On Tue, 29 Jan 2019 21:02:16 -0800 Dan Williams <dan.j.williams@xxxxxxxxx> wrote: > Randomization of the page allocator improves the average utilization of > a direct-mapped memory-side-cache. Memory side caching is a platform > capability that Linux has been previously exposed to in HPC > (high-performance computing) environments on specialty platforms. In > that instance it was a smaller pool of high-bandwidth-memory relative to > higher-capacity / lower-bandwidth DRAM. Now, this capability is going to > be found on general purpose server platforms where DRAM is a cache in > front of higher latency persistent memory [1]. > > Robert offered an explanation of the state of the art of Linux > interactions with memory-side-caches [2], and I copy it here: > > It's been a problem in the HPC space: > http://www.nersc.gov/research-and-development/knl-cache-mode-performance-coe/ > > A kernel module called zonesort is available to try to help: > https://software.intel.com/en-us/articles/xeon-phi-software > > and this abandoned patch series proposed that for the kernel: > https://lkml.kernel.org/r/20170823100205.17311-1-lukasz.daniluk@xxxxxxxxx > > Dan's patch series doesn't attempt to ensure buffers won't conflict, but > also reduces the chance that the buffers will. This will make performance > more consistent, albeit slower than "optimal" (which is near impossible > to attain in a general-purpose kernel). That's better than forcing > users to deploy remedies like: > "To eliminate this gradual degradation, we have added a Stream > measurement to the Node Health Check that follows each job; > nodes are rebooted whenever their measured memory bandwidth > falls below 300 GB/s." > > A replacement for zonesort was merged upstream in commit cc9aec03e58f > "x86/numa_emulation: Introduce uniform split capability". With this > numa_emulation capability, memory can be split into cache sized > ("near-memory" sized) numa nodes. A bind operation to such a node, and > disabling workloads on other nodes, enables full cache performance. > However, once the workload exceeds the cache size then cache conflicts > are unavoidable. While HPC environments might be able to tolerate > time-scheduling of cache sized workloads, for general purpose server > platforms, the oversubscribed cache case will be the common case. > > The worst case scenario is that a server system owner benchmarks a > workload at boot with an un-contended cache only to see that performance > degrade over time, even below the average cache performance due to > excessive conflicts. Randomization clips the peaks and fills in the > valleys of cache utilization to yield steady average performance. > > Here are some performance impact details of the patches: > > 1/ An Intel internal synthetic memory bandwidth measurement tool, saw a > 3X speedup in a contrived case that tries to force cache conflicts. The > contrived cased used the numa_emulation capability to force an instance > of the benchmark to be run in two of the near-memory sized numa nodes. > If both instances were placed on the same emulated they would fit and > cause zero conflicts. While on separate emulated nodes without > randomization they underutilized the cache and conflicted unnecessarily > due to the in-order allocation per node. > > 2/ A well known Java server application benchmark was run with a heap > size that exceeded cache size by 3X. The cache conflict rate was 8% for > the first run and degraded to 21% after page allocator aging. With > randomization enabled the rate levelled out at 11%. > > 3/ A MongoDB workload did not observe measurable difference in > cache-conflict rates, but the overall throughput dropped by 7% with > randomization in one case. > > 4/ Mel Gorman ran his suite of performance workloads with randomization > enabled on platforms without a memory-side-cache and saw a mix of some > improvements and some losses [3]. > > While there is potentially significant improvement for applications that > depend on low latency access across a wide working-set, the performance > may be negligible to negative for other workloads. For this reason the > shuffle capability defaults to off unless a direct-mapped > memory-side-cache is detected. Even then, the page_alloc.shuffle=0 > parameter can be specified to disable the randomization on those > systems. > > Outside of memory-side-cache