Ksm is driver that allow merging identical pages between one or more
applications in way unvisible to the application that use it.
Pages that are merged are marked as readonly and are COWed when any
application try to change them.
Ksm is used for cases where using fork() is not suitable,
one of this cases is where the pages of the application keep changing
dynamicly and the application cannot know in advance what pages are
going to be identical.
Ksm works by walking over the memory pages of the applications it
scan in order to find identical pages.
It uses a two sorted data strctures called stable and unstable trees
to find in effective way the identical pages.
When ksm finds two identical pages, it marks them as readonly and merges
them into single one page,
after the pages are marked as readonly and merged into one page, linux
will treat this pages as normal copy_on_write pages and will fork them
when write access will happen to them.
Ksm scan just memory areas that were registred to be scanned by it.
Ksm api:
KSM_GET_API_VERSION:
Give the userspace the api version of the module.
KSM_CREATE_SHARED_MEMORY_AREA:
Create shared memory reagion fd, that latter allow the user to register
the memory region to scan by using:
KSM_REGISTER_MEMORY_REGION and KSM_REMOVE_MEMORY_REGION
KSM_START_STOP_KTHREAD:
Return information about the kernel thread, the inforamtion is returned
using the ksm_kthread_info structure:
ksm_kthread_info:
__u32 sleep:
number of microsecoends to sleep between each iteration of
scanning.
__u32 pages_to_scan:
number of pages to scan for each iteration of scanning.
__u32 max_pages_to_merge:
maximum number of pages to merge in each iteration of scanning
(so even if there are still more pages to scan, we stop this
iteration)
__u32 flags:
flags to control ksmd (right now just ksm_control_flags_run
available)
KSM_REGISTER_MEMORY_REGION:
Register userspace virtual address range to be scanned by ksm.
This ioctl is using the ksm_memory_region structure:
ksm_memory_region:
__u32 npages;
number of pages to share inside this memory region.
__u32 pad;
__u64 addr:
the begining of the virtual address of this region.
KSM_REMOVE_MEMORY_REGION:
Remove memory region from ksm.
Signed-off-by: Izik Eidus <iei...@redhat.com>
---
include/linux/ksm.h | 69 +++
include/linux/miscdevice.h | 1 +
mm/Kconfig | 6 +
mm/Makefile | 1 +
mm/ksm.c | 1431 ++++++++++++++++++++++++++++++++++++++++++++
5 files changed, 1508 insertions(+), 0 deletions(-)
create mode 100644 include/linux/ksm.h
create mode 100644 mm/ksm.c
diff --git a/include/linux/ksm.h b/include/linux/ksm.h
new file mode 100644
index 0000000..5776dce
--- /dev/null
+++ b/include/linux/ksm.h
@@ -0,0 +1,69 @@
+#ifndef __LINUX_KSM_H
+#define __LINUX_KSM_H
+
+/*
+ * Userspace interface for /dev/ksm - kvm shared memory
+ */
+
+#include <linux/types.h>
+#include <linux/ioctl.h>
+
+#include <asm/types.h>
+
+#define KSM_API_VERSION 1
+
+#define ksm_control_flags_run 1
+
+/* for KSM_REGISTER_MEMORY_REGION */
+struct ksm_memory_region {
+ __u32 npages; /* number of pages to share */
+ __u32 pad;
+ __u64 addr; /* the begining of the virtual address */
+ __u64 reserved_bits;
+};
+
+struct ksm_kthread_info {
+ __u32 sleep; /* number of microsecoends to sleep */
+ __u32 pages_to_scan; /* number of pages to scan */
+ __u32 flags; /* control flags */
+ __u32 pad;
+ __u64 reserved_bits;
+};
+
+#define KSMIO 0xAB
+
+/* ioctls for /dev/ksm */
+
+#define KSM_GET_API_VERSION _IO(KSMIO, 0x00)
+/*
+ * KSM_CREATE_SHARED_MEMORY_AREA - create the shared memory reagion fd
+ */
+#define KSM_CREATE_SHARED_MEMORY_AREA _IO(KSMIO, 0x01) /* return SMA fd */
+/*
+ * KSM_START_STOP_KTHREAD - control the kernel thread scanning speed
+ * (can stop the kernel thread from working by setting running = 0)
+ */
+#define KSM_START_STOP_KTHREAD _IOW(KSMIO, 0x02,\
+ struct ksm_kthread_info)
+/*
+ * KSM_GET_INFO_KTHREAD - return information about the kernel thread
+ * scanning speed.
