From: Zi Yan <z...@nvidia.com>
It exchanges two pages by unmapping both first, then exchanging the
data of the pages using a u64 register, and finally remapping both
pages.
It saves the overheads of allocating two new pages in two
back-to-back migrate_pages().
Signed-off-by: Zi Yan <z...@nvidia.com>
---
include/linux/exchange.h | 23 ++
include/linux/ksm.h | 4 +
mm/Makefile | 1 +
mm/exchange.c | 597 +++++++++++++++++++++++++++++++++++++++++++++++
mm/ksm.c | 35 +++
5 files changed, 660 insertions(+)
create mode 100644 include/linux/exchange.h
create mode 100644 mm/exchange.c
diff --git a/include/linux/exchange.h b/include/linux/exchange.h
new file mode 100644
index 0000000..778068e
--- /dev/null
+++ b/include/linux/exchange.h
@@ -0,0 +1,23 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _LINUX_EXCHANGE_H
+#define _LINUX_EXCHANGE_H
+
+#include <linux/migrate.h>
+
+struct exchange_page_info {
+ struct page *from_page;
+ struct page *to_page;
+
+ struct anon_vma *from_anon_vma;
+ struct anon_vma *to_anon_vma;
+
+ int from_page_was_mapped;
+ int to_page_was_mapped;
+
+ struct list_head list;
+};
+
+int exchange_pages(struct list_head *exchange_list,
+ enum migrate_mode mode,
+ int reason);
+#endif /* _LINUX_EXCHANGE_H */
diff --git a/include/linux/ksm.h b/include/linux/ksm.h
index e48b1e4..170312d 100644
--- a/include/linux/ksm.h
+++ b/include/linux/ksm.h
@@ -55,6 +55,7 @@ void rmap_walk_ksm(struct page *page, struct
rmap_walk_control *rwc);
void ksm_migrate_page(struct page *newpage, struct page *oldpage);
bool reuse_ksm_page(struct page *page,
struct vm_area_struct *vma, unsigned long address);
+void ksm_exchange_page(struct page *to_page, struct page *from_page);
#else /* !CONFIG_KSM */
@@ -92,6 +93,9 @@ static inline bool reuse_ksm_page(struct page *page,
struct vm_area_struct *vma, unsigned long address)
{
return false;
+static inline void ksm_exchange_page(struct page *to_page,
+ struct page *from_page)
+{
}
#endif /* CONFIG_MMU */
#endif /* !CONFIG_KSM */
diff --git a/mm/Makefile b/mm/Makefile
index fa02a9f..5e6c591 100644
--- a/mm/Makefile
+++ b/mm/Makefile
@@ -45,6 +45,7 @@ obj-y += init-mm.o
obj-y += memblock.o
obj-y += copy_page.o
+obj-y += exchange.o
ifdef CONFIG_MMU
obj-$(CONFIG_ADVISE_SYSCALLS) += madvise.o
diff --git a/mm/exchange.c b/mm/exchange.c
new file mode 100644
index 0000000..626bbea
--- /dev/null
+++ b/mm/exchange.c
@@ -0,0 +1,597 @@
+/*
+ * Exchange two in-use pages. Page flags and page->mapping are exchanged
+ * as well. Only anonymous pages are supported.
+ *
+ * Copyright (C) 2016 NVIDIA, Zi Yan <z...@nvidia.com>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2.
