linux-kernel - Re: [PATCH v4 11/23] device-dax: Kill dax_kmem

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Date:   Thu, 24 Sep 2020 23:41:41 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     David Hildenbrand <david@...hat.com>,
        Joao Martins <joao.m.martins@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Linux MM <linux-mm@...ck.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux ACPI <linux-acpi@...r.kernel.org>,
        Maling list - DRI developers 
        <dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH v4 11/23] device-dax: Kill dax_kmem_res



> Am 24.09.2020 um 23:26 schrieb Dan Williams <dan.j.williams@...el.com>:
> 
> [..]
>>> 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.

I think in that unlikely event (rather impossible), add_memory_driver_managed() 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 with flags here.
>