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ksm: hold anon_vma in rmap_item
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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@tiscali.co.uk>
Cc: Izik Eidus <ieidus@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Chris Wright <chrisw@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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Hugh Dickins authored and Linus Torvalds committed Dec 15, 2009
1 parent 5ad6468 commit db114b8
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Showing 3 changed files with 122 additions and 64 deletions.
24 changes: 24 additions & 0 deletions include/linux/rmap.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -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
Expand All @@ -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)
{
Expand Down Expand Up @@ -70,6 +93,7 @@ void __anon_vma_merge(struct vm_area_struct *, struct vm_area_struct *);
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
Expand Down
157 changes: 95 additions & 62 deletions mm/ksm.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -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
Expand All @@ -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 */
Expand Down Expand Up @@ -196,13 +196,6 @@ static DECLARE_WAIT_QUEUE_HEAD(ksm_thread_wait);
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)
Expand Down Expand Up @@ -323,6 +316,25 @@ static inline int in_stable_tree(struct rmap_item *rmap_item)
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,
Expand Down Expand Up @@ -472,6 +484,7 @@ static void remove_rmap_item_from_tree(struct rmap_item *rmap_item)
ksm_pages_shared--;
}

drop_anon_vma(rmap_item);
rmap_item->address &= PAGE_MASK;

} else if (rmap_item->address & UNSTABLE_FLAG) {
Expand Down Expand Up @@ -752,6 +765,9 @@ static int try_to_merge_one_page(struct vm_area_struct *vma,
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))
Expand Down Expand Up @@ -805,9 +821,6 @@ static int try_to_merge_with_ksm_page(struct rmap_item *rmap_item,
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;
Expand All @@ -816,6 +829,11 @@ static int try_to_merge_with_ksm_page(struct rmap_item *rmap_item,
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;
Expand Down Expand Up @@ -869,6 +887,11 @@ static struct page *try_to_merge_two_pages(struct rmap_item *rmap_item,
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);

Expand All @@ -879,8 +902,10 @@ static struct page *try_to_merge_two_pages(struct rmap_item *rmap_item,
* 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);
Expand Down Expand Up @@ -1155,7 +1180,9 @@ static void cmp_and_merge_page(struct page *page, struct rmap_item *rmap_item)
* 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);
}
}
Expand Down Expand Up @@ -1490,44 +1517,48 @@ int page_referenced_ksm(struct page *page, struct mem_cgroup *memcg,
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));

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) {
if (memcg && !mm_match_cgroup(rmap_item->mm, memcg))
continue;
struct anon_vma *anon_vma = rmap_item->anon_vma;
struct vm_area_struct *vma;

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;

vma->vm_mm = rmap_item->mm;
vma->vm_start = rmap_item->address;
vma->vm_end = vma->vm_start + PAGE_SIZE;
if (memcg && !mm_match_cgroup(vma->vm_mm, memcg))
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;
}

Expand All @@ -1537,43 +1568,45 @@ int try_to_unmap_ksm(struct page *page, enum ttu_flags flags)
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));

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;
}

Expand Down
5 changes: 3 additions & 2 deletions mm/rmap.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -68,7 +68,7 @@ static inline struct anon_vma *anon_vma_alloc(void)
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);
}
Expand Down Expand Up @@ -172,7 +172,7 @@ void anon_vma_unlink(struct vm_area_struct *vma)
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)
Expand All @@ -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);
}

Expand Down

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