep

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Date:   Mon, 28 Nov 2022 18:50:09 +0100
From:   Paolo Abeni <pabeni@...hat.com>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     linux-fsdevel@...r.kernel.org,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Davidlohr Bueso <dave@...olabs.net>,
        Jason Baron <jbaron@...mai.com>,
        Roman Penyaev <rpenyaev@...e.de>, netdev@...r.kernel.org,
        Carlos Maiolino <cmaiolino@...hat.com>
Subject: Re: [PATCH v2] epoll: use refcount to reduce ep_mutex contention

Hello,

On Thu, 2022-11-24 at 15:02 -0800, Eric Biggers wrote:
[...]
> The behavior after this patch is that the resources aren't freed until the epoll
> file *and* all files that were added to it have been closed.
> 
> Is that okay? 

[...]

> But probably some users do things the other way around.  I.e., they have a
> long-lived file descriptor that is repeatedly polled using different epoll
> instances that have a shorter lifetime.
> 
> In that case, the number of 'struct eventpoll' and 'struct epitem' in kernel
> memory will keep growing until 'max_user_watches' is hit, at which point
> EPOLL_CTL_ADD will start failing with ENOSPC.
> 
> Are you sure that is fine?

I have a new version of this patch which hopefully should address both
concerns. I'll share it soon.

Thanks!

Paolo