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From:   Dan Williams <dan.j.williams@intel.com>
Date:   Thu, 24 Sep 2020 14:26:19 -0700
Message-ID: <CAPcyv4iQ4VnXMU0+_7rfXwPowgcdoABSFUH4WO_3P9vHtWAzPg@mail.gmail.com>
Subject: Re: [PATCH v4 11/23] device-dax: Kill dax_kmem_res
To:     David Hildenbrand <david@redhat.com>
Cc:     Joao Martins <joao.m.martins@oracle.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vishal Verma <vishal.l.verma@intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Pavel Tatashin <pasha.tatashin@soleen.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Linux MM <linux-mm@kvack.org>,
        linux-nvdimm <linux-nvdimm@lists.01.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux ACPI <linux-acpi@vger.kernel.org>,
        Maling list - DRI developers 
        <dri-devel@lists.freedesktop.org>
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[..]
> > I'm not suggesting to busy the whole "virtio" range, just the portion
> > that's about to be passed to add_memory_driver_managed().
>
> I'm afraid I don't get your point. For virtio-mem:
>
> Before:
>
> 1. Create virtio0 container resource
>
> 2. (somewhen in the future) add_memory_driver_managed()
>  - Create resource (System RAM (virtio_mem)), marking it busy/driver
>    managed
>
> After:
>
> 1. Create virtio0 container resource
>
> 2. (somewhen in the future) Create resource (System RAM (virtio_mem)),
>    marking it busy/driver managed
> 3. add_memory_driver_managed()
>
> Not helpful or simpler IMHO.

The concern I'm trying to address is the theoretical race window and
layering violation in this sequence in the kmem driver:

1/ res = request_mem_region(...);
2/ res->flags = IORESOURCE_MEM;
3/ add_memory_driver_managed();

Between 2/ and 3/ something can race and think that it owns the
region. Do I think it will happen in practice, no, but it's still a
pattern that deserves come cleanup.