[PATCH] thp: avoid VM_BUG_ON page_count(page) false positives in __collapse_huge_page_copy
From: Andrea Arcangeli <hidden>
Date: 2012-09-25 18:11:25
Subsystem:
memory management, memory management - thp (transparent huge page), the rest · Maintainers:
Andrew Morton, David Hildenbrand, Lorenzo Stoakes, Linus Torvalds
Use page_freeze_refs to prevent speculative pagecache lookups to trigger the false positives, so we're still able to check the page_count to be exact. Signed-off-by: Andrea Arcangeli <redacted> --- mm/huge_memory.c | 19 ++++++++++++++++++- 1 files changed, 18 insertions(+), 1 deletions(-)
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 1598708..7eca652 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c@@ -1704,6 +1704,9 @@ void __khugepaged_exit(struct mm_struct *mm) static void release_pte_page(struct page *page) { +#ifdef CONFIG_DEBUG_VM + page_unfreeze_refs(page, 2); +#endif /* 0 stands for page_is_file_cache(page) == false */ dec_zone_page_state(page, NR_ISOLATED_ANON + 0); unlock_page(page);
@@ -1784,6 +1787,20 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma, VM_BUG_ON(!PageLocked(page)); VM_BUG_ON(PageLRU(page)); +#ifdef CONFIG_DEBUG_VM + /* + * For the VM_BUG_ON check on page_count(page) in + * __collapse_huge_page_copy not to trigger false + * positives we've to prevent the speculative + * pagecache lookups too with page_freeze_refs. We + * could check for >= 2 instead but this provides for + * a more strict debugging behavior. + */ + if (!page_freeze_refs(page, 2)) { + release_pte_pages(pte, _pte+1); + goto out; + } +#endif /* If there is no mapped pte young don't collapse the page */ if (pte_young(pteval) || PageReferenced(page) || mmu_notifier_test_young(vma->vm_mm, address))
@@ -1814,7 +1831,7 @@ static void __collapse_huge_page_copy(pte_t *pte, struct page *page, src_page = pte_page(pteval); copy_user_highpage(page, src_page, address, vma); VM_BUG_ON(page_mapcount(src_page) != 1); - VM_BUG_ON(page_count(src_page) != 2); + VM_BUG_ON(page_count(src_page) != 0); release_pte_page(src_page); /* * ptl mostly unnecessary, but preempt has to --
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