+ */
+
+#include <linux/syscalls.h>
+#include <linux/migrate.h>
+#include <linux/exchange.h>
+#include <linux/security.h>
+#include <linux/cpuset.h>
+#include <linux/hugetlb.h>
+#include <linux/mm_inline.h>
+#include <linux/page_idle.h>
+#include <linux/page-flags.h>
+#include <linux/ksm.h>
+#include <linux/memcontrol.h>
+#include <linux/balloon_compaction.h>
+#include <linux/buffer_head.h>
+
+
+#include "internal.h"
+
+/*
+ * Move a list of individual pages
+ */
+struct pages_to_node {
+ unsigned long from_addr;
+ int from_status;
+
+ unsigned long to_addr;
+ int to_status;
+};
+
+struct page_flags {
+ unsigned int page_error :1;
+ unsigned int page_referenced:1;
+ unsigned int page_uptodate:1;
+ unsigned int page_active:1;
+ unsigned int page_unevictable:1;
+ unsigned int page_checked:1;
+ unsigned int page_mappedtodisk:1;
+ unsigned int page_dirty:1;
+ unsigned int page_is_young:1;
+ unsigned int page_is_idle:1;
+ unsigned int page_swapcache:1;
+ unsigned int page_writeback:1;
+ unsigned int page_private:1;
+ unsigned int __pad:3;
+};
+
+
+static void exchange_page(char *to, char *from)
+{
+ u64 tmp;
+ int i;
+
+ for (i = 0; i < PAGE_SIZE; i += sizeof(tmp)) {
+ tmp = *((u64*)(from + i));
+ *((u64*)(from + i)) = *((u64*)(to + i));
+ *((u64*)(to + i)) = tmp;
+ }
+}
+
+static inline void exchange_highpage(struct page *to, struct page *from)
+{
+ char *vfrom, *vto;
+
+ vfrom = kmap_atomic(from);
+ vto = kmap_atomic(to);
+ exchange_page(vto, vfrom);
+ kunmap_atomic(vto);
+ kunmap_atomic(vfrom);
+}
+
+static void __exchange_gigantic_page(struct page *dst, struct page *src,
+ int nr_pages)
+{
+ int i;
+ struct page *dst_base = dst;
+ struct page *src_base = src;
+
+ for (i = 0; i < nr_pages; ) {
+ cond_resched();
+ exchange_highpage(dst, src);
+
+ i++;
+ dst = mem_map_next(dst, dst_base, i);
+ src = mem_map_next(src, src_base, i);
+ }
+}
+
+static void exchange_huge_page(struct page *dst, struct page *src)
+{
+ int i;
+ int nr_pages;
+
+ if (PageHuge(src)) {
+ /* hugetlbfs page */
+ struct hstate *h = page_hstate(src);
+ nr_pages = pages_per_huge_page(h);
+
+ if (unlikely(nr_pages > MAX_ORDER_NR_PAGES)) {
+ __exchange_gigantic_page(dst, src, nr_pages);
+ return;
+ }
+ } else {
+ /* thp page */
+ BUG_ON(!PageTransHuge(src));
+ nr_pages = hpage_nr_pages(src);
+ }
+
+ for (i = 0; i < nr_pages; i++) {
+ cond_resched();
+ exchange_highpage(dst + i, src + i);
+ }
+}
+
+/*
+ * Copy the page to its new location without polluting cache
+ */
+static void exchange_page_flags(struct page *to_page, struct page *from_page)
+{
+ int from_cpupid, to_cpupid;
+ struct page_flags from_page_flags, to_page_flags;
+ struct mem_cgroup *to_memcg = page_memcg(to_page),
+ *from_memcg = page_memcg(from_page);
+
+ from_cpupid = page_cpupid_xchg_last(from_page, -1);
+
+ from_page_flags.page_error = TestClearPageError(from_page);
+ from_page_flags.page_referenced = TestClearPageReferenced(from_page);
+ from_page_flags.page_uptodate = PageUptodate(from_page);
+ ClearPageUptodate(from_page);
+ from_page_flags.page_active = TestClearPageActive(from_page);
+ from_page_flags.page_unevictable = TestClearPageUnevictable(from_page);
+ from_page_flags.page_checked = PageChecked(from_page);
+ ClearPageChecked(from_page);
+ from_page_flags.page_mappedtodisk = PageMappedToDisk(from_page);
+ ClearPageMappedToDisk(from_page);
+ from_page_flags.page_dirty = PageDirty(from_page);
+ ClearPageDirty(from_page);
+ from_page_flags.page_is_young = test_and_clear_page_young(from_page);
+ from_page_flags.page_is_idle = page_is_idle(from_page);
+ clear_page_idle(from_page);
+ from_page_flags.page_swapcache = PageSwapCache(from_page);
+ from_page_flags.page_private = PagePrivate(from_page);
+ ClearPagePrivate(from_page);
