Re: [PATCH v5 00/10] mm: Sub-section memory hotplug support

[Dan Williams <dan.j.williams@intel.com>]

On Wed, Mar 27, 2019 at 9:13 AM Michal Hocko <mhocko@kernel.org> wrote:
>
> On Tue 26-03-19 17:20:41, Dan Williams wrote:
> > On Tue, Mar 26, 2019 at 1:04 AM Michal Hocko <mhocko@kernel.org> wrote:
> > >
> > > On Mon 25-03-19 13:03:47, Dan Williams wrote:
> > > > On Mon, Mar 25, 2019 at 3:20 AM Michal Hocko <mhocko@kernel.org> wrote:
> > > [...]
> > > > > > User-defined memory namespaces have this problem, but 2MB is the
> > > > > > default alignment and is sufficient for most uses.
> > > > >
> > > > > What does prevent users to go and use a larger alignment?
> > > >
> > > > Given that we are living with 64MB granularity on mainstream platforms
> > > > for the foreseeable future, the reason users can't rely on a larger
> > > > alignment to address the issue is that the physical alignment may
> > > > change from one boot to the next.
> > >
> > > I would love to learn more about this inter boot volatility. Could you
> > > expand on that some more? I though that the HW configuration presented
> > > to the OS would be more or less stable unless the underlying HW changes.
> >
> > Even if the configuration is static there can be hardware failures
> > that prevent a DIMM, or a PCI device to be included in the memory map.
> > When that happens the BIOS needs to re-layout the map and the result
> > is not guaranteed to maintain the previous alignment.
> >
> > > > No, you can't just wish hardware / platform firmware won't do this,
> > > > because there are not enough platform resources to give every hardware
> > > > device a guaranteed alignment.
> > >
> > > Guarantee is one part and I can see how nobody wants to give you
> > > something as strong but how often does that happen in the real life?
> >
> > I expect a "rare" event to happen everyday in a data-center fleet.
> > Failure rates tend towards 100% daily occurrence at scale and in this
> > case the kernel has everything it needs to mitigate such an event.
> >
> > Setting aside the success rate of a software-alignment mitigation, the
> > reason I am charging this hill again after a 2 year hiatus is the
> > realization that this problem is wider spread than the original
> > failing scenario. Back in 2017 the problem seemed limited to custom
> > memmap= configurations, and collisions between PMEM and System RAM.
> > Now it is clear that the collisions can happen between PMEM regions
> > and namespaces as well, and the problem spans platforms from multiple
> > vendors. Here is the most recent collision problem:
> > https://github.com/pmem/ndctl/issues/76, from a third-party platform.
> >
> > The fix for that issue uncovered a bug in the padding implementation,
> > and a fix for that bug would result in even more hacks in the nvdimm
> > code for what is a core kernel deficiency. Code review of those
> > changes resulted in changing direction to go after the core
> > deficiency.
>
> This kind of information along with real world examples is exactly what
> you should have added into the cover letter. A previous very vague
> claims were not really convincing or something that can be considered a
> proper justification. Please do realize that people who are not working
> with the affected HW are unlikely to have an idea how serious/relevant
> those problems really are.
>
> People are asking for a smaller memory hotplug granularity for other
> usecases (e.g. memory ballooning into VMs) which are quite dubious to
> be honest and not really worth all the code rework. If we are talking
> about something that can be worked around elsewhere then it is preferred
> because the code base is not in an excellent shape and putting more on
> top is just going to cause more headaches.
>
> I will try to find some time to review this more deeply (no promises
> though because time is hectic and this is not a simple feature). For the
> future, please try harder to write up a proper justification and a
> highlevel design description which tells a bit about all important parts
> of the new scheme.

Fair enough. I've been steeped in this for too long, and should have
taken a wider view to bring reviewers up to speed.