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thp: implement refcounting for huge zero page

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H.  Peter Anvin doesn't like huge zero page which sticks in memory forever
after the first allocation.  Here's implementation of lockless refcounting
for huge zero page.

We have two basic primitives: {get,put}_huge_zero_page(). They
manipulate reference counter.

If counter is 0, get_huge_zero_page() allocates a new huge page and takes
two references: one for caller and one for shrinker.  We free the page
only in shrinker callback if counter is 1 (only shrinker has the
reference).

put_huge_zero_page() only decrements counter.  Counter is never zero in
put_huge_zero_page() since shrinker holds on reference.

Freeing huge zero page in shrinker callback helps to avoid frequent
allocate-free.

Refcounting has cost.  On 4 socket machine I observe ~1% slowdown on
parallel (40 processes) read page faulting comparing to lazy huge page
allocation.  I think it's pretty reasonable for synthetic benchmark.

[lliubbo@gmail.com: fix mismerge]
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@linux.intel.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Bob Liu <lliubbo@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Kirill A. Shutemov 2012-12-12 13:51:06 -08:00 committed by Linus Torvalds
parent 78ca0e6792
commit 97ae17497e
1 changed files with 89 additions and 26 deletions
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115
mm/huge_memory.c
View file

@ -12,12 +12,14 @@
#include <linux/mmu_notifier.h>
#include <linux/rmap.h>
#include <linux/swap.h>
#include <linux/shrinker.h>
#include <linux/mm_inline.h>
#include <linux/kthread.h>
#include <linux/khugepaged.h>
#include <linux/freezer.h>
#include <linux/mman.h>
#include <linux/pagemap.h>
#include <asm/tlb.h>
#include <asm/pgalloc.h>
#include "internal.h"
@ -47,7 +49,6 @@ static unsigned int khugepaged_scan_sleep_millisecs __read_mostly = 10000;
/* during fragmentation poll the hugepage allocator once every minute */
static unsigned int khugepaged_alloc_sleep_millisecs __read_mostly = 60000;
static struct task_struct *khugepaged_thread __read_mostly;
static unsigned long huge_zero_pfn __read_mostly;
static DEFINE_MUTEX(khugepaged_mutex);
static DEFINE_SPINLOCK(khugepaged_mm_lock);
static DECLARE_WAIT_QUEUE_HEAD(khugepaged_wait);
@ -160,24 +161,13 @@ static int start_khugepaged(void)
return err;
}
static int init_huge_zero_pfn(void)
{
struct page *hpage;
unsigned long pfn;
hpage = alloc_pages((GFP_TRANSHUGE | __GFP_ZERO) & ~__GFP_MOVABLE,
HPAGE_PMD_ORDER);
if (!hpage)
return -ENOMEM;
pfn = page_to_pfn(hpage);
if (cmpxchg(&huge_zero_pfn, 0, pfn))
__free_page(hpage);
return 0;
}
static atomic_t huge_zero_refcount;
static unsigned long huge_zero_pfn __read_mostly;
static inline bool is_huge_zero_pfn(unsigned long pfn)
{
return huge_zero_pfn && pfn == huge_zero_pfn;
unsigned long zero_pfn = ACCESS_ONCE(huge_zero_pfn);
return zero_pfn && pfn == zero_pfn;
}
static inline bool is_huge_zero_pmd(pmd_t pmd)
@ -185,6 +175,60 @@ static inline bool is_huge_zero_pmd(pmd_t pmd)
return is_huge_zero_pfn(pmd_pfn(pmd));
}
static unsigned long get_huge_zero_page(void)
{
struct page *zero_page;
retry:
if (likely(atomic_inc_not_zero(&huge_zero_refcount)))
return ACCESS_ONCE(huge_zero_pfn);
zero_page = alloc_pages((GFP_TRANSHUGE | __GFP_ZERO) & ~__GFP_MOVABLE,
HPAGE_PMD_ORDER);
if (!zero_page)
return 0;
preempt_disable();
if (cmpxchg(&huge_zero_pfn, 0, page_to_pfn(zero_page))) {
preempt_enable();
__free_page(zero_page);
goto retry;
}
/* We take additional reference here. It will be put back by shrinker */
atomic_set(&huge_zero_refcount, 2);
preempt_enable();
return ACCESS_ONCE(huge_zero_pfn);
}
static void put_huge_zero_page(void)
{
/*
* Counter should never go to zero here. Only shrinker can put