+ from_page_flags.page_writeback = test_clear_page_writeback(from_page);
+
+
+ to_cpupid = page_cpupid_xchg_last(to_page, -1);
+
+ to_page_flags.page_error = TestClearPageError(to_page);
+ to_page_flags.page_referenced = TestClearPageReferenced(to_page);
+ to_page_flags.page_uptodate = PageUptodate(to_page);
+ ClearPageUptodate(to_page);
+ to_page_flags.page_active = TestClearPageActive(to_page);
+ to_page_flags.page_unevictable = TestClearPageUnevictable(to_page);
+ to_page_flags.page_checked = PageChecked(to_page);
+ ClearPageChecked(to_page);
+ to_page_flags.page_mappedtodisk = PageMappedToDisk(to_page);
+ ClearPageMappedToDisk(to_page);
+ to_page_flags.page_dirty = PageDirty(to_page);
+ ClearPageDirty(to_page);
+ to_page_flags.page_is_young = test_and_clear_page_young(to_page);
+ to_page_flags.page_is_idle = page_is_idle(to_page);
+ clear_page_idle(to_page);
+ to_page_flags.page_swapcache = PageSwapCache(to_page);
+ to_page_flags.page_private = PagePrivate(to_page);
+ ClearPagePrivate(to_page);
+ to_page_flags.page_writeback = test_clear_page_writeback(to_page);
+
+ /* set to_page */
+ if (from_page_flags.page_error)
+ SetPageError(to_page);
+ if (from_page_flags.page_referenced)
+ SetPageReferenced(to_page);
+ if (from_page_flags.page_uptodate)
+ SetPageUptodate(to_page);
+ if (from_page_flags.page_active) {
+ VM_BUG_ON_PAGE(from_page_flags.page_unevictable, from_page);
+ SetPageActive(to_page);
+ } else if (from_page_flags.page_unevictable)
+ SetPageUnevictable(to_page);
+ if (from_page_flags.page_checked)
+ SetPageChecked(to_page);
+ if (from_page_flags.page_mappedtodisk)
+ SetPageMappedToDisk(to_page);
+
+ /* Move dirty on pages not done by migrate_page_move_mapping() */
+ if (from_page_flags.page_dirty)
+ SetPageDirty(to_page);
+
+ if (from_page_flags.page_is_young)
+ set_page_young(to_page);
+ if (from_page_flags.page_is_idle)
+ set_page_idle(to_page);
+
+ /* set from_page */
+ if (to_page_flags.page_error)
+ SetPageError(from_page);
+ if (to_page_flags.page_referenced)
+ SetPageReferenced(from_page);
+ if (to_page_flags.page_uptodate)
+ SetPageUptodate(from_page);
+ if (to_page_flags.page_active) {
+ VM_BUG_ON_PAGE(to_page_flags.page_unevictable, from_page);
+ SetPageActive(from_page);
+ } else if (to_page_flags.page_unevictable)
+ SetPageUnevictable(from_page);
+ if (to_page_flags.page_checked)
+ SetPageChecked(from_page);
+ if (to_page_flags.page_mappedtodisk)
+ SetPageMappedToDisk(from_page);
+
+ /* Move dirty on pages not done by migrate_page_move_mapping() */
+ if (to_page_flags.page_dirty)
+ SetPageDirty(from_page);
+
+ if (to_page_flags.page_is_young)
+ set_page_young(from_page);
+ if (to_page_flags.page_is_idle)
+ set_page_idle(from_page);
+
+ /*
+ * Copy NUMA information to the new page, to prevent over-eager
+ * future migrations of this same page.
+ */
+ page_cpupid_xchg_last(to_page, from_cpupid);
+ page_cpupid_xchg_last(from_page, to_cpupid);
+
+ ksm_exchange_page(to_page, from_page);
+ /*
+ * Please do not reorder this without considering how mm/ksm.c's
+ * get_ksm_page() depends upon ksm_migrate_page() and PageSwapCache().
+ */
+ ClearPageSwapCache(to_page);
+ ClearPageSwapCache(from_page);
+ if (from_page_flags.page_swapcache)
+ SetPageSwapCache(to_page);
+ if (to_page_flags.page_swapcache)
+ SetPageSwapCache(from_page);
+
+
+#ifdef CONFIG_PAGE_OWNER
+ /* exchange page owner */
+ BUG();
+#endif
+ /* exchange mem cgroup */
+ to_page->mem_cgroup = from_memcg;
+ from_page->mem_cgroup = to_memcg;
+
+}
+
+/*
+ * Replace the page in the mapping.
+ *
+ * The number of remaining references must be:
+ * 1 for anonymous pages without a mapping
+ * 2 for pages with a mapping
+ * 3 for pages with a mapping and PagePrivate/PagePrivate2 set.