+ */
+#define KSM_GET_INFO_KTHREAD _IOW(KSMIO, 0x03,\
+ struct ksm_kthread_info)
+
+
+/* ioctls for SMA fds */
+
+/*
+ * KSM_REGISTER_MEMORY_REGION - register virtual address memory area to be
+ * scanned by kvm.
+ */
+#define KSM_REGISTER_MEMORY_REGION _IOW(KSMIO, 0x20,\
+ struct ksm_memory_region)
+/*
+ * KSM_REMOVE_MEMORY_REGION - remove virtual address memory area from ksm.
+ */
+#define KSM_REMOVE_MEMORY_REGION _IO(KSMIO, 0x21)
+
+#endif
diff --git a/include/linux/miscdevice.h b/include/linux/miscdevice.h
index a820f81..6d4f8df 100644
--- a/include/linux/miscdevice.h
+++ b/include/linux/miscdevice.h
@@ -29,6 +29,7 @@
#define HPET_MINOR 228
#define FUSE_MINOR 229
#define KVM_MINOR 232
+#define KSM_MINOR 233
#define MISC_DYNAMIC_MINOR 255
struct device;
diff --git a/mm/Kconfig b/mm/Kconfig
index a5b7781..2818223 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -216,3 +216,9 @@ config UNEVICTABLE_LRU
config MMU_NOTIFIER
bool
+
+config KSM
+ tristate "Enable KSM for page sharing"
+ help
+ Enable the KSM kernel module to allow page sharing of equal pages
+ among different tasks.
diff --git a/mm/Makefile b/mm/Makefile
index 72255be..e3bf7bf 100644
--- a/mm/Makefile
+++ b/mm/Makefile
@@ -24,6 +24,7 @@ obj-$(CONFIG_SPARSEMEM_VMEMMAP) += sparse-vmemmap.o
obj-$(CONFIG_TMPFS_POSIX_ACL) += shmem_acl.o
obj-$(CONFIG_SLOB) += slob.o
obj-$(CONFIG_MMU_NOTIFIER) += mmu_notifier.o
+obj-$(CONFIG_KSM) += ksm.o
obj-$(CONFIG_SLAB) += slab.o
obj-$(CONFIG_SLUB) += slub.o
obj-$(CONFIG_FAILSLAB) += failslab.o
diff --git a/mm/ksm.c b/mm/ksm.c
new file mode 100644
index 0000000..eba4c09
--- /dev/null
+++ b/mm/ksm.c
@@ -0,0 +1,1431 @@
+/*
+ * Memory merging driver for Linux
+ *
+ * This module enables dynamic sharing of identical pages found in different
+ * memory areas, even if they are not shared by fork()
+ *
+ * Copyright (C) 2008 Red Hat, Inc.
+ * Authors:
+ * Izik Eidus
+ * Andrea Arcangeli
+ * Chris Wright
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2.
