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Date: Mon, 28 Mar 2011 22:17:44 +0800
From: Nai Xia <nai.xia@...il.com>
To: "linux-kernel" <linux-kernel@...r.kernel.org>
Cc: Izik Eidus <izik.eidus@...ellosystems.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Hugh Dickins <hughd@...gle.com>,
Johannes Weiner <hannes@...xchg.org>,
Chris Wright <chrisw@...s-sol.org>,
Andrea Arcangeli <aarcange@...hat.com>,
Rik van Riel <riel@...hat.com>,
"linux-mm" <linux-mm@...ck.org>
Subject: [PATCH 2/2] ksm: take dirty bit as reference to avoid volatile pages
Introduced ksm_page_changed() to reference the dirty bit of a pte. We clear
the dirty bit for each pte scanned but don't flush the tlb. For a huge page,
if one of the subpage has changed, we try to skip the whole huge page
assuming(this is true by now) that ksmd linearly scans the address space.
A NEW_FLAG is also introduced as a status of rmap_item to make ksmd scan
more aggressively for new VMAs - only skip the pages considered to be volatile
by the dirty bits.
Suggested-by: Izik Eidus <izik.eidus@...ellosystems.com>
Signed-off-by: Nai Xia <nai.xia@...il.com>
---
diff --git a/mm/ksm.c b/mm/ksm.c
index c2b2a94..2350cc6 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -107,6 +107,7 @@ struct ksm_scan {
unsigned long address;
struct rmap_item **rmap_list;
unsigned long seqnr;
+ unsigned long huge_skip; /* if a huge pte is dirty skip page */
};
/**
@@ -150,6 +151,7 @@ struct rmap_item {
#define SEQNR_MASK 0x0ff /* low bits of unstable tree seqnr */
#define UNSTABLE_FLAG 0x100 /* is a node of the unstable tree */
#define STABLE_FLAG 0x200 /* is listed from the stable tree */
+#define NEW_FLAG 0x400 /* this rmap_item is new */
/* The stable and unstable tree heads */
static struct rb_root root_stable_tree = RB_ROOT;
@@ -301,6 +303,11 @@ static inline int in_stable_tree(struct rmap_item *rmap_item)
return rmap_item->address & STABLE_FLAG;
}
+static inline unsigned long get_address(struct rmap_item *rmap_item)
+{
+ return rmap_item->address & PAGE_MASK;
+}
+
static void hold_anon_vma(struct rmap_item *rmap_item,
struct anon_vma *anon_vma)
{
@@ -390,7 +397,7 @@ static int break_ksm(struct vm_area_struct *vma, unsigned long addr)
static void break_cow(struct rmap_item *rmap_item)
{
struct mm_struct *mm = rmap_item->mm;
- unsigned long addr = rmap_item->address;
+ unsigned long addr = get_address(rmap_item);
struct vm_area_struct *vma;
/*
@@ -429,7 +436,7 @@ static struct page *page_trans_compound_anon(struct page *page)
static struct page *get_mergeable_page(struct rmap_item *rmap_item)
{
struct mm_struct *mm = rmap_item->mm;
- unsigned long addr = rmap_item->address;
+ unsigned long addr = get_address(rmap_item);
struct vm_area_struct *vma;
struct page *page;
@@ -467,7 +474,7 @@ static void remove_node_from_stable_tree(struct stable_node *stable_node)
else
ksm_pages_shared--;
ksm_drop_anon_vma(rmap_item);
- rmap_item->address &= PAGE_MASK;
+ rmap_item->address &= ~STABLE_FLAG;
cond_resched();
}
@@ -555,8 +562,7 @@ static void remove_rmap_item_from_tree(struct rmap_item *rmap_item)
ksm_pages_shared--;
ksm_drop_anon_vma(rmap_item);
- rmap_item->address &= PAGE_MASK;
-
+ rmap_item->address &= ~STABLE_FLAG;
} else if (rmap_item->address & UNSTABLE_FLAG) {
unsigned char age;
/*
@@ -568,11 +574,13 @@ static void remove_rmap_item_from_tree(struct rmap_item *rmap_item)
*/
age = (unsigned char)(ksm_scan.seqnr - rmap_item->address);
BUG_ON(age > 1);
+
if (!age)
rb_erase(&rmap_item->node, &root_unstable_tree);
ksm_pages_unshared--;
- rmap_item->address &= PAGE_MASK;
+ rmap_item->address &= ~UNSTABLE_FLAG;
+ rmap_item->address &= ~SEQNR_MASK;
}
out:
cond_resched(); /* we're called from many long loops */
@@ -682,15 +690,6 @@ error:
}
#endif /* CONFIG_SYSFS */