+ */
+
+static int exchange_page_move_mapping(struct address_space *to_mapping,
+ struct address_space *from_mapping,
+ struct page *to_page, struct page *from_page,
+ enum migrate_mode mode,
+ int to_extra_count, int from_extra_count)
+{
+ int to_expected_count = 1 + to_extra_count,
+ from_expected_count = 1 + from_extra_count;
+ unsigned long from_page_index = page_index(from_page),
+ to_page_index = page_index(to_page);
+ int to_swapbacked = PageSwapBacked(to_page),
+ from_swapbacked = PageSwapBacked(from_page);
+ struct address_space *to_mapping_value = to_page->mapping,
+ *from_mapping_value =
from_page->mapping;
+
+
+ if (!to_mapping) {
+ /* Anonymous page without mapping */
+ if (page_count(to_page) != to_expected_count)
+ return -EAGAIN;
+ }
+
+ if (!from_mapping) {
+ /* Anonymous page without mapping */
+ if (page_count(from_page) != from_expected_count)
+ return -EAGAIN;
+ }
+
+ /*
+ * Now we know that no one else is looking at the page:
+ * no turning back from here.
+ */
+ /* from_page */
+ from_page->index = to_page_index;
+ from_page->mapping = to_mapping_value;
+
+ ClearPageSwapBacked(from_page);
+ if (to_swapbacked)
+ SetPageSwapBacked(from_page);
+
+
+ /* to_page */
+ to_page->index = from_page_index;
+ to_page->mapping = from_mapping_value;
+
+ ClearPageSwapBacked(to_page);
+ if (from_swapbacked)
+ SetPageSwapBacked(to_page);
+
+ return MIGRATEPAGE_SUCCESS;
+}
+
+static int exchange_from_to_pages(struct page *to_page, struct page *from_page,
+ enum migrate_mode mode)
+{
+ int rc = -EBUSY;
+ struct address_space *to_page_mapping, *from_page_mapping;
+
+ VM_BUG_ON_PAGE(!PageLocked(from_page), from_page);
+ VM_BUG_ON_PAGE(!PageLocked(to_page), to_page);
+
+ /* copy page->mapping not use page_mapping() */
+ to_page_mapping = page_mapping(to_page);
+ from_page_mapping = page_mapping(from_page);
+
+ BUG_ON(from_page_mapping);
+ BUG_ON(to_page_mapping);
+
+ BUG_ON(PageWriteback(from_page));
+ BUG_ON(PageWriteback(to_page));
+
+ /* actual page mapping exchange */
+ rc = exchange_page_move_mapping(to_page_mapping, from_page_mapping,
+ to_page, from_page, mode, 0, 0);
+ /* actual page data exchange */
+ if (rc != MIGRATEPAGE_SUCCESS)
+ return rc;
+
+ rc = -EFAULT;
+
+ if (PageHuge(from_page) || PageTransHuge(from_page))
+ exchange_huge_page(to_page, from_page);
+ else
+ exchange_highpage(to_page, from_page);
+ rc = 0;
+
+ exchange_page_flags(to_page, from_page);
+
+ return rc;
+}
+
+static int unmap_and_exchange(struct page *from_page, struct page *to_page,
+ enum migrate_mode mode)
+{
+ int rc = -EAGAIN;
+ int from_page_was_mapped = 0, to_page_was_mapped = 0;
+ pgoff_t from_index, to_index;
+ struct anon_vma *from_anon_vma = NULL, *to_anon_vma = NULL;
+
+ /* from_page lock down */
+ if (!trylock_page(from_page)) {
+ if ((mode & MIGRATE_MODE_MASK) == MIGRATE_ASYNC)
+ goto out;
+
+ lock_page(from_page);
+ }
+
+ BUG_ON(PageWriteback(from_page));
+
+ /*
+ * By try_to_unmap(), page->mapcount goes down to 0 here. In this case,
+ * we cannot notice that anon_vma is freed while we migrates a page.
+ * This get_anon_vma() delays freeing anon_vma pointer until the end
+ * of migration. File cache pages are no problem because of page_lock()
+ * File Caches may use write_page() or lock_page() in migration, then,
+ * just care Anon page here.
+ *
+ * Only page_get_anon_vma() understands the subtleties of
+ * getting a hold on an anon_vma from outside one of its mms.
+ * But if we cannot get anon_vma, then we won't need it anyway,
+ * because that implies that the anon page is no longer mapped
+ * (and cannot be remapped so long as we hold the page lock).
+ */
+ if (PageAnon(from_page) && !PageKsm(from_page))
+ from_anon_vma = page_get_anon_vma(from_page);
+
+ /* to_page lock down */
+ if (!trylock_page(to_page)) {
+ if ((mode & MIGRATE_MODE_MASK) == MIGRATE_ASYNC)
+ goto out_unlock;
+
+ lock_page(to_page);
+ }
+
+ BUG_ON(PageWriteback(to_page));
+
+ /*
+ * By try_to_unmap(), page->mapcount goes down to 0 here. In this case,
+ * we cannot notice that anon_vma is freed while we migrates a page.
