commit: 1c641e84719429bbfe62a95ed3545ee7fe24408f From: Andrea Arcangeli <[email protected]> Date: Mon, 5 Mar 2012 14:59:20 -0800 Subject: mm: thp: fix BUG on mm->nr_ptes
Dave Jones reports a few Fedora users hitting the BUG_ON(mm->nr_ptes...) in exit_mmap() recently. Quoting Hugh's discovery and explanation of the SMP race condition: "mm->nr_ptes had unusual locking: down_read mmap_sem plus page_table_lock when incrementing, down_write mmap_sem (or mm_users 0) when decrementing; whereas THP is careful to increment and decrement it under page_table_lock. Now most of those paths in THP also hold mmap_sem for read or write (with appropriate checks on mm_users), but two do not: when split_huge_page() is called by hwpoison_user_mappings(), and when called by add_to_swap(). It's conceivable that the latter case is responsible for the exit_mmap() BUG_ON mm->nr_ptes that has been reported on Fedora." The simplest way to fix it without having to alter the locking is to make split_huge_page() a noop in nr_ptes terms, so by counting the preallocated pagetables that exists for every mapped hugepage. It was an arbitrary choice not to count them and either way is not wrong or right, because they are not used but they're still allocated. Reported-by: Dave Jones <[email protected]> Reported-by: Hugh Dickins <[email protected]> Signed-off-by: Andrea Arcangeli <[email protected]> Acked-by: Hugh Dickins <[email protected]> Cc: David Rientjes <[email protected]> Cc: Josh Boyer <[email protected]> Cc: <[email protected]> [3.0.x, 3.1.x, 3.2.x] Signed-off-by: Andrew Morton <[email protected]> Signed-off-by: Linus Torvalds <[email protected]> --- mm/huge_memory.c | 6 +++--- 1 files changed, 3 insertions(+), 3 deletions(-) diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 91d3efb..8f7fc39 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -671,6 +671,7 @@ static int __do_huge_pmd_anonymous_page(struct mm_struct *mm, set_pmd_at(mm, haddr, pmd, entry); prepare_pmd_huge_pte(pgtable, mm); add_mm_counter(mm, MM_ANONPAGES, HPAGE_PMD_NR); + mm->nr_ptes++; spin_unlock(&mm->page_table_lock); } @@ -789,6 +790,7 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm, pmd = pmd_mkold(pmd_wrprotect(pmd)); set_pmd_at(dst_mm, addr, dst_pmd, pmd); prepare_pmd_huge_pte(pgtable, dst_mm); + dst_mm->nr_ptes++; ret = 0; out_unlock: @@ -887,7 +889,6 @@ static int do_huge_pmd_wp_page_fallback(struct mm_struct *mm, } kfree(pages); - mm->nr_ptes++; smp_wmb(); /* make pte visible before pmd */ pmd_populate(mm, pmd, pgtable); page_remove_rmap(page); @@ -1047,6 +1048,7 @@ int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma, VM_BUG_ON(page_mapcount(page) < 0); add_mm_counter(tlb->mm, MM_ANONPAGES, -HPAGE_PMD_NR); VM_BUG_ON(!PageHead(page)); + tlb->mm->nr_ptes--; spin_unlock(&tlb->mm->page_table_lock); tlb_remove_page(tlb, page); pte_free(tlb->mm, pgtable); @@ -1375,7 +1377,6 @@ static int __split_huge_page_map(struct page *page, pte_unmap(pte); } - mm->nr_ptes++; smp_wmb(); /* make pte visible before pmd */ /* * Up to this point the pmd is present and huge and @@ -1988,7 +1989,6 @@ static void collapse_huge_page(struct mm_struct *mm, set_pmd_at(mm, address, pmd, _pmd); update_mmu_cache(vma, address, _pmd); prepare_pmd_huge_pte(pgtable, mm); - mm->nr_ptes--; spin_unlock(&mm->page_table_lock); #ifndef CONFIG_NUMA -- 1.7.3.4 -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
