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Subject[PATCH 2/2] thp: avoid race on multiple parallel page faults to the same page
Date
From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>

pmd value is stable only with mm->page_table_lock taken. After taking
the lock we need to check that nobody modified the pmd before change it.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
mm/huge_memory.c | 29 ++++++++++++++++++++++++-----
1 file changed, 24 insertions(+), 5 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 9d6f521..51cb8fe 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -770,17 +770,20 @@ static inline struct page *alloc_hugepage(int defrag)
}
#endif

-static void set_huge_zero_page(pgtable_t pgtable, struct mm_struct *mm,
+static bool set_huge_zero_page(pgtable_t pgtable, struct mm_struct *mm,
struct vm_area_struct *vma, unsigned long haddr, pmd_t *pmd,
unsigned long zero_pfn)
{
pmd_t entry;
+ if (!pmd_none(*pmd))
+ return false;
entry = pfn_pmd(zero_pfn, vma->vm_page_prot);
entry = pmd_wrprotect(entry);
entry = pmd_mkhuge(entry);
set_pmd_at(mm, haddr, pmd, entry);
pgtable_trans_huge_deposit(mm, pgtable);
mm->nr_ptes++;
+ return true;
}

int do_huge_pmd_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma,
@@ -800,6 +803,7 @@ int do_huge_pmd_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma,
transparent_hugepage_use_zero_page()) {
pgtable_t pgtable;
unsigned long zero_pfn;
+ bool set;
pgtable = pte_alloc_one(mm, haddr);
if (unlikely(!pgtable))
return VM_FAULT_OOM;
@@ -810,9 +814,13 @@ int do_huge_pmd_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma,
goto out;
}
spin_lock(&mm->page_table_lock);
- set_huge_zero_page(pgtable, mm, vma, haddr, pmd,
+ set = set_huge_zero_page(pgtable, mm, vma, haddr, pmd,
zero_pfn);
spin_unlock(&mm->page_table_lock);
+ if (!set) {
+ pte_free(mm, pgtable);
+ put_huge_zero_page();
+ }
return 0;
}
page = alloc_hugepage_vma(transparent_hugepage_defrag(vma),
@@ -1046,14 +1054,16 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
*/
if (is_huge_zero_pmd(pmd)) {
unsigned long zero_pfn;
+ bool set;
/*
* 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,
+ set = set_huge_zero_page(pgtable, dst_mm, vma, addr, dst_pmd,
zero_pfn);
+ BUG_ON(!set); /* unexpected !pmd_none(dst_pmd) */
ret = 0;
goto out_unlock;
}
@@ -1110,7 +1120,7 @@ unlock:

static int do_huge_pmd_wp_zero_page_fallback(struct mm_struct *mm,
struct vm_area_struct *vma, unsigned long address,
- pmd_t *pmd, unsigned long haddr)
+ pmd_t *pmd, pmd_t orig_pmd, unsigned long haddr)
{
pgtable_t pgtable;
pmd_t _pmd;
@@ -1139,6 +1149,9 @@ static int do_huge_pmd_wp_zero_page_fallback(struct mm_struct *mm,
mmu_notifier_invalidate_range_start(mm, mmun_start, mmun_end);

spin_lock(&mm->page_table_lock);
+ if (unlikely(!pmd_same(*pmd, orig_pmd)))
+ goto out_free_page;
+
pmdp_clear_flush(vma, haddr, pmd);
/* leave pmd empty until pte is filled */

@@ -1171,6 +1184,12 @@ static int do_huge_pmd_wp_zero_page_fallback(struct mm_struct *mm,
ret |= VM_FAULT_WRITE;
out:
return ret;
+out_free_page:
+ spin_unlock(&mm->page_table_lock);
+ mmu_notifier_invalidate_range_end(mm, mmun_start, mmun_end);
+ mem_cgroup_uncharge_page(page);
+ put_page(page);
+ goto out;
}

static int do_huge_pmd_wp_page_fallback(struct mm_struct *mm,
@@ -1317,7 +1336,7 @@ alloc:
count_vm_event(THP_FAULT_FALLBACK);
if (is_huge_zero_pmd(orig_pmd)) {
ret = do_huge_pmd_wp_zero_page_fallback(mm, vma,
- address, pmd, haddr);
+ address, pmd, orig_pmd, haddr);
} else {
ret = do_huge_pmd_wp_page_fallback(mm, vma, address,
pmd, orig_pmd, page, haddr);
--
1.7.11.7


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