linux-kernel - Re: [PATCH v9 11/12] libnvdimm/pfn: Fix fsdax-mode namespace info-block zero-fields

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Message-ID: <CAPcyv4iSndjxgQZq1HtsyY5=h837b-MY3FNDzAdrBGiKJGwOvw@mail.gmail.com>
Date:   Fri, 7 Jun 2019 13:09:58 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     stable <stable@...r.kernel.org>, Linux MM <linux-mm@...ck.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Oscar Salvador <osalvador@...e.de>,
        Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH v9 11/12] libnvdimm/pfn: Fix fsdax-mode namespace
 info-block zero-fields

On Fri, Jun 7, 2019 at 12:54 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
>
> On Thu, 6 Jun 2019 15:06:26 -0700 Dan Williams <dan.j.williams@...el.com> wrote:
>
> > On Thu, Jun 6, 2019 at 2:46 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
> > >
> > > On Wed, 05 Jun 2019 14:58:58 -0700 Dan Williams <dan.j.williams@...el.com> wrote:
> > >
> > > > At namespace creation time there is the potential for the "expected to
> > > > be zero" fields of a 'pfn' info-block to be filled with indeterminate
> > > > data. While the kernel buffer is zeroed on allocation it is immediately
> > > > overwritten by nd_pfn_validate() filling it with the current contents of
> > > > the on-media info-block location. For fields like, 'flags' and the
> > > > 'padding' it potentially means that future implementations can not rely
> > > > on those fields being zero.
> > > >
> > > > In preparation to stop using the 'start_pad' and 'end_trunc' fields for
> > > > section alignment, arrange for fields that are not explicitly
> > > > initialized to be guaranteed zero. Bump the minor version to indicate it
> > > > is safe to assume the 'padding' and 'flags' are zero. Otherwise, this
> > > > corruption is expected to benign since all other critical fields are
> > > > explicitly initialized.
> > > >
> > > > Fixes: 32ab0a3f5170 ("libnvdimm, pmem: 'struct page' for pmem")
> > > > Cc: <stable@...r.kernel.org>
> > > > Signed-off-by: Dan Williams <dan.j.williams@...el.com>
> > >
> > > The cc:stable in [11/12] seems odd.  Is this independent of the other
> > > patches?  If so, shouldn't it be a standalone thing which can be
> > > prioritized?
> > >
> >
> > The cc: stable is about spreading this new policy to as many kernels
> > as possible not fixing an issue in those kernels. It's not until patch
> > 12 "libnvdimm/pfn: Stop padding pmem namespaces to section alignment"
> > as all previous kernel do initialize all fields.
> >
> > I'd be ok to drop that cc: stable, my concern is distros that somehow
> > pickup and backport patch 12 and miss patch 11.
>
> Could you please propose a changelog paragraph which explains all this
> to those who will be considering this patch for backports?
>

Will do. I'll resend the series with this and the fixups proposed by Oscar.