+ */
+
+#include <linux/module.h>
+#include <linux/errno.h>
+#include <linux/mm.h>
+#include <linux/fs.h>
+#include <linux/miscdevice.h>
+#include <linux/vmalloc.h>
+#include <linux/file.h>
+#include <linux/mman.h>
+#include <linux/sched.h>
+#include <linux/rwsem.h>
+#include <linux/pagemap.h>
+#include <linux/sched.h>
+#include <linux/rmap.h>
+#include <linux/spinlock.h>
+#include <linux/jhash.h>
+#include <linux/delay.h>
+#include <linux/kthread.h>
+#include <linux/wait.h>
+#include <linux/scatterlist.h>
+#include <linux/random.h>
+#include <linux/slab.h>
+#include <linux/swap.h>
+#include <linux/rbtree.h>
+#include <linux/anon_inodes.h>
+#include <linux/ksm.h>
+
+#include <asm/tlbflush.h>
+
+MODULE_AUTHOR("Red Hat, Inc.");
+MODULE_LICENSE("GPL");
+
+static int rmap_hash_size;
+module_param(rmap_hash_size, int, 0);
+MODULE_PARM_DESC(rmap_hash_size, "Hash table size for the reverse mapping");
+
+/*
+ * ksm_mem_slot - hold information for an userspace scanning range
+ * (the scanning for this region will be from addr untill addr +
+ * npages * PAGE_SIZE inside mm)
+ */
+struct ksm_mem_slot {
+ struct list_head link;
+ struct list_head sma_link;
+ struct mm_struct *mm;
+ unsigned long addr; /* the begining of the virtual address */
+ unsigned npages; /* number of pages to share */
+};
+
+/*
+ * ksm_sma - shared memory area, each process have its own sma that contain the
+ * information about the slots that it own
+ */
+struct ksm_sma {
+ struct list_head sma_slots;
+};
+
+/**
+ * struct ksm_scan - cursor for scanning
+ * @slot_index: the current slot we are scanning
+ * @page_index: the page inside the sma that is currently being scanned
+ *
+ * ksm uses it to know what are the next pages it need to scan
+ */
+struct ksm_scan {
+ struct ksm_mem_slot *slot_index;
+ unsigned long page_index;
+};
+
+/*
+ * Few notes about ksm scanning progress (make it easier to understand the
+ * data structures below):
+ *
+ * In order to reduce excessive scanning, ksm sort the memory pages by their
+ * contents into a data strcture that hold pointer into the pages.
+ *
+ * Since the contents of the pages may change at any moment, ksm cant just
+ * insert the pages into normal sorted tree and expect it to find anything.
+ *
+ * For this purpuse ksm use two data strctures - stable and unstable trees,
+ * the stable tree hold pointers into all the merged pages (KsmPage) sorted by
+ * their contents, beacuse that each such page have to be write-protected,
+ * searching on this tree is fully assuranced to be working and therefore this
+ * tree is called the stable tree.
+ *
+ * In addition to the stable tree, ksm use another data strcture called the
+ * unstable tree, this specific tree hold pointers into pages that have
+ * been found to be "unchanged for period of time", the unstable tree sort this
+ * pages by their contents, but given the fact that this pages are not
+ * write-protected, ksm cant trust the unstable tree to be fully assuranced to
+ * work.
+ * For the reason that the unstable tree would become corrupted when some of
+ * the page inside itself would change, the tree is called unstable.
+ * Ksm solve this problem by two ways:
+ * 1) the unstable tree get flushed every time ksm finish to scan the whole
+ * memory, and then the tree is rebuild from the begining.
+ * 2) Ksm will only insert into the unstable tree, pages that their hash value
+ * was not changed during the whole progress of one circuler scanning of the
+ * memory.
+ * 3) The unstable tree is RedBlack Tree - meaning its balancing is based on
+ * the colors of the nodes and not their content, this assure that even when
+ * the tree get "corrupted" we wont get out of balance and the timing of
+ * scanning is the same, another issue is that searching and inserting nodes
+ * into rbtree is the same algorithem, therefore we have no overhead when we
+ * flush the tree and rebuild it.