utilization concerns there is potentially > security benefit from randomization. Some data exfiltration and > return-oriented-programming attacks rely on the ability to infer the > location of sensitive data objects. The kernel page allocator, > especially early in system boot, has predictable first-in-first out > behavior for physical pages. Pages are freed in physical address order > when first onlined. > > Quoting Kees: > "While we already have a base-address randomization > (CONFIG_RANDOMIZE_MEMORY), attacks against the same hardware and > memory layouts would certainly be using the predictability of > allocation ordering (i.e. for attacks where the base address isn't > important: only the relative positions between allocated memory). > This is common in lots of heap-style attacks. They try to gain > control over ordering by spraying allocations, etc. > > I'd really like to see this because it gives us something similar > to CONFIG_SLAB_FREELIST_RANDOM but for the page allocator." > > While SLAB_FREELIST_RANDOM reduces the predictability of some local slab > caches it leaves vast bulk of memory to be predictably in order > allocated. However, it should be noted, the concrete security benefits > are hard to quantify, and no known CVE is mitigated by this > randomization. > > Introduce shuffle_free_memory(), and its helper shuffle_zone(), to > perform a Fisher-Yates shuffle of the page allocator 'free_area' lists > when they are initially populated with free memory at boot and at > hotplug time. Do this based on either the presence of a > page_alloc.shuffle=Y command line parameter, or autodetection of a > memory-side-cache (to be added in a follow-on patch). This is unfortunate from a testing and coverage point of view. At least initially it is desirable that all testers run this feature. Also, it's unfortunate that enableing the feature requires a reboot. What happens if we do away with the boot-time (and maybe hotplug-time) randomization and permit the feature to be switched on/off at runtime? > The shuffling is done in terms of CONFIG_SHUFFLE_PAGE_ORDER sized free > pages where the default CONFIG_SHUFFLE_PAGE_ORDER is MAX_ORDER-1 i.e. > 10, 4MB this trades off randomization granularity for time spent > shuffling. MAX_ORDER-1 was chosen to be minimally invasive to the page > allocator while still showing memory-side cache behavior improvements, > and the expectation that the security implications of finer granularity > randomization is mitigated by CONFIG_SLAB_FREELIST_RANDOM. > > The performance impact of the shuffling appears to be in the noise > compared to other memory initialization work. Also the bulk of the work > is done in the background as a part of deferred_init_memmap(). > > This initial randomization can be undone over time so a follow-on patch > is introduced to inject entropy on page free decisions. It is reasonable > to ask if the page free entropy is sufficient, but it is not enough due > to the in-order initial freeing of pages. At the start of that process > putting page1 in front or behind page0 still keeps them close together, > page2 is still near page1 and has a high chance of being adjacent. As > more pages are added ordering diversity improves, but there is still > high page locality for the low address pages and this leads to no > significant impact to the cache conflict rate. > > ... > > include/linux/list.h | 17 ++++ > include/linux/mmzone.h | 4 + > include/linux/shuffle.h | 45 +++++++++++ > init/Kconfig | 23 ++++++ > mm/Makefile | 7 ++ > mm/memblock.c | 1 > mm/memory_hotplug.c | 3 + > mm/page_alloc.c | 6 +- > mm/shuffle.c | 188 +++++++++++++++++++++++++++++++++++++++++++++++ Can we get a Documentation update for the new kernel parameter? > > ... > > --- /dev/null > +++ b/mm/shuffle.c > @@ -0,0 +1,188 @@ > +// SPDX-License-Identifier: GPL-2.0 > +// Copyright(c) 2018 Intel Corporation. All rights reserved. > + > +#include <linux/mm.h> > +#include <linux/init.h> > +#include <linux/mmzone.h> > +#include <linux/random.h> > +#include <linux/shuffle.h> Does shuffle.h need to be available to the whole kernel or can we put it in mm/? > +#include <linux/moduleparam.h> > +#include "internal.h" > + > +DEFINE_STATIC_KEY_FALSE(page_alloc_shuffle_key); > +static unsigned long shuffle_state __ro_after_init; > + > +/* > + * Depending on the architecture, module parameter