+ * This get_anon_vma() delays freeing anon_vma pointer until the end
+ * of migration. File cache pages are no problem because of page_lock()
+ * File Caches may use write_page() or lock_page() in migration, then,
+ * just care Anon page here.
+ *
+ * Only page_get_anon_vma() understands the subtleties of
+ * getting a hold on an anon_vma from outside one of its mms.
+ * But if we cannot get anon_vma, then we won't need it anyway,
+ * because that implies that the anon page is no longer mapped
+ * (and cannot be remapped so long as we hold the page lock).
+ */
+ if (PageAnon(to_page) && !PageKsm(to_page))
+ to_anon_vma = page_get_anon_vma(to_page);
+
+ from_index = from_page->index;
+ to_index = to_page->index;
+
+ /*
+ * Corner case handling:
+ * 1. When a new swap-cache page is read into, it is added to the LRU
+ * and treated as swapcache but it has no rmap yet.
+ * Calling try_to_unmap() against a page->mapping==NULL page will
+ * trigger a BUG. So handle it here.
+ * 2. An orphaned page (see truncate_complete_page) might have
+ * fs-private metadata. The page can be picked up due to memory
+ * offlining. Everywhere else except page reclaim, the page is
+ * invisible to the vm, so the page can not be migrated. So try to
+ * free the metadata, so the page can be freed.
+ */
+ if (!from_page->mapping) {
+ VM_BUG_ON_PAGE(PageAnon(from_page), from_page);
+ if (page_has_private(from_page)) {
+ try_to_free_buffers(from_page);
+ goto out_unlock_both;
+ }
+ } else if (page_mapped(from_page)) {
+ /* Establish migration ptes */
+ VM_BUG_ON_PAGE(PageAnon(from_page) && !PageKsm(from_page) &&
+ !from_anon_vma, from_page);
+ try_to_unmap(from_page,
+ TTU_MIGRATION|TTU_IGNORE_MLOCK|TTU_IGNORE_ACCESS);
+ from_page_was_mapped = 1;
+ }
+
+ if (!to_page->mapping) {
+ VM_BUG_ON_PAGE(PageAnon(to_page), to_page);
+ if (page_has_private(to_page)) {
+ try_to_free_buffers(to_page);
+ goto out_unlock_both_remove_from_migration_pte;
+ }
+ } else if (page_mapped(to_page)) {
+ /* Establish migration ptes */
+ VM_BUG_ON_PAGE(PageAnon(to_page) && !PageKsm(to_page) &&
+ !to_anon_vma, to_page);
+ try_to_unmap(to_page,
+ TTU_MIGRATION|TTU_IGNORE_MLOCK|TTU_IGNORE_ACCESS);
+ to_page_was_mapped = 1;
+ }
+
+ if (!page_mapped(from_page) && !page_mapped(to_page))
+ rc = exchange_from_to_pages(to_page, from_page, mode);
+
+ /* In remove_migration_ptes(), page_walk_vma() assumes
+ * from_page and to_page have the same index.
+ * Thus, we restore old_page->index here.
+ * Here to_page is the old_page.
+ */
+ if (to_page_was_mapped) {
+ if (rc == MIGRATEPAGE_SUCCESS)
+ swap(to_page->index, to_index);
+
+ remove_migration_ptes(to_page,
+ rc == MIGRATEPAGE_SUCCESS ? from_page : to_page, false);
+
+ if (rc == MIGRATEPAGE_SUCCESS)
+ swap(to_page->index, to_index);
+ }
+
+out_unlock_both_remove_from_migration_pte:
+ if (from_page_was_mapped) {
+ if (rc == MIGRATEPAGE_SUCCESS)
+ swap(from_page->index, from_index);
+
+ remove_migration_ptes(from_page,
+ rc == MIGRATEPAGE_SUCCESS ? to_page : from_page, false);
+
+ if (rc == MIGRATEPAGE_SUCCESS)
+ swap(from_page->index, from_index);
+ }
+
+
+
+out_unlock_both:
+ if (to_anon_vma)
+ put_anon_vma(to_anon_vma);
+ unlock_page(to_page);
+out_unlock:
+ /* Drop an anon_vma reference if we took one */
+ if (from_anon_vma)
+ put_anon_vma(from_anon_vma);
+ unlock_page(from_page);
+out:
+
+ return rc;
+}
+
+/*
+ * Exchange pages in the exchange_list
+ *
+ * Caller should release the exchange_list resource.