+ * 4) Ksm never flush the stable tree, this mean that even if it would take 10
+ * times to find page inside the unstable tree, as soon as we would find it,
+ * it will be secured inside the stable tree,
+ * (When we scan new page, we first compare it against the stable tree, and
+ * then against the unstable tree)
+ */
+
+struct rmap_item;
+
+/*
+ * tree_item - object of the stable and unstable trees
+ */
+struct tree_item {
+ struct rb_node node;
+ struct rmap_item *rmap_item;
+};
+
+/*
+ * rmap_item - object of the rmap_hash hash table
+ * (it is holding the previous hash value (oldindex),
+ * pointer into the page_hash_item, and pointer into the tree_item)
+ */
+
+/**
+ * struct rmap_item - reverse mapping item for virtual addresses
+ * @link: link into the rmap_hash hash table.
+ * @mm: the memory strcture the rmap_item is pointing to.
+ * @address: the virtual address the rmap_item is pointing to.
+ * @oldchecksum: old checksum result for the page belong the virtual address
+ * @stable_tree: when 1 rmap_item is used for stable_tree, 0 unstable tree
+ * @tree_item: pointer into the stable/unstable tree that hold the virtual
+ * address that the rmap_item is pointing to.
+ * @next: the next rmap item inside the stable/unstable tree that have that is
+ * found inside the same tree node.
+ */
+
+struct rmap_item {
+ struct hlist_node link;
+ struct mm_struct *mm;
+ unsigned long address;
+ unsigned int oldchecksum; /* old checksum value */
+ unsigned char stable_tree; /* 1 stable_tree 0 unstable tree */
+ struct tree_item *tree_item;
+ struct rmap_item *next;
+ struct rmap_item *prev;
+};
+
+/*
+ * slots is linked list that hold all the memory regions that were registred
+ * to be scanned.
+ */
+static LIST_HEAD(slots);
+/*
+ * slots_lock protect against removing and adding memory regions while a
scanner
+ * is in the middle of scanning.
+ */
+static DECLARE_RWSEM(slots_lock);
+
+/* The stable and unstable trees heads. */
+struct rb_root root_stable_tree = RB_ROOT;
+struct rb_root root_unstable_tree = RB_ROOT;
+
+
+/* The number of linked list members inside the hash table */
+static int nrmaps_hash;
+/* rmap_hash hash table */
+static struct hlist_head *rmap_hash;
+
+static struct kmem_cache *tree_item_cache;
+static struct kmem_cache *rmap_item_cache;
+
+static int kthread_sleep; /* sleep time of the kernel thread */
+static int kthread_pages_to_scan; /* npages to scan for the kernel thread */
+static struct ksm_scan kthread_ksm_scan;
+static int ksmd_flags;
+static struct task_struct *kthread;
+static DECLARE_WAIT_QUEUE_HEAD(kthread_wait);
+static DECLARE_RWSEM(kthread_lock);
+
+static int ksm_slab_init(void)
+{
+ int ret = -ENOMEM;
+
+ tree_item_cache = KMEM_CACHE(tree_item, 0);
+ if (!tree_item_cache)
+ goto out;
+
+ rmap_item_cache = KMEM_CACHE(rmap_item, 0);
+ if (!rmap_item_cache)
+ goto out_free;
+
+ return 0;
+
+out_free:
+ kmem_cache_destroy(tree_item_cache);
+out:
+ return ret;
+}
+
+static void ksm_slab_free(void)
+{
+ kmem_cache_destroy(rmap_item_cache);
+ kmem_cache_destroy(tree_item_cache);
+}
+
+static inline struct tree_item *alloc_tree_item(void)
+{
+ return kmem_cache_zalloc(tree_item_cache, GFP_KERNEL);
+}
+
+static void free_tree_item(struct tree_item *tree_item)
+{
+ kmem_cache_free(tree_item_cache, tree_item);
+}
+
+static inline struct rmap_item *alloc_rmap_item(void)
+{
+ return kmem_cache_zalloc(rmap_item_cache, GFP_KERNEL);
+}
+
+static inline void free_rmap_item(struct rmap_item *rmap_item)
+{
+ kmem_cache_free(rmap_item_cache, rmap_item);
+}
+
+/*
+ * PageKsm - this type of pages are the write protected pages that ksm map
+ * into multiple vmas (this is the "shared page")
+ * this page was allocated using alloc_page(), and every pte that point to it
+ * is always write protected (therefore its data content cant ever be changed)
+ * and this page cant be swapped.
