linux-kernel - Re: [PATCH 1/1] RDMA/odp: convert to use HMM for ODP

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Date:   Thu, 21 Feb 2019 15:59:37 -0700
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Jerome Glisse <jglisse@...hat.com>
Cc:     Haggai Eran <haggaie@...lanox.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        Leon Romanovsky <leonro@...lanox.com>,
        Doug Ledford <dledford@...hat.com>,
        Artemy Kovalyov <artemyko@...lanox.com>,
        Moni Shoua <monis@...lanox.com>,
        Mike Marciniszyn <mike.marciniszyn@...el.com>,
        Kaike Wan <kaike.wan@...el.com>,
        Dennis Dalessandro <dennis.dalessandro@...el.com>,
        Aviad Yehezkel <aviadye@...lanox.com>
Subject: Re: [PATCH 1/1] RDMA/odp: convert to use HMM for ODP

On Wed, Feb 20, 2019 at 05:29:24PM -0500, Jerome Glisse wrote:
> > > 
> > > Yes it is safe, the hmm struct has its own refcount and mirror holds a
> > > reference on it, the mm struct itself has a reference on the mm
> > > struct.
> > 
> > The issue here is that that hmm_mirror_unregister() must be a strong
> > fence that guarentees no callback is running or will run after
> > return. mmu_notifier_unregister did not provide that.
> > 
> > I think I saw locking in hmm that was doing this..
> 
> So pattern is:
>     hmm_mirror_register(mirror);
> 
>     // Safe for driver to call within HMM with mirror no matter what
> 
>     hmm_mirror_unregister(mirror)
> 
>     // Driver must no stop calling within HMM, it would be a use after
>     // free scenario

This statement is the opposite direction

I want to know that HMM doesn't allow any driver callbacks to be
running after unregister - because I am going to kfree mirror and
other memory touched by the driver callbacks.

Jason