On Fri, May 31, 2019 at 12:33 AM Vlastimil Babka <vba...@suse.cz> 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.
