[PATCH 3/9] ksm: hold anon_vma in rmap_item
From: Hugh Dickins
Date: Tue Nov 24 2009 - 11:43:55 EST
For full functionality, page_referenced_one() and try_to_unmap_one()
need to know the vma: to pass vma down to arch-dependent flushes,
or to observe VM_LOCKED or VM_EXEC. But KSM keeps no record of vma:
nor can it, since vmas get split and merged without its knowledge.
Instead, note page's anon_vma in its rmap_item when adding to stable
tree: all the vmas which might map that page are listed by its anon_vma.
page_referenced_ksm() and try_to_unmap_ksm() then traverse the anon_vma,
first to find the probable vma, that which matches rmap_item's mm; but
if that is not enough to locate all instances, traverse again to try the
others. This catches those occasions when fork has duplicated a pte of
a ksm page, but ksmd has not yet come around to assign it an rmap_item.
But each rmap_item in the stable tree which refers to an anon_vma needs
to take a reference to it. Andrea's anon_vma design cleverly avoided a
reference count (an anon_vma was free when its list of vmas was empty),
but KSM now needs to add that. Is a 32-bit count sufficient? I believe
so - the anon_vma is only free when both count is 0 and list is empty.
Signed-off-by: Hugh Dickins <hugh.dickins@xxxxxxxxxxxxx>
---
include/linux/rmap.h | 24 ++++++
mm/ksm.c | 153 ++++++++++++++++++++++++-----------------
mm/rmap.c | 5 -
3 files changed, 120 insertions(+), 62 deletions(-)
--- ksm2/include/linux/rmap.h 2009-11-22 20:40:04.000000000 +0000
+++ ksm3/include/linux/rmap.h 2009-11-22 20:40:11.000000000 +0000
@@ -26,6 +26,9 @@
*/
struct anon_vma {
spinlock_t lock; /* Serialize access to vma list */
+#ifdef CONFIG_KSM
+ atomic_t ksm_refcount;
+#endif
/*
* NOTE: the LSB of the head.next is set by
* mm_take_all_locks() _after_ taking the above lock. So the
@@ -38,6 +41,26 @@ struct anon_vma {
};
#ifdef CONFIG_MMU
+#ifdef CONFIG_KSM
+static inline void ksm_refcount_init(struct anon_vma *anon_vma)
+{
+ atomic_set(&anon_vma->ksm_refcount, 0);
+}
+
+static inline int ksm_refcount(struct anon_vma *anon_vma)
+{
+ return atomic_read(&anon_vma->ksm_refcount);
+}
+#else
+static inline void ksm_refcount_init(struct anon_vma *anon_vma)
+{
+}
+
+static inline int ksm_refcount(struct anon_vma *anon_vma)
+{
+ return 0;
+}
+#endif /* CONFIG_KSM */
static inline struct anon_vma *page_anon_vma(struct page *page)
{
@@ -70,6 +93,7 @@ void __anon_vma_merge(struct vm_area_str
void anon_vma_unlink(struct vm_area_struct *);
void anon_vma_link(struct vm_area_struct *);
void __anon_vma_link(struct vm_area_struct *);
+void anon_vma_free(struct anon_vma *);
/*
* rmap interfaces called when adding or removing pte of page
--- ksm2/mm/ksm.c 2009-11-22 20:40:04.000000000 +0000
+++ ksm3/mm/ksm.c 2009-11-22 20:40:11.000000000 +0000
@@ -121,7 +121,7 @@ struct stable_node {
/**
* struct rmap_item - reverse mapping item for virtual addresses
* @rmap_list: next rmap_item in mm_slot's singly-linked rmap_list
- * @filler: unused space we're making available in this patch
+ * @anon_vma: pointer to anon_vma for this mm,address, when in stable tree
* @mm: the memory structure this rmap_item is pointing into
* @address: the virtual address this rmap_item tracks (+ flags in low bits)
* @oldchecksum: previous checksum of the page at that virtual address
@@ -131,7 +131,7 @@ struct stable_node {
*/
struct rmap_item {
struct rmap_item *rmap_list;
- unsigned long filler;
+ struct anon_vma *anon_vma; /* when stable */
struct mm_struct *mm;
unsigned long address; /* + low bits used for flags below */
unsigned int oldchecksum; /* when unstable */
@@ -196,13 +196,6 @@ static DECLARE_WAIT_QUEUE_HEAD(ksm_threa
static DEFINE_MUTEX(ksm_thread_mutex);
static DEFINE_SPINLOCK(ksm_mmlist_lock);
-/*
- * Temporary hack for page_referenced_ksm() and try_to_unmap_ksm(),
- * later we rework things a little to get the right vma to them.
