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a2129f2479
Patch series "mm / virtio: Provide support for free page reporting", v17. This series provides an asynchronous means of reporting free guest pages to a hypervisor so that the memory associated with those pages can be dropped and reused by other processes and/or guests on the host. Using this it is possible to avoid unnecessary I/O to disk and greatly improve performance in the case of memory overcommit on the host. When enabled we will be performing a scan of free memory every 2 seconds while pages of sufficiently high order are being freed. In each pass at least one sixteenth of each free list will be reported. By doing this we avoid racing against other threads that may be causing a high amount of memory churn. The lowest page order currently scanned when reporting pages is pageblock_order so that this feature will not interfere with the use of Transparent Huge Pages in the case of virtualization. Currently this is only in use by virtio-balloon however there is the hope that at some point in the future other hypervisors might be able to make use of it. In the virtio-balloon/QEMU implementation the hypervisor is currently using MADV_DONTNEED to indicate to the host kernel that the page is currently free. It will be zeroed and faulted back into the guest the next time the page is accessed. To track if a page is reported or not the Uptodate flag was repurposed and used as a Reported flag for Buddy pages. We walk though the free list isolating pages and adding them to the scatterlist until we either encounter the end of the list or have processed at least one sixteenth of the pages that were listed in nr_free prior to us starting. If we fill the scatterlist before we reach the end of the list we rotate the list so that the first unreported page we encounter is moved to the head of the list as that is where we will resume after we have freed the reported pages back into the tail of the list. Below are the results from various benchmarks. I primarily focused on two tests. The first is the will-it-scale/page_fault2 test, and the other is a modified version of will-it-scale/page_fault1 that was enabled to use THP. I did this as it allows for better visibility into different parts of the memory subsystem. The guest is running with 32G for RAM on one node of a E5-2630 v3. The host has had some features such as CPU turbo disabled in the BIOS. Test page_fault1 (THP) page_fault2 Name tasks Process Iter STDEV Process Iter STDEV Baseline 1 1012402.50 0.14% 361855.25 0.81% 16 8827457.25 0.09% 3282347.00 0.34% Patches Applied 1 1007897.00 0.23% 361887.00 0.26% 16 8784741.75 0.39% 3240669.25 0.48% Patches Enabled 1 1010227.50 0.39% 359749.25 0.56% 16 8756219.00 0.24% 3226608.75 0.97% Patches Enabled 1 1050982.00 4.26% 357966.25 0.14% page shuffle 16 8672601.25 0.49% 3223177.75 0.40% Patches enabled 1 1003238.00 0.22% 360211.00 0.22% shuffle w/ RFC 16 8767010.50 0.32% 3199874.00 0.71% The results above are for a baseline with a linux-next-20191219 kernel, that kernel with this patch set applied but page reporting disabled in virtio-balloon, the patches applied and page reporting fully enabled, the patches enabled with page shuffling enabled, and the patches applied with page shuffling enabled and an RFC patch that makes used of MADV_FREE in QEMU. These results include the deviation seen between the average value reported here versus the high and/or low value. I observed that during the test memory usage for the first three tests never dropped whereas with the patches fully enabled the VM would drop to using only a few GB of the host's memory when switching from memhog to page fault tests. Any of the overhead visible with this patch set enabled seems due to page faults caused by accessing the reported pages and the host zeroing the page before giving it back to the guest. This overhead is much more visible when using THP than with standard 4K pages. In addition page shuffling seemed to increase the amount of faults generated due to an increase in memory churn. The overehad is reduced when using MADV_FREE as we can avoid the extra zeroing of the pages when they are reintroduced to the host, as can be seen when the RFC is applied with shuffling enabled. The overall guest size is kept fairly small to only a few GB while the test is running. If the host memory were oversubscribed this patch set should result in a performance improvement as swapping memory in the host can be avoided. A brief history on the background of free page reporting can be found at: https://lore.kernel.org/lkml/29f43d5796feed0dec8e8bb98b187d9dac03b900.camel@linux.intel.com/ This patch (of 9): Move the head/tail adding logic out of the shuffle code and into the __free_one_page function since ultimately that is where it is really needed anyway. By doing this we should be able to reduce the overhead and can consolidate all of the list addition bits in one spot. Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Reviewed-by: Dan Williams <dan.j.williams@intel.com> Acked-by: Mel Gorman <mgorman@techsingularity.net> Acked-by: David Hildenbrand <david@redhat.com> Cc: Yang Zhang <yang.zhang.wz@gmail.com> Cc: Pankaj Gupta <pagupta@redhat.com> Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> Cc: Nitesh Narayan Lal <nitesh@redhat.com> Cc: Rik van Riel <riel@surriel.com> Cc: Matthew Wilcox <willy@infradead.org> Cc: Luiz Capitulino <lcapitulino@redhat.com> Cc: Dave Hansen <dave.hansen@intel.com> Cc: Wei Wang <wei.w.wang@intel.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Paolo Bonzini <pbonzini@redhat.com> Cc: Michal Hocko <mhocko@kernel.org> Cc: Vlastimil Babka <vbabka@suse.cz> Cc: Oscar Salvador <osalvador@suse.de> Cc: Michael S. Tsirkin <mst@redhat.com> Cc: wei qi <weiqi4@huawei.com> Link: http://lkml.kernel.org/r/20200211224602.29318.84523.stgit@localhost.localdomain Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
