From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759285AbaIOWki (ORCPT ); Mon, 15 Sep 2014 18:40:38 -0400 Received: from mx1.redhat.com ([209.132.183.28]:15674 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755034AbaIOWkf (ORCPT ); Mon, 15 Sep 2014 18:40:35 -0400 From: Naoya Horiguchi To: Andrew Morton , Hugh Dickins Cc: David Rientjes , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Naoya Horiguchi , Subject: [PATCