From c3870662e6be07c04e3cbd057b6a3b1a89fe3e03 Mon Sep 17 00:00:00 2001 From: Mel Gorman Date: Mon, 7 Apr 2014 15:36:56 -0700 Subject: [PATCH] mm: numa: recheck for transhuge pages under lock during protection changes Sasha reported the following bug using trinity kernel BUG at mm/mprotect.c:149! invalid opcode: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC Dumping ftrace buffer: (ftrace buffer empty) Modules linked in: CPU: 20 PID: 26219 Comm: trinity-c216 Tainted: G W 3.14.0-rc5-next-20140305-sasha-00011-ge06f5f3-dirty #105 task: ffff8800b6c80000 ti: ffff880228436000 task.ti: ffff880228436000 RIP: change_protection_range+0x3b3/0x500 Call Trace: change_protection+0x25/0x30 change_prot_numa+0x1b/0x30 task_numa_work+0x279/0x360 task_work_run+0xae/0xf0 do_notify_resume+0x8e/0xe0 retint_signal+0x4d/0x92 The VM_BUG_ON was added in -mm by the patch "mm,numa: reorganize change_pmd_range". The race existed without the patch but was just harder to hit. The problem is that a transhuge check is made without holding the PTL. It's possible at the time of the check that a parallel fault clears the pmd and inserts a new one which then triggers the VM_BUG_ON check. This patch removes the VM_BUG_ON but fixes the race by rechecking transhuge under the PTL when marking page tables for NUMA hinting and bailing if a race occurred. It is not a problem for calls to mprotect() as they hold mmap_sem for write. Signed-off-by: Mel Gorman Reported-by: Sasha Levin Reviewed-by: Rik van Riel Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/mprotect.c | 36 ++++++++++++++++++++++++++++++++++-- 1 file changed, 34 insertions(+), 2 deletions(-) diff --git a/mm/mprotect.c b/mm/mprotect.c index b8c7d69a503275..1ed954115c9c99 100644 --- a/mm/mprotect.c +++ b/mm/mprotect.c @@ -36,6 +36,34 @@ static inline pgprot_t pgprot_modify(pgprot_t oldprot, pgprot_t newprot) } #endif +/* + * For a prot_numa update we only hold mmap_sem for read so there is a + * potential race with faulting where a pmd was temporarily none. This + * function checks for a transhuge pmd under the appropriate lock. It + * returns a pte if it was successfully locked or NULL if it raced with + * a transhuge insertion. + */ +static pte_t *lock_pte_protection(struct vm_area_struct *vma, pmd_t *pmd, + unsigned long addr, int prot_numa, spinlock_t **ptl) +{ + pte_t *pte; + spinlock_t *pmdl; + + /* !prot_numa is protected by mmap_sem held for write */ + if (!prot_numa) + return pte_offset_map_lock(vma->vm_mm, pmd, addr, ptl); + + pmdl = pmd_lock(vma->vm_mm, pmd); + if (unlikely(pmd_trans_huge(*pmd) || pmd_none(*pmd))) { + spin_unlock(pmdl); + return NULL; + } + + pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, ptl); + spin_unlock(pmdl); + return pte; +} + static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd, unsigned long addr, unsigned long end, pgprot_t newprot, int dirty_accountable, int prot_numa) @@ -45,7 +73,10 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd, spinlock_t *ptl; unsigned long pages = 0; - pte = pte_offset_map_lock(mm, pmd, addr, &ptl); + pte = lock_pte_protection(vma, pmd, addr, prot_numa, &ptl); + if (!pte) + return 0; + arch_enter_lazy_mmu_mode(); do { oldpte = *pte; @@ -132,12 +163,13 @@ static inline unsigned long change_pmd_range(struct vm_area_struct *vma, pages += HPAGE_PMD_NR; nr_huge_updates++; } + + /* huge pmd was handled */ continue; } } /* fall through, the trans huge pmd just split */ } - VM_BUG_ON(pmd_trans_huge(*pmd)); this_pages = change_pte_range(vma, pmd, addr, next, newprot, dirty_accountable, prot_numa); pages += this_pages;