208 lines
5.5 KiB
C
208 lines
5.5 KiB
C
// SPDX-License-Identifier: GPL-2.0
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// Copyright(c) 2018 Intel Corporation. All rights reserved.
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#include <linux/mm.h>
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#include <linux/init.h>
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#include <linux/mmzone.h>
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#include <linux/random.h>
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#include <linux/moduleparam.h>
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#include "internal.h"
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#include "shuffle.h"
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DEFINE_STATIC_KEY_FALSE(page_alloc_shuffle_key);
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static unsigned long shuffle_state __ro_after_init;
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/*
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* Depending on the architecture, module parameter parsing may run
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* before, or after the cache detection. SHUFFLE_FORCE_DISABLE prevents,
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* or reverts the enabling of the shuffle implementation. SHUFFLE_ENABLE
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* attempts to turn on the implementation, but aborts if it finds
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* SHUFFLE_FORCE_DISABLE already set.
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*/
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__meminit void page_alloc_shuffle(enum mm_shuffle_ctl ctl)
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{
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if (ctl == SHUFFLE_FORCE_DISABLE)
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set_bit(SHUFFLE_FORCE_DISABLE, &shuffle_state);
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if (test_bit(SHUFFLE_FORCE_DISABLE, &shuffle_state)) {
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if (test_and_clear_bit(SHUFFLE_ENABLE, &shuffle_state))
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static_branch_disable(&page_alloc_shuffle_key);
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} else if (ctl == SHUFFLE_ENABLE
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&& !test_and_set_bit(SHUFFLE_ENABLE, &shuffle_state))
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static_branch_enable(&page_alloc_shuffle_key);
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}
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static bool shuffle_param;
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static int shuffle_show(char *buffer, const struct kernel_param *kp)
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{
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return sprintf(buffer, "%c\n", test_bit(SHUFFLE_ENABLE, &shuffle_state)
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? 'Y' : 'N');
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}
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static __meminit int shuffle_store(const char *val,
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const struct kernel_param *kp)
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{
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int rc = param_set_bool(val, kp);
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if (rc < 0)
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return rc;
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if (shuffle_param)
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page_alloc_shuffle(SHUFFLE_ENABLE);
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else
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page_alloc_shuffle(SHUFFLE_FORCE_DISABLE);
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return 0;
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}
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module_param_call(shuffle, shuffle_store, shuffle_show, &shuffle_param, 0400);
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/*
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* For two pages to be swapped in the shuffle, they must be free (on a
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* 'free_area' lru), have the same order, and have the same migratetype.
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*/
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static struct page * __meminit shuffle_valid_page(unsigned long pfn, int order)
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{
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struct page *page;
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/*
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* Given we're dealing with randomly selected pfns in a zone we
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* need to ask questions like...