- */
-static DEFINE_SPINLOCK(ksm_fallback_vma_lock);
-static struct vm_area_struct ksm_fallback_vma;
-
#define KSM_KMEM_CACHE(__struct, __flags) kmem_cache_create("ksm_"#__struct,\
sizeof(struct __struct), __alignof__(struct __struct),\
(__flags), NULL)
@@ -323,6 +316,25 @@ static inline int in_stable_tree(struct
return rmap_item->address & STABLE_FLAG;
}
+static void hold_anon_vma(struct rmap_item *rmap_item,
+ struct anon_vma *anon_vma)
+{
+ rmap_item->anon_vma = anon_vma;
+ atomic_inc(&anon_vma->ksm_refcount);
+}
+
+static void drop_anon_vma(struct rmap_item *rmap_item)
+{
+ struct anon_vma *anon_vma = rmap_item->anon_vma;
+
+ if (atomic_dec_and_lock(&anon_vma->ksm_refcount, &anon_vma->lock)) {
+ int empty = list_empty(&anon_vma->head);
+ spin_unlock(&anon_vma->lock);
+ if (empty)
+ anon_vma_free(anon_vma);
+ }
+}
+
/*
* ksmd, and unmerge_and_remove_all_rmap_items(), must not touch an mm's
* page tables after it has passed through ksm_exit() - which, if necessary,
@@ -472,6 +484,7 @@ static void remove_rmap_item_from_tree(s
ksm_pages_shared--;
}
+ drop_anon_vma(rmap_item);
rmap_item->address &= PAGE_MASK;
} else if (rmap_item->address & UNSTABLE_FLAG) {
@@ -752,6 +765,9 @@ static int try_to_merge_one_page(struct
pte_t orig_pte = __pte(0);
int err = -EFAULT;
+ if (page == kpage) /* ksm page forked */
+ return 0;
+
if (!(vma->vm_flags & VM_MERGEABLE))
goto out;
if (!PageAnon(page))
@@ -805,9 +821,6 @@ static int try_to_merge_with_ksm_page(st
struct vm_area_struct *vma;
int err = -EFAULT;
- if (page == kpage) /* ksm page forked */
- return 0;
-
down_read(&mm->mmap_sem);
if (ksm_test_exit(mm))
goto out;
@@ -816,6 +829,11 @@ static int try_to_merge_with_ksm_page(st
goto out;
err = try_to_merge_one_page(vma, page, kpage);
+ if (err)
+ goto out;
+
+ /* Must get reference to anon_vma while still holding mmap_sem */
+ hold_anon_vma(rmap_item, vma->anon_vma);
out:
up_read(&mm->mmap_sem);
return err;
@@ -869,6 +887,11 @@ static struct page *try_to_merge_two_pag
lru_cache_add_lru(kpage, LRU_ACTIVE_ANON);
err = try_to_merge_one_page(vma, page, kpage);
+ if (err)
+ goto up;
+
+ /* Must get reference to anon_vma while still holding mmap_sem */
+ hold_anon_vma(rmap_item, vma->anon_vma);
up:
up_read(&mm->mmap_sem);
@@ -879,8 +902,10 @@ up:
* If that fails, we have a ksm page with only one pte
* pointing to it: so break it.
*/
- if (err)
+ if (err) {
+ drop_anon_vma(rmap_item);
break_cow(rmap_item);
+ }
}
if (err) {
put_page(kpage);
@@ -1155,7 +1180,9 @@ static void cmp_and_merge_page(struct pa
* in which case we need to break_cow on both.
*/
if (!stable_node) {
+ drop_anon_vma(tree_rmap_item);
break_cow(tree_rmap_item);
+ drop_anon_vma(rmap_item);
break_cow(rmap_item);
}
}
@@ -1490,7 +1517,7 @@ int page_referenced_ksm(struct page *pag
struct hlist_node *hlist;
unsigned int mapcount = page_mapcount(page);
int referenced = 0;
- struct vm_area_struct *vma;
+ int search_new_forks = 0;
VM_BUG_ON(!PageKsm(page));
VM_BUG_ON(!PageLocked(page));
@@ -1498,36 +1525,40 @@ int page_referenced_ksm(struct page *pag
stable_node = page_stable_node(page);
if (!stable_node)
return 0;
-
- /*
- * Temporary hack: really we need anon_vma in rmap_item, to
- * provide the correct vma, and to find recently forked instances.
- * Use zalloc to avoid weirdness if any other fields are involved.
- */
- vma = kmem_cache_zalloc(vm_area_cachep, GFP_ATOMIC);
- if (!vma) {
- spin_lock(&ksm_fallback_vma_lock);
- vma = &ksm_fallback_vma;
- }
-
+again:
hlist_for_each_entry(rmap_item, hlist, &stable_node->hlist, hlist) {
+ struct anon_vma *anon_vma = rmap_item->anon_vma;
+ struct vm_area_struct *vma;
+
if (memcg && !mm_match_cgroup(rmap_item->mm, memcg))
continue;
- vma->vm_mm = rmap_item->mm;
- vma->vm_start = rmap_item->address;
- vma->vm_end = vma->vm_start + PAGE_SIZE;
+ spin_lock(&anon_vma->lock);
+ list_for_each_entry(vma, &anon_vma->head, anon_vma_node) {
+ if (rmap_item->address < vma->vm_start ||
+ rmap_item->address >= vma->vm_end)
+ continue;
+ /*
+ * Initially we examine only the vma which covers this
+ * rmap_item; but later, if there is still work to do,
+ * we examine covering vmas in other mms: in case they
+ * were forked from the original since ksmd passed.