+ */
+static inline int PageKsm(struct page *page)
+{
+ /*
+ * When ksm create new shared page, it create kernel allocated page
+ * using alloc_page(), therefore this page is not anonymous, taking into
+ * account that ksm scan just anonymous pages, we can relay on the fact
+ * that each time we see !PageAnon(page) we are hitting shared page.
+ */
+ return !PageAnon(page);
+}
+
+static int rmap_hash_init(void)
+{
+ if (!rmap_hash_size) {
+ struct sysinfo sinfo;
+
+ si_meminfo(&sinfo);
+ rmap_hash_size = sinfo.totalram / 10;
+ }
+ nrmaps_hash = rmap_hash_size;
+ rmap_hash = vmalloc(nrmaps_hash * sizeof(struct hlist_head));
+ if (!rmap_hash)
+ return -ENOMEM;
+ memset(rmap_hash, 0, nrmaps_hash * sizeof(struct hlist_head));
+ return 0;
+}
+
+static void rmap_hash_free(void)
+{
+ int i;
+ struct hlist_head *bucket;
+ struct hlist_node *node, *n;
+ struct rmap_item *rmap_item;
+
+ for (i = 0; i < nrmaps_hash; ++i) {
+ bucket = &rmap_hash[i];
+ hlist_for_each_entry_safe(rmap_item, node, n, bucket, link) {
+ hlist_del(&rmap_item->link);
+ free_rmap_item(rmap_item);
+ }
+ }
+ vfree(rmap_hash);
+}
+
+static inline u32 calc_checksum(struct page *page)
+{
+ u32 checksum;
+ void *addr = kmap_atomic(page, KM_USER0);
+ checksum = jhash(addr, PAGE_SIZE, 17);
+ kunmap_atomic(addr, KM_USER0);
+ return checksum;
+}
+
+/*
+ * Return rmap_item for a given virtual address.
+ */
+static struct rmap_item *get_rmap_item(struct mm_struct *mm, unsigned long
addr)
+{
+ struct rmap_item *rmap_item;
+ struct hlist_head *bucket;
+ struct hlist_node *node;
+
+ bucket = &rmap_hash[addr % nrmaps_hash];
+ hlist_for_each_entry(rmap_item, node, bucket, link) {
+ if (mm == rmap_item->mm && rmap_item->address == addr) {
+ return rmap_item;
+ }
+ }
+ return NULL;
+}
+
+/*
+ * Removing rmap_item from stable or unstable tree.
+ * This function will free the rmap_item object, and if that rmap_item was
+ * insde the stable or unstable trees, it would remove the link from there
+ * as well.
