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From:   David Hildenbrand <david@redhat.com>
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Subject: Re: [PATCH v4 11/23] device-dax: Kill dax_kmem_res
Date:   Thu, 24 Sep 2020 23:41:41 +0200
Message-Id: <79BEC711-C769-432B-9A50-63C6A3AEB0E3@redhat.com>
References: <CAPcyv4iQ4VnXMU0+_7rfXwPowgcdoABSFUH4WO_3P9vHtWAzPg@mail.gmail.com>
Cc:     David Hildenbrand <david@redhat.com>,
        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>
In-Reply-To: <CAPcyv4iQ4VnXMU0+_7rfXwPowgcdoABSFUH4WO_3P9vHtWAzPg@mail.gmail.com>
To:     Dan Williams <dan.j.williams@intel.com>
Precedence: bulk


> Am 24.09.2020 um 23:26 schrieb Dan Williams <dan.j.williams@intel.com>:
>=20
> =EF=BB=BF[..]
>>> I'm not suggesting to busy the whole "virtio" range, just the portion
>>> that's about to be passed to add_memory_driver_managed().
>>=20
>> I'm afraid I don't get your point. For virtio-mem:
>>=20
>> Before:
>>=20
>> 1. Create virtio0 container resource
>>=20
>> 2. (somewhen in the future) add_memory_driver_managed()
>> - Create resource (System RAM (virtio_mem)), marking it busy/driver
>>   managed
>>=20
>> After:
>>=20
>> 1. Create virtio0 container resource
>>=20
>> 2. (somewhen in the future) Create resource (System RAM (virtio_mem)),
>>   marking it busy/driver managed
>> 3. add_memory_driver_managed()
>>=20
>> Not helpful or simpler IMHO.
>=20
> The concern I'm trying to address is the theoretical race window and
> layering violation in this sequence in the kmem driver:
>=20
> 1/ res =3D request_mem_region(...);
> 2/ res->flags =3D IORESOURCE_MEM;
> 3/ add_memory_driver_managed();
>=20
> 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.

I think in that unlikely event (rather impossible), add_memory_driver_manage=
d() should fail, detecting a conflicting (busy) resource. Not sure what will=
 happen next ( and did not double-check).

But yeah, the way the BUSY bit is cleared here is wrong - simply overwriting=
 other bits. And it would be even better if we could avoid manually messing w=
ith flags here.
>=20