* last reference.
*/
BUG_ON(atomic_dec_and_test(&huge_zero_refcount));
}
static int shrink_huge_zero_page(struct shrinker *shrink,
struct shrink_control *sc)
{
if (!sc->nr_to_scan)
/* we can free zero page only if last reference remains */
return atomic_read(&huge_zero_refcount) == 1 ? HPAGE_PMD_NR : 0;
if (atomic_cmpxchg(&huge_zero_refcount, 1, 0) == 1) {
unsigned long zero_pfn = xchg(&huge_zero_pfn, 0);
BUG_ON(zero_pfn == 0);
__free_page(__pfn_to_page(zero_pfn));
}
return 0;
}
static struct shrinker huge_zero_page_shrinker = {
.shrink = shrink_huge_zero_page,
.seeks = DEFAULT_SEEKS,
};
#ifdef CONFIG_SYSFS
static ssize_t double_flag_show(struct kobject *kobj,
@ -576,6 +620,8 @@ static int __init hugepage_init(void)
goto out;
}
register_shrinker(&huge_zero_page_shrinker);
/*
* By default disable transparent hugepages on smaller systems,
* where the extra memory used could hurt more than TLB overhead
@ -705,10 +751,11 @@ static inline struct page *alloc_hugepage(int defrag)
#endif
static void set_huge_zero_page(pgtable_t pgtable, struct mm_struct *mm,
struct vm_area_struct *vma, unsigned long haddr, pmd_t *pmd)
struct vm_area_struct *vma, unsigned long haddr, pmd_t *pmd,
unsigned long zero_pfn)
{
pmd_t entry;
entry = pfn_pmd(huge_zero_pfn, vma->vm_page_prot);
entry = pfn_pmd(zero_pfn, vma->vm_page_prot);
entry = pmd_wrprotect(entry);
entry = pmd_mkhuge(entry);
set_pmd_at(mm, haddr, pmd, entry);
@ -731,15 +778,19 @@ int do_huge_pmd_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma,
return VM_FAULT_OOM;
if (!(flags & FAULT_FLAG_WRITE)) {
pgtable_t pgtable;
if (unlikely(!huge_zero_pfn && init_huge_zero_pfn())) {
count_vm_event(THP_FAULT_FALLBACK);
goto out;
}
unsigned long zero_pfn;
pgtable = pte_alloc_one(mm, haddr);
if (unlikely(!pgtable))
return VM_FAULT_OOM;
zero_pfn = get_huge_zero_page();
if (unlikely(!zero_pfn)) {
pte_free(mm, pgtable);
count_vm_event(THP_FAULT_FALLBACK);
goto out;
}
spin_lock(&mm->page_table_lock);
set_huge_zero_page(pgtable, mm, vma, haddr, pmd);
set_huge_zero_page(pgtable, mm, vma, haddr, pmd,
zero_pfn);
spin_unlock(&mm->page_table_lock);
return 0;
}
@ -813,7 +864,15 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
* a page table.
*/
if (is_huge_zero_pmd(pmd)) {
set_huge_zero_page(pgtable, dst_mm, vma, addr, dst_pmd);
unsigned long zero_pfn;
/*
* get_huge_zero_page() will never allocate a new page here,
* since we already have a zero page to copy. It just takes a
* reference.
*/
zero_pfn = get_huge_zero_page();
set_huge_zero_page(pgtable, dst_mm, vma, addr, dst_pmd,
zero_pfn);
ret = 0;
goto out_unlock;
}
@ -923,6 +982,7 @@ static int do_huge_pmd_wp_zero_page_fallback(struct mm_struct *mm,
smp_wmb(); /* make pte visible before pmd */
pmd_populate(mm, pmd, pgtable);
spin_unlock(&mm->page_table_lock);
put_huge_zero_page();
inc_mm_counter(mm, MM_ANONPAGES);
mmu_notifier_invalidate_range_end(mm, mmun_start, mmun_end);
@ -1123,9 +1183,10 @@ alloc:
page_add_new_anon_rmap(new_page, vma, haddr);
set_pmd_at(mm, haddr, pmd, entry);
update_mmu_cache_pmd(vma, address, pmd);
if (is_huge_zero_pmd(orig_pmd))
if (is_huge_zero_pmd(orig_pmd)) {
add_mm_counter(mm, MM_ANONPAGES, HPAGE_PMD_NR);
else {
put_huge_zero_page();
} else {
VM_BUG_ON(!PageHead(page));
page_remove_rmap(page);
put_page(page);
@ -1202,6 +1263,7 @@ int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
if (is_huge_zero_pmd(orig_pmd)) {
tlb->mm->nr_ptes--;
spin_unlock(&tlb->mm->page_table_lock);
put_huge_zero_page();
} else {
page = pmd_page(orig_pmd);
page_remove_rmap(page);
@ -2511,6 +2573,7 @@ static void __split_huge_zero_page_pmd(struct vm_area_struct *vma,
}
smp_wmb(); /* make pte visible before pmd */
pmd_populate(mm, pmd, pgtable);
put_huge_zero_page();
}
void __split_huge_page_pmd(struct vm_area_struct *vma, unsigned long address,