+ */
+static void remove_rmap_item_from_tree(struct rmap_item *rmap_item)
+{
+ struct tree_item *tree_item;
+
+ tree_item = rmap_item->tree_item;
+ rmap_item->tree_item = NULL;
+
+ if (rmap_item->stable_tree) {
+ if (rmap_item->prev) {
+ BUG_ON(rmap_item->prev->next != rmap_item);
+ rmap_item->prev->next = rmap_item->next;
+ }
+ if (rmap_item->next) {
+ BUG_ON(rmap_item->next->prev != rmap_item);
+ rmap_item->next->prev = rmap_item->prev;
+ }
+ }
+
+ if (tree_item) {
+ if (rmap_item->stable_tree) {
+ if (!rmap_item->next && !rmap_item->prev) {
+ rb_erase(&tree_item->node, &root_stable_tree);
+ free_tree_item(tree_item);
+ } else if (!rmap_item->prev) {
+ tree_item->rmap_item = rmap_item->next;
+ } else {
+ tree_item->rmap_item = rmap_item->prev;
+ }
+ } else if (!rmap_item->stable_tree) {
+ free_tree_item(tree_item);
+ }
+ }
+
+ hlist_del(&rmap_item->link);
+ free_rmap_item(rmap_item);
+}
+
+static void remove_page_from_tree(struct mm_struct *mm,
+ unsigned long addr)
+{
+ struct rmap_item *rmap_item;
+
+ rmap_item = get_rmap_item(mm, addr);
+ if (!rmap_item)
+ return;
+ remove_rmap_item_from_tree(rmap_item);
+ return;
+}
+
+static int ksm_sma_ioctl_register_memory_region(struct ksm_sma *ksm_sma,
+ struct ksm_memory_region *mem)
+{
+ struct ksm_mem_slot *slot;
+ int ret = -EPERM;
+
+ slot = kzalloc(sizeof(struct ksm_mem_slot), GFP_KERNEL);
+ if (!slot) {
+ ret = -ENOMEM;
+ goto out;
+ }
+
+ slot->mm = get_task_mm(current);
+ if (!slot->mm)
+ goto out_free;
+ slot->addr = mem->addr;
+ slot->npages = mem->npages;
+
+ down_write(&slots_lock);
+
+ list_add_tail(&slot->link, &slots);
+ list_add_tail(&slot->sma_link, &ksm_sma->sma_slots);
+
+ up_write(&slots_lock);
+ return 0;
+
+out_free:
+ kfree(slot);
+out:
+ return ret;
+}
+
+static void remove_mm_from_hash_and_tree(struct mm_struct *mm)
+{
+ struct ksm_mem_slot *slot;
+ int pages_count;
+
+ list_for_each_entry(slot, &slots, link)
+ if (slot->mm == mm)
+ break;
+ BUG_ON(!slot);
+
+ root_unstable_tree = RB_ROOT;
+ for (pages_count = 0; pages_count < slot->npages; ++pages_count)
+ remove_page_from_tree(mm, slot->addr +
+ pages_count * PAGE_SIZE);
+ list_del(&slot->link);
+}
+
+static int ksm_sma_ioctl_remove_memory_region(struct ksm_sma *ksm_sma)
+{
+ struct ksm_mem_slot *slot, *node;
+
+ down_write(&slots_lock);
+ list_for_each_entry_safe(slot, node, &ksm_sma->sma_slots, sma_link) {
+ remove_mm_from_hash_and_tree(slot->mm);
+ mmput(slot->mm);
+ list_del(&slot->sma_link);
+ kfree(slot);
+ }
+ up_write(&slots_lock);
+ return 0;
+}
+
+static int ksm_sma_release(struct inode *inode, struct file *filp)
+{
+ struct ksm_sma *ksm_sma = filp->private_data;
+ int r;
+
+ r = ksm_sma_ioctl_remove_memory_region(ksm_sma);
+ kfree(ksm_sma);
+ return r;
+}
+
+static long ksm_sma_ioctl(struct file *filp,
+ unsigned int ioctl, unsigned long arg)
+{
+ struct ksm_sma *sma = filp->private_data;
+ void __user *argp = (void __user *)arg;
+ int r = EINVAL;
+
+ switch (ioctl) {
+ case KSM_REGISTER_MEMORY_REGION: {
+ struct ksm_memory_region ksm_memory_region;
+
+ r = -EFAULT;
+ if (copy_from_user(&ksm_memory_region, argp,
+ sizeof(ksm_memory_region)))
+ goto out;
+ r = ksm_sma_ioctl_register_memory_region(sma,
+ &ksm_memory_region);
+ break;
+ }
+ case KSM_REMOVE_MEMORY_REGION:
+ r = ksm_sma_ioctl_remove_memory_region(sma);
+ break;
+ }
+
+out:
+ return r;
+}
+
+static unsigned long addr_in_vma(struct vm_area_struct *vma, struct page *page)