parsing may run > + * before, or after the cache detection. SHUFFLE_FORCE_DISABLE prevents, > + * or reverts the enabling of the shuffle implementation. SHUFFLE_ENABLE > + * attempts to turn on the implementation, but aborts if it finds > + * SHUFFLE_FORCE_DISABLE already set. > + */ > +void page_alloc_shuffle(enum mm_shuffle_ctl ctl) > +{ > + if (ctl == SHUFFLE_FORCE_DISABLE) > + set_bit(SHUFFLE_FORCE_DISABLE, &shuffle_state); > + > + if (test_bit(SHUFFLE_FORCE_DISABLE, &shuffle_state)) { > + if (test_and_clear_bit(SHUFFLE_ENABLE, &shuffle_state)) > + static_branch_disable(&page_alloc_shuffle_key); > + } else if (ctl == SHUFFLE_ENABLE > + && !test_and_set_bit(SHUFFLE_ENABLE, &shuffle_state)) > + static_branch_enable(&page_alloc_shuffle_key); > +} Can this be __meminit? > +static bool shuffle_param; > +extern int shuffle_show(char *buffer, const struct kernel_param *kp) > +{ > + return sprintf(buffer, "%c\n", test_bit(SHUFFLE_ENABLE, &shuffle_state) > + ? 'Y' : 'N'); > +} > +static int shuffle_store(const char *val, const struct kernel_param *kp) > +{ > + int rc = param_set_bool(val, kp); > + > + if (rc < 0) > + return rc; > + if (shuffle_param) > + page_alloc_shuffle(SHUFFLE_ENABLE); > + else > + page_alloc_shuffle(SHUFFLE_FORCE_DISABLE); > + return 0; > +} > +module_param_call(shuffle, shuffle_store, shuffle_show, &shuffle_param, 0400); > > ... > > +/* > + * Fisher-Yates shuffle the freelist which prescribes iterating through > + * an array, pfns in this case, and randomly swapping each entry with > + * another in the span, end_pfn - start_pfn. > + * > + * To keep the implementation simple it does not attempt to correct for > + * sources of bias in the distribution, like modulo bias or > + * pseudo-random number generator bias. I.e. the expectation is that > + * this shuffling raises the bar for attacks that exploit the > + * predictability of page allocations, but need not be a perfect > + * shuffle. Reflowing the comment to use all 80 cols would save a line :) > + */ > +#define SHUFFLE_RETRY 10 > +void __meminit __shuffle_zone(struct zone *z) > +{ > + unsigned long i, flags; > + unsigned long start_pfn = z->zone_start_pfn; > + unsigned long end_pfn = zone_end_pfn(z); > + const int order = SHUFFLE_ORDER; > + const int order_pages = 1 << order; > + > + spin_lock_irqsave(&z->lock, flags); > + start_pfn = ALIGN(start_pfn, order_pages); > + for (i = start_pfn; i < end_pfn; i += order_pages) { > + unsigned long j; > + int migratetype, retry; > + struct page *page_i, *page_j; > + > + /* > + * We expect page_i, in the sub-range of a zone being > + * added (@start_pfn to @end_pfn), to more likely be > + * valid compared to page_j randomly selected in the > + * span @zone_start_pfn to @spanned_pages. > + */ > + page_i = shuffle_valid_page(i, order); > + if (!page_i) > + continue; > + > + for (retry = 0; retry < SHUFFLE_RETRY; retry++) { > + /* > + * Pick a random order aligned page from the > + * start of the zone. Use the *whole* zone here > + * so that if it is freed in tiny pieces that we > + * randomize in the whole zone, not just within > + * those fragments. Second sentence is hard to parse. > + * > + * Since page_j comes from a potentially sparse > + * address range we want to try a bit harder to > + * find a shuffle point for page_i. > + */ Reflow the comment... > + j = z->zone_start_pfn + > + ALIGN_DOWN(get_random_long() % z->spanned_pages, > + order_pages); > + page_j = shuffle_valid_page(j, order); > + if (page_j && page_j != page_i) > + break; > + } > + if (retry >= SHUFFLE_RETRY) { > + pr_debug("%s: failed to swap %#lx\n", __func__, i); > + continue; > + } > + > + /* > + * Each migratetype corresponds to its own list, make > + * sure the types match otherwise we're moving pages to > + * lists where they do not belong. > + */ Reflow. > + migratetype = get_pageblock_migratetype(page_i); > + if (get_pageblock_migratetype(page_j) != migratetype) { > + pr_debug("%s: migratetype mismatch %#lx\n", __func__, i); > + continue; > + } > + > + list_swap(&page_i->lru, &page_j->lru); > + > + pr_debug("%s: swap: %#lx -> %#lx\n", __func__, i, j); > + > + /* take it easy on the zone lock */ > + if ((i % (100 * order_pages)) == 0) { > + spin_unlock_irqrestore(&z->lock, flags); > + cond_resched(); > + spin_lock_irqsave(&z->lock, flags); > + } > + } > + spin_unlock_irqrestore(&z->lock, flags); > +} > > ... >
