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SubjectRe: [PATCH 2/6] thp: optimize away unnecessary page table locking
Date
On Sun, Feb 19, 2012 at 01:21:02PM -0800, Hugh Dickins wrote:
> On Wed, 8 Feb 2012, Naoya Horiguchi wrote:
> > Currently when we check if we can handle thp as it is or we need to
> > split it into regular sized pages, we hold page table lock prior to
> > check whether a given pmd is mapping thp or not. Because of this,
> > when it's not "huge pmd" we suffer from unnecessary lock/unlock overhead.
> > To remove it, this patch introduces a optimized check function and
> > replace several similar logics with it.
> >
> > Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> > Cc: David Rientjes <rientjes@google.com>
> >
> > Changes since v4:
> > - Rethink returned value of __pmd_trans_huge_lock()
>
> [snip]
>
> > --- 3.3-rc2.orig/mm/mremap.c
> > +++ 3.3-rc2/mm/mremap.c
> > @@ -155,8 +155,6 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
> > if (err > 0) {
> > need_flush = true;
> > continue;
> > - } else if (!err) {
> > - split_huge_page_pmd(vma->vm_mm, old_pmd);
> > }
> > VM_BUG_ON(pmd_trans_huge(*old_pmd));
> > }

Thanks for reporting,

> Is that what you intended to do there?

No. This is a bug.

> I just hit that VM_BUG_ON on rc3-next-20120217.

I found that when extend != HPAGE_PMD_SIZE, thp is not split so
it hits the VM_BUG_ON.
The following patch cancels the change in returned value in v4->v5
and I confirmed this fixes the problem in my simple test.
Andrew, could you add it on top of this optimization patch?

Naoya
----------------------------------------------------
From 3c49816cab7d8cb072d9dffb97242e40f5124230 Mon Sep 17 00:00:00 2001
From: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Date: Mon, 20 Feb 2012 01:48:12 -0500
Subject: [PATCH] fix mremap bug of failing to split thp

The patch "thp: optimize away unnecessary page table locking" introduced
a bug to move_page_tables(), where we fail to split thp when move_huge_pmd()
is not called. To fix it, this patch reverts the return value changes and
readd if (!err) block.

Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
---
fs/proc/task_mmu.c | 6 +++---
mm/huge_memory.c | 13 ++++++-------
mm/mremap.c | 2 ++
3 files changed, 11 insertions(+), 10 deletions(-)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 7810281..2d12325 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -394,7 +394,7 @@ static int smaps_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
pte_t *pte;
spinlock_t *ptl;

- if (pmd_trans_huge_lock(pmd, vma)) {
+ if (pmd_trans_huge_lock(pmd, vma) == 1) {
smaps_pte_entry(pmd_to_pte_t(pmd), addr, HPAGE_PMD_SIZE, walk);
spin_unlock(&walk->mm->page_table_lock);
mss->anonymous_thp += HPAGE_PMD_SIZE;
@@ -696,7 +696,7 @@ static int pagemap_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
/* find the first VMA at or above 'addr' */
vma = find_vma(walk->mm, addr);

- if (pmd_trans_huge_lock(pmd, vma)) {
+ if (pmd_trans_huge_lock(pmd, vma) == 1) {
for (; addr != end; addr += PAGE_SIZE) {
unsigned long offset = (addr & ~PAGEMAP_WALK_MASK)
>> PAGE_SHIFT;
@@ -973,7 +973,7 @@ static int gather_pte_stats(pmd_t *pmd, unsigned long addr,

md = walk->private;

- if (pmd_trans_huge_lock(pmd, md->vma)) {
+ if (pmd_trans_huge_lock(pmd, md->vma) == 1) {
pte_t huge_pte = pmd_to_pte_t(pmd);
struct page *page;

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index bbf57b5..f342bb2 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1030,7 +1030,7 @@ int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
{
int ret = 0;

- if (__pmd_trans_huge_lock(pmd, vma)) {
+ if (__pmd_trans_huge_lock(pmd, vma) == 1) {
struct page *page;
pgtable_t pgtable;
pgtable = get_pmd_huge_pte(tlb->mm);
@@ -1056,7 +1056,7 @@ int mincore_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
{
int ret = 0;

- if (__pmd_trans_huge_lock(pmd, vma)) {
+ if (__pmd_trans_huge_lock(pmd, vma) == 1) {
/*
* All logical pages in the range are present
* if backed by a huge page.
@@ -1094,12 +1094,11 @@ int move_huge_pmd(struct vm_area_struct *vma, struct vm_area_struct *new_vma,
goto out;
}

- if (__pmd_trans_huge_lock(old_pmd, vma)) {
+ if ((ret = __pmd_trans_huge_lock(old_pmd, vma)) == 1) {
pmd = pmdp_get_and_clear(mm, old_addr, old_pmd);
VM_BUG_ON(!pmd_none(*new_pmd));
set_pmd_at(mm, new_addr, new_pmd, pmd);
spin_unlock(&mm->page_table_lock);
- ret = 1;
}
out:
return ret;
@@ -1111,7 +1110,7 @@ int change_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
struct mm_struct *mm = vma->vm_mm;
int ret = 0;

- if (__pmd_trans_huge_lock(pmd, vma)) {
+ if (__pmd_trans_huge_lock(pmd, vma) == 1) {
pmd_t entry;
entry = pmdp_get_and_clear(mm, addr, pmd);
entry = pmd_modify(entry, newprot);
@@ -1125,7 +1124,7 @@ int change_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,

/*
* Returns 1 if a given pmd maps a stable (not under splitting) thp.
- * Returns 0 otherwise.
+ * Returns -1 if it maps a thp under splitting. Returns 0 otherwise.
*
* Note that if it returns 1, this routine returns without unlocking page
* table locks. So callers must unlock them.
@@ -1137,7 +1136,7 @@ int __pmd_trans_huge_lock(pmd_t *pmd, struct vm_area_struct *vma)
if (unlikely(pmd_trans_splitting(*pmd))) {
spin_unlock(&vma->vm_mm->page_table_lock);
wait_split_huge_page(vma->anon_vma, pmd);
- return 0;
+ return -1;
} else {
/* Thp mapped by 'pmd' is stable, so we can
* handle it as it is. */
diff --git a/mm/mremap.c b/mm/mremap.c
index 22458b9..87bb839 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -155,6 +155,8 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
if (err > 0) {
need_flush = true;
continue;
+ } else if (!err) {
+ split_huge_page_pmd(vma->vm_mm, old_pmd);
}
VM_BUG_ON(pmd_trans_huge(*old_pmd));
}
--
1.7.7.6


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