-static u32 calc_checksum(struct page *page)
-{
- u32 checksum;
- void *addr = kmap_atomic(page, KM_USER0);
- checksum = jhash2(addr, PAGE_SIZE / 4, 17);
- kunmap_atomic(addr, KM_USER0);
- return checksum;
-}
-
static int memcmp_pages(struct page *page1, struct page *page2)
{
char *addr1, *addr2;
@@ -718,13 +717,14 @@ static int write_protect_page(struct vm_area_struct *vma, struct page *page,
spinlock_t *ptl;
int swapped;
int err = -EFAULT;
+ int need_pte_unmap;
addr = page_address_in_vma(page, vma);
if (addr == -EFAULT)
goto out;
BUG_ON(PageTransCompound(page));
- ptep = page_check_address(page, mm, addr, &ptl, 0);
+ ptep = page_check_address(page, mm, addr, &ptl, 0, &need_pte_unmap);
if (!ptep)
goto out;
@@ -760,7 +760,7 @@ static int write_protect_page(struct vm_area_struct *vma, struct page *page,
err = 0;
out_unlock:
- pte_unmap_unlock(ptep, ptl);
+ page_check_address_unmap_unlock(ptl, ptep, need_pte_unmap);
out:
return err;
}
@@ -936,12 +936,13 @@ static int try_to_merge_with_ksm_page(struct rmap_item *rmap_item,
struct mm_struct *mm = rmap_item->mm;
struct vm_area_struct *vma;
int err = -EFAULT;
+ unsigned long address = get_address(rmap_item);
down_read(&mm->mmap_sem);
if (ksm_test_exit(mm))
goto out;
- vma = find_vma(mm, rmap_item->address);
- if (!vma || vma->vm_start > rmap_item->address)
+ vma = find_vma(mm, address);
+ if (!vma || vma->vm_start > address)
goto out;
err = try_to_merge_one_page(vma, page, kpage);
@@ -1171,6 +1172,62 @@ static void stable_tree_append(struct rmap_item *rmap_item,
ksm_pages_shared++;
}
+static inline unsigned long get_huge_end_addr(unsigned long address)
+{
+ return (address & HPAGE_PMD_MASK) + HPAGE_SIZE;
+}
+
+/*
+ * ksm_page_changed - take the dirty bit of the pte as a hint for volatile
+ * pages. We clear the dirty bit for each pte scanned but don't flush the
+ * tlb. For huge pages, if one of the subpage has changed, we try to skip
+ * the whole huge page.
+ */
+static int ksm_page_changed(struct page *page, struct rmap_item *rmap_item)
+{
+ int ret = 1;
+ unsigned long address = get_address(rmap_item);
+ struct mm_struct *mm = rmap_item->mm;
+ pte_t *ptep, entry;
+ spinlock_t *ptl;
+ int need_pte_unmap;
+
+ if (ksm_scan.huge_skip) {
+ /* in process of skipping a huge page */
+ if (ksm_scan.mm_slot->mm == rmap_item->mm &&
+ PageTail(page) && address < ksm_scan.huge_skip) {
+ ret = 1;
+ goto out;
+ } else {
+ ksm_scan.huge_skip = 0;
+ }
+ }
+
+ ptep = page_check_address(page, mm, address, &ptl, 0, &need_pte_unmap);
+ if (!ptep)
+ goto out;
+
+ entry = *ptep;
+ if (!pte_dirty(entry)) {
+ ret = 0;
+ } else {
+ set_page_dirty(page);
+ entry = pte_mkclean(entry);
+ set_pte_at(mm, address, ptep, entry);
+ if (PageTransCompound(page))
+ ksm_scan.huge_skip = get_huge_end_addr(address);
+ }
+
+ if (rmap_item->address & NEW_FLAG) {
+ rmap_item->address &= ~NEW_FLAG;
+ ret = 0;
+ }
+
+ page_check_address_unmap_unlock(ptl, ptep, need_pte_unmap);
+out:
+ return ret;
+}
+
/*
* cmp_and_merge_page - first see if page can be merged into the stable tree;
* if not, compare checksum to previous and if it's the same, see if page can
@@ -1186,7 +1243,6 @@ static void cmp_and_merge_page(struct page *page, struct rmap_item *rmap_item)
struct page *tree_page = NULL;
struct stable_node *stable_node;
struct page *kpage;
- unsigned int checksum;
int err;
remove_rmap_item_from_tree(rmap_item);
@@ -1208,17 +1264,8 @@ static void cmp_and_merge_page(struct page *page, struct rmap_item *rmap_item)
return;
}
- /*
- * If the hash value of the page has changed from the last time
- * we calculated it, this page is changing frequently: therefore we
- * don't want to insert it in the unstable tree, and we don't want
- * to waste our time searching for something identical to it there.
