Re: [PATCH v10 1/3] mm: Shuffle initial free memory to improve memory-side-cache utilization

[Dan Williams]

On Fri, May 31, 2019 at 12:33 AM Vlastimil Babka <vbabka@xxxxxxx> wrote:
>
> On 2/1/19 6:15 AM, Dan Williams wrote:
> > --- a/init/Kconfig
> > +++ b/init/Kconfig
> > @@ -1714,6 +1714,29 @@ config SLAB_FREELIST_HARDENED
> >         sacrifies to harden the kernel slab allocator against common
> >         freelist exploit methods.
> >
> > +config SHUFFLE_PAGE_ALLOCATOR
> > +     bool "Page allocator randomization"
> > +     default SLAB_FREELIST_RANDOM && ACPI_NUMA
> > +     help
> > +       Randomization of the page allocator improves the average
> > +       utilization of a direct-mapped memory-side-cache. See section
> > +       5.2.27 Heterogeneous Memory Attribute Table (HMAT) in the ACPI
> > +       6.2a specification for an example of how a platform advertises
> > +       the presence of a memory-side-cache. There are also incidental
> > +       security benefits as it reduces the predictability of page
> > +       allocations to compliment SLAB_FREELIST_RANDOM, but the
> > +       default granularity of shuffling on 4MB (MAX_ORDER) pages is
> > +       selected based on cache utilization benefits.
> > +
> > +       While the randomization improves cache utilization it may
> > +       negatively impact workloads on platforms without a cache. For
> > +       this reason, by default, the randomization is enabled only
> > +       after runtime detection of a direct-mapped memory-side-cache.
> > +       Otherwise, the randomization may be force enabled with the
> > +       'page_alloc.shuffle' kernel command line parameter.
> > +
> > +       Say Y if unsure.
>
> It says "Say Y if unsure", yet if I run make oldconfig, the default is
> N. Does that make sense?

The default is due to the general policy of not forcing users into new
kernel functionality (i.e. the common Linus objection when a new
config symbol is default 'y') . However, if someone is actively
considering whether to enable it I think there's no harm in
recommending 'y' because the facility currently needs to be paired
with the page_alloc.shuffle=1 command line option.