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

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Date:   Thu, 24 Sep 2020 14:50:03 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     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

On Thu, Sep 24, 2020 at 2:42 PM David Hildenbrand <david@...hat.com> wrote:
>
>
>
> > 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).

add_memory_driver_managed() will fail, but the release_mem_region() in
kmem to unwind on the error path will do the wrong thing because that
other driver thinks it got ownership of the region.

> 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.

I'm ok to leave it alone for now (hasn't been and likely never will be
a problem in practice), but I think it was still worth grumbling
about. I'll leave that part of kmem alone in the upcoming split of
dax_kmem_res removal.