- */
- checksum = calc_checksum(page);
- if (rmap_item->oldchecksum != checksum) {
- rmap_item->oldchecksum = checksum;
+ if (ksm_page_changed(page, rmap_item))
return;
- }
tree_rmap_item =
unstable_tree_search_insert(rmap_item, page, &tree_page);
@@ -1264,9 +1311,9 @@ static struct rmap_item *get_next_rmap_item(struct mm_slot *mm_slot,
while (*rmap_list) {
rmap_item = *rmap_list;
- if ((rmap_item->address & PAGE_MASK) == addr)
+ if (get_address(rmap_item) == addr)
return rmap_item;
- if (rmap_item->address > addr)
+ if (get_address(rmap_item) > addr)
break;
*rmap_list = rmap_item->rmap_list;
remove_rmap_item_from_tree(rmap_item);
@@ -1278,6 +1325,7 @@ static struct rmap_item *get_next_rmap_item(struct mm_slot *mm_slot,
/* It has already been zeroed */
rmap_item->mm = mm_slot->mm;
rmap_item->address = addr;
+ rmap_item->address |= NEW_FLAG;
rmap_item->rmap_list = *rmap_list;
*rmap_list = rmap_item;
}
@@ -1614,12 +1662,12 @@ again:
struct anon_vma *anon_vma = rmap_item->anon_vma;
struct anon_vma_chain *vmac;
struct vm_area_struct *vma;
+ unsigned long address = get_address(rmap_item);
anon_vma_lock(anon_vma);
list_for_each_entry(vmac, &anon_vma->head, same_anon_vma) {
vma = vmac->vma;
- if (rmap_item->address < vma->vm_start ||
- rmap_item->address >= vma->vm_end)
+ if (address < vma->vm_start || address >= vma->vm_end)
continue;
/*
* Initially we examine only the vma which covers this
@@ -1633,8 +1681,8 @@ again:
if (memcg && !mm_match_cgroup(vma->vm_mm, memcg))
continue;
- referenced += page_referenced_one(page, vma,
- rmap_item->address, &mapcount, vm_flags);
+ referenced += page_referenced_one(page, vma, address,
+ &mapcount, vm_flags);
if (!search_new_forks || !mapcount)
break;
}
@@ -1667,12 +1715,12 @@ again:
struct anon_vma *anon_vma = rmap_item->anon_vma;
struct anon_vma_chain *vmac;
struct vm_area_struct *vma;
+ unsigned long address = get_address(rmap_item);
anon_vma_lock(anon_vma);
list_for_each_entry(vmac, &anon_vma->head, same_anon_vma) {
vma = vmac->vma;
- if (rmap_item->address < vma->vm_start ||
- rmap_item->address >= vma->vm_end)
+ if (address < vma->vm_start || address >= vma->vm_end)
continue;
/*
* Initially we examine only the vma which covers this
@@ -1683,8 +1731,7 @@ again:
if ((rmap_item->mm == vma->vm_mm) == search_new_forks)
continue;
- ret = try_to_unmap_one(page, vma,
- rmap_item->address, flags);
+ ret = try_to_unmap_one(page, vma, address, flags);
if (ret != SWAP_AGAIN || !page_mapped(page)) {
anon_vma_unlock(anon_vma);
goto out;
@@ -1719,12 +1766,12 @@ again:
struct anon_vma *anon_vma = rmap_item->anon_vma;
struct anon_vma_chain *vmac;
struct vm_area_struct *vma;
+ unsigned long address = get_address(rmap_item);
anon_vma_lock(anon_vma);
list_for_each_entry(vmac, &anon_vma->head, same_anon_vma) {
vma = vmac->vma;
- if (rmap_item->address < vma->vm_start ||
- rmap_item->address >= vma->vm_end)
+ if (address < vma->vm_start || address >= vma->vm_end)
continue;
/*
* Initially we examine only the vma which covers this
@@ -1735,7 +1782,7 @@ again:
if ((rmap_item->mm == vma->vm_mm) == search_new_forks)
continue;
- ret = rmap_one(page, vma, rmap_item->address, arg);
+ ret = rmap_one(page, vma, address, arg);
if (ret != SWAP_AGAIN) {
anon_vma_unlock(anon_vma);
goto out;
---
--
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