+ */
+ if ((rmap_item->mm == vma->vm_mm) == search_new_forks)
+ continue;
- referenced += page_referenced_one(page, vma,
+ referenced += page_referenced_one(page, vma,
rmap_item->address, &mapcount, vm_flags);
+ if (!search_new_forks || !mapcount)
+ break;
+ }
+ spin_unlock(&anon_vma->lock);
if (!mapcount)
goto out;
}
+ if (!search_new_forks++)
+ goto again;
out:
- if (vma == &ksm_fallback_vma)
- spin_unlock(&ksm_fallback_vma_lock);
- else
- kmem_cache_free(vm_area_cachep, vma);
return referenced;
}
@@ -1537,7 +1568,7 @@ int try_to_unmap_ksm(struct page *page,
struct hlist_node *hlist;
struct rmap_item *rmap_item;
int ret = SWAP_AGAIN;
- struct vm_area_struct *vma;
+ int search_new_forks = 0;
VM_BUG_ON(!PageKsm(page));
VM_BUG_ON(!PageLocked(page));
@@ -1545,35 +1576,37 @@ int try_to_unmap_ksm(struct page *page,
stable_node = page_stable_node(page);
if (!stable_node)
return SWAP_FAIL;
-
- /*
- * Temporary hack: really we need anon_vma in rmap_item, to
- * provide the correct vma, and to find recently forked instances.
- * Use zalloc to avoid weirdness if any other fields are involved.
- */
- if (TTU_ACTION(flags) != TTU_UNMAP)
- return SWAP_FAIL;
-
- vma = kmem_cache_zalloc(vm_area_cachep, GFP_ATOMIC);
- if (!vma) {
- spin_lock(&ksm_fallback_vma_lock);
- vma = &ksm_fallback_vma;
- }
-
+again:
hlist_for_each_entry(rmap_item, hlist, &stable_node->hlist, hlist) {
- vma->vm_mm = rmap_item->mm;
- vma->vm_start = rmap_item->address;
- vma->vm_end = vma->vm_start + PAGE_SIZE;
+ struct anon_vma *anon_vma = rmap_item->anon_vma;
+ struct vm_area_struct *vma;
- ret = try_to_unmap_one(page, vma, rmap_item->address, flags);
- if (ret != SWAP_AGAIN || !page_mapped(page))
- goto out;
+ spin_lock(&anon_vma->lock);
+ list_for_each_entry(vma, &anon_vma->head, anon_vma_node) {
+ if (rmap_item->address < vma->vm_start ||
+ rmap_item->address >= vma->vm_end)
+ continue;
+ /*
+ * Initially we examine only the vma which covers this
+ * rmap_item; but later, if there is still work to do,
+ * we examine covering vmas in other mms: in case they
+ * were forked from the original since ksmd passed.
+ */
+ if ((rmap_item->mm == vma->vm_mm) == search_new_forks)
+ continue;
+
+ ret = try_to_unmap_one(page, vma,
+ rmap_item->address, flags);
+ if (ret != SWAP_AGAIN || !page_mapped(page)) {
+ spin_unlock(&anon_vma->lock);
+ goto out;
+ }
+ }
+ spin_unlock(&anon_vma->lock);
}
+ if (!search_new_forks++)
+ goto again;
out:
- if (vma == &ksm_fallback_vma)
- spin_unlock(&ksm_fallback_vma_lock);
- else
- kmem_cache_free(vm_area_cachep, vma);
return ret;
}
--- ksm2/mm/rmap.c 2009-11-22 20:40:04.000000000 +0000
+++ ksm3/mm/rmap.c 2009-11-22 20:40:11.000000000 +0000
@@ -68,7 +68,7 @@ static inline struct anon_vma *anon_vma_
return kmem_cache_alloc(anon_vma_cachep, GFP_KERNEL);
}
-static inline void anon_vma_free(struct anon_vma *anon_vma)
+void anon_vma_free(struct anon_vma *anon_vma)
{
kmem_cache_free(anon_vma_cachep, anon_vma);
}
@@ -172,7 +172,7 @@ void anon_vma_unlink(struct vm_area_stru
list_del(&vma->anon_vma_node);
/* We must garbage collect the anon_vma if it's empty */
- empty = list_empty(&anon_vma->head);
+ empty = list_empty(&anon_vma->head) && !ksm_refcount(anon_vma);
spin_unlock(&anon_vma->lock);
if (empty)
@@ -184,6 +184,7 @@ static void anon_vma_ctor(void *data)
struct anon_vma *anon_vma = data;
spin_lock_init(&anon_vma->lock);
+ ksm_refcount_init(anon_vma);
INIT_LIST_HEAD(&anon_vma->head);
}
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