+{
+ pgoff_t pgoff = page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT);
+ unsigned long addr;
+
+ addr = vma->vm_start + ((pgoff - vma->vm_pgoff) << PAGE_SHIFT);
+ if (unlikely(addr < vma->vm_start || addr >= vma->vm_end))
+ return -EFAULT;
+ return addr;
+}
+
+static pte_t *get_pte(struct mm_struct *mm, unsigned long addr)
+{
+ pgd_t *pgd;
+ pud_t *pud;
+ pmd_t *pmd;
+ pte_t *ptep = NULL;
+
+ pgd = pgd_offset(mm, addr);
+ if (!pgd_present(*pgd))
+ goto out;
+
+ pud = pud_offset(pgd, addr);
+ if (!pud_present(*pud))
+ goto out;
+
+ pmd = pmd_offset(pud, addr);
+ if (!pmd_present(*pmd))
+ goto out;
+
+ ptep = pte_offset_map(pmd, addr);
+out:
+ return ptep;
+}
+
+static int is_present_pte(struct mm_struct *mm, unsigned long addr)
+{
+ pte_t *ptep;
+ int r;
+
+ ptep = get_pte(mm, addr);
+ if (!ptep)
+ return 0;
+
+ r = pte_present(*ptep);
+ pte_unmap(ptep);
+
+ return r;
+}
+
+static int memcmp_pages(struct page *page1, struct page *page2)
+{
+ char *addr1, *addr2;
+ int r;
+
+ addr1 = kmap_atomic(page1, KM_USER0);
+ addr2 = kmap_atomic(page2, KM_USER1);
+ r = memcmp(addr1, addr2, PAGE_SIZE);
+ kunmap_atomic(addr1, KM_USER0);
+ kunmap_atomic(addr2, KM_USER1);
+ return r;
+}
+
+/* pages_identical
+ * return 1 if identical, 0 otherwise.
+ */
+static inline int pages_identical(struct page *page1, struct page *page2)
+{
+ return !memcmp_pages(page1, page2);
+}
+
+/*
+ * try_to_merge_one_page - take two pages and merge them into one
+ * @mm: mm_struct that hold vma pointing into oldpage
+ * @vma: the vma that hold the pte pointing into oldpage
+ * @oldpage: the page that we want to replace with newpage
+ * @newpage: the page that we want to map instead of oldpage
+ * @newprot: the new permission of the pte inside vma
+ * note:
+ * oldpage should be anon page while newpage should be file mapped page
+ *
+ * this function return 0 if the pages were merged, 1 otherwise.
+ */
+static int try_to_merge_one_page(struct mm_struct *mm,
+ struct vm_area_struct *vma,
+ struct page *oldpage,
+ struct page *newpage,
+ pgprot_t newprot)
+{
+ int ret = 1;
+ int odirect_sync;
+ unsigned long page_addr_in_vma;
+ pte_t orig_pte, *orig_ptep;
+
+ get_page(newpage);
+ get_page(oldpage);
+
+ down_read(&mm->mmap_sem);
+
+ page_addr_in_vma = addr_in_vma(vma, oldpage);
+ if (page_addr_in_vma == -EFAULT)
+ goto out_unlock;
+
+ orig_ptep = get_pte(mm, page_addr_in_vma);
+ if (!orig_ptep)
+ goto out_unlock;
+ orig_pte = *orig_ptep;
+ pte_unmap(orig_ptep);
+ if (!pte_present(orig_pte))
+ goto out_unlock;
+ if (page_to_pfn(oldpage) != pte_pfn(orig_pte))
+ goto out_unlock;
+ /*
+ * we need the page lock to read a stable PageSwapCache in
+ * page_wrprotect()
+ */
+ if (!trylock_page(oldpage))
+ goto out_unlock;
+ /*
+ * page_wrprotect check if the page is swapped or in swap cache,
+ * in the future we might want to run here if_present_pte and then
+ * swap_free
+ */
+ if (!page_wrprotect(oldpage, &odirect_sync, 2)) {
+ unlock_page(oldpage);
+ goto out_unlock;
+ }
+ unlock_page(oldpage);
+ if (!odirect_sync)
+ goto out_unlock;
+
+ orig_pte = pte_wrprotect(orig_pte);
+
+ if (pages_identical(oldpage, newpage))
+ ret = replace_page(vma, oldpage, newpage, orig_pte, newprot);
+
+out_unlock:
+ up_read(&mm->mmap_sem);
+ put_page(oldpage);
+ put_page(newpage);
+ return ret;
+}
+
+/*
+ * try_to_merge_two_pages - take two identical pages and prepare them to be
+ * merged into one page.