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*/
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/* ...is the pfn even in the memmap? */
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if (!pfn_valid_within(pfn))
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return NULL;
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/* ...is the pfn in a present section or a hole? */
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if (!pfn_in_present_section(pfn))
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return NULL;
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/* ...is the page free and currently on a free_area list? */
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page = pfn_to_page(pfn);
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if (!PageBuddy(page))
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return NULL;
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/*
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* ...is the page on the same list as the page we will
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* shuffle it with?
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*/
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if (page_order(page) != order)
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return NULL;
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return page;
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}
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/*
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* Fisher-Yates shuffle the freelist which prescribes iterating through an
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* array, pfns in this case, and randomly swapping each entry with another in
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* the span, end_pfn - start_pfn.
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*
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* To keep the implementation simple it does not attempt to correct for sources
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* of bias in the distribution, like modulo bias or pseudo-random number
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* generator bias. I.e. the expectation is that this shuffling raises the bar
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* for attacks that exploit the predictability of page allocations, but need not
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* be a perfect shuffle.
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*/
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#define SHUFFLE_RETRY 10
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void __meminit __shuffle_zone(struct zone *z)
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{
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unsigned long i, flags;
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unsigned long start_pfn = z->zone_start_pfn;
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unsigned long end_pfn = zone_end_pfn(z);
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const int order = SHUFFLE_ORDER;
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const int order_pages = 1 << order;
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spin_lock_irqsave(&z->lock, flags);
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start_pfn = ALIGN(start_pfn, order_pages);
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for (i = start_pfn; i < end_pfn; i += order_pages) {
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unsigned long j;
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int migratetype, retry;
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struct page *page_i, *page_j;
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/*
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* We expect page_i, in the sub-range of a zone being added
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* (@start_pfn to @end_pfn), to more likely be valid compared to
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* page_j randomly selected in the span @zone_start_pfn to
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* @spanned_pages.
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*/
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page_i = shuffle_valid_page(i, order);
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if (!page_i)
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continue;
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for (retry = 0; retry < SHUFFLE_RETRY; retry++) {
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/*
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* Pick a random order aligned page in the zone span as
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* a swap target. If the selected pfn is a hole, retry
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* up to SHUFFLE_RETRY attempts find a random valid pfn
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* in the zone.
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*/
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j = z->zone_start_pfn +
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ALIGN_DOWN(get_random_long() % z->spanned_pages,
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order_pages);
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page_j = shuffle_valid_page(j, order);
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if (page_j && page_j != page_i)
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break;
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}
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if (retry >= SHUFFLE_RETRY) {
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pr_debug("%s: failed to swap %#lx\n", __func__, i);
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continue;
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}
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/*
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* Each migratetype corresponds to its own list, make sure the
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* types match otherwise we're moving pages to lists where they
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* do not belong.
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*/
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migratetype = get_pageblock_migratetype(page_i);
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if (get_pageblock_migratetype(page_j) != migratetype) {
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pr_debug("%s: migratetype mismatch %#lx\n", __func__, i);
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continue;
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}
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list_swap(&page_i->lru, &page_j->lru);
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pr_debug("%s: swap: %#lx -> %#lx\n", __func__, i, j);
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/* take it easy on the zone lock */
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if ((i % (100 * order_pages)) == 0) {
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spin_unlock_irqrestore(&z->lock, flags);
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cond_resched();
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spin_lock_irqsave(&z->lock, flags);
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}
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}
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spin_unlock_irqrestore(&z->lock, flags);
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}
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/**
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* shuffle_free_memory - reduce the predictability of the page allocator
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* @pgdat: node page data
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*/
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void __meminit __shuffle_free_memory(pg_data_t *pgdat)
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{
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struct zone *z;
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for (z = pgdat->node_zones; z < pgdat->node_zones + MAX_NR_ZONES; z++)
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shuffle_zone(z);
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}
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bool shuffle_pick_tail(void)
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{
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static u64 rand;
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static u8 rand_bits;
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bool ret;
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/*
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* The lack of locking is deliberate. If 2 threads race to
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* update the rand state it just adds to the entropy.
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*/
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if (rand_bits == 0) {
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rand_bits = 64;
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rand = get_random_u64();
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}
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ret = rand & 1;
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rand_bits--;
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rand >>= 1;
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return ret;
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}
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