+ *
+ * */
+int exchange_pages(struct list_head *exchange_list,
+ enum migrate_mode mode,
+ int reason)
+{
+ struct exchange_page_info *one_pair, *one_pair2;
+ int failed = 0;
+
+ list_for_each_entry_safe(one_pair, one_pair2, exchange_list, list) {
+ struct page *from_page = one_pair->from_page;
+ struct page *to_page = one_pair->to_page;
+ int rc;
+ int retry = 0;
+
+again:
+ if (page_count(from_page) == 1) {
+ /* page was freed from under us. So we are done */
+ ClearPageActive(from_page);
+ ClearPageUnevictable(from_page);
+
+ put_page(from_page);
+ dec_node_page_state(from_page, NR_ISOLATED_ANON +
+ page_is_file_cache(from_page));
+
+ if (page_count(to_page) == 1) {
+ ClearPageActive(to_page);
+ ClearPageUnevictable(to_page);
+ put_page(to_page);
+ } else
+ goto putback_to_page;
+
+ continue;
+ }
+
+ if (page_count(to_page) == 1) {
+ /* page was freed from under us. So we are done */
+ ClearPageActive(to_page);
+ ClearPageUnevictable(to_page);
+
+ put_page(to_page);
+
+ dec_node_page_state(to_page, NR_ISOLATED_ANON +
+ page_is_file_cache(to_page));
+
+ dec_node_page_state(from_page, NR_ISOLATED_ANON +
+ page_is_file_cache(from_page));
+ putback_lru_page(from_page);
+ continue;
+ }
+
+ /* TODO: compound page not supported */
+ if (PageCompound(from_page) || page_mapping(from_page)) {
+ ++failed;
+ goto putback;
+ }
+
+ rc = unmap_and_exchange(from_page, to_page, mode);
+
+ if (rc == -EAGAIN && retry < 3) {
+ ++retry;
+ goto again;
+ }
+
+ if (rc != MIGRATEPAGE_SUCCESS)
+ ++failed;
+
+putback:
+ dec_node_page_state(from_page, NR_ISOLATED_ANON +
+ page_is_file_cache(from_page));
+
+ putback_lru_page(from_page);
+putback_to_page:
+ dec_node_page_state(to_page, NR_ISOLATED_ANON +
+ page_is_file_cache(to_page));
+
+ putback_lru_page(to_page);
+
+ }
+ return failed;
+}
diff --git a/mm/ksm.c b/mm/ksm.c
index fc64874..e5b492b 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -2716,6 +2716,41 @@ void ksm_migrate_page(struct page *newpage, struct page
*oldpage)
set_page_stable_node(oldpage, NULL);
}
}
+
+void ksm_exchange_page(struct page *to_page, struct page *from_page)
+{
+ struct stable_node *to_stable_node, *from_stable_node;
+
+ VM_BUG_ON_PAGE(!PageLocked(to_page), to_page);
+ VM_BUG_ON_PAGE(!PageLocked(from_page), from_page);
+
+ to_stable_node = page_stable_node(to_page);
+ from_stable_node = page_stable_node(from_page);
+ if (to_stable_node) {
+ VM_BUG_ON_PAGE(to_stable_node->kpfn != page_to_pfn(from_page),
+ from_page);
+ to_stable_node->kpfn = page_to_pfn(to_page);
+ /*
+ * newpage->mapping was set in advance; now we need smp_wmb()
+ * to make sure that the new stable_node->kpfn is visible
+ * to get_ksm_page() before it can see that oldpage->mapping
+ * has gone stale (or that PageSwapCache has been cleared).
+ */
+ smp_wmb();
+ }
+ if (from_stable_node) {
+ VM_BUG_ON_PAGE(from_stable_node->kpfn != page_to_pfn(to_page),
+ to_page);
+ from_stable_node->kpfn = page_to_pfn(from_page);
+ /*
+ * newpage->mapping was set in advance; now we need smp_wmb()
+ * to make sure that the new stable_node->kpfn is visible
+ * to get_ksm_page() before it can see that oldpage->mapping
+ * has gone stale (or that PageSwapCache has been cleared).
+ */
+ smp_wmb();
+ }
+}
#endif /* CONFIG_MIGRATION */
#ifdef CONFIG_MEMORY_HOTREMOVE
--
2.7.4