+ *
+ * this function return 0 if we successfully mapped two identical pages into
one
+ * page, 1 otherwise.
+ * (note in case we created KsmPage and mapped one page into it but the second
+ * page was not mapped we consider it as a failure and return 1)
+ */
+static int try_to_merge_two_pages(struct mm_struct *mm1, struct page *page1,
+ struct mm_struct *mm2, struct page *page2,
+ unsigned long addr1, unsigned long addr2)
+{
+ struct vm_area_struct *vma;
+ pgprot_t prot;
+ int ret = 1;
+
+ /*
+ * If page2 isn't shared (it isn't PageKsm) we have to allocate a new
+ * file mapped page and make the two ptes of mm1(page1) and mm2(page2)
+ * point to it. If page2 is shared, we can just make the pte of
+ * mm1(page1) point to page2
+ */
+ if (PageKsm(page2)) {
+ down_read(&mm1->mmap_sem);
+ vma = find_vma(mm1, addr1);
+ up_read(&mm1->mmap_sem);
+ if (!vma)
+ return ret;
+ prot = vma->vm_page_prot;
+ pgprot_val(prot) &= ~_PAGE_RW;
+ ret = try_to_merge_one_page(mm1, vma, page1, page2, prot);
+ } else {
+ struct page *kpage;
+
+ kpage = alloc_page(GFP_HIGHUSER);
+ if (!kpage)
+ return ret;
+ down_read(&mm1->mmap_sem);
+ vma = find_vma(mm1, addr1);
+ up_read(&mm1->mmap_sem);
+ if (!vma) {
+ put_page(kpage);
+ return ret;
+ }
+ prot = vma->vm_page_prot;
+ pgprot_val(prot) &= ~_PAGE_RW;
+
+ copy_user_highpage(kpage, page1, addr1, vma);
+ ret = try_to_merge_one_page(mm1, vma, page1, kpage, prot);
+
+ if (!ret) {
+ down_read(&mm2->mmap_sem);
+ vma = find_vma(mm2, addr2);
+ up_read(&mm2->mmap_sem);
+ if (!vma) {
+ put_page(kpage);
+ ret = 1;
+ return ret;
+ }
+
+ prot = vma->vm_page_prot;
+ pgprot_val(prot) &= ~_PAGE_RW;
+
+ ret = try_to_merge_one_page(mm2, vma, page2, kpage,
+ prot);
+ /*
+ * If the secoend try_to_merge_one_page call was failed,
+ * we are in situation where we have Ksm page that have
+ * just one pte pointing to it, in this case we break
+ * it.
+ */
+ if (ret) {
+ struct page *tmppage[1];
+
+ down_read(&mm1->mmap_sem);
+ if (get_user_pages(current, mm1, addr1, 1, 1,
+ 0, tmppage, NULL)) {
+ put_page(tmppage[0]);
+ }
+ up_read(&mm1->mmap_sem);
+ }
+ }
+ put_page(kpage);
+ }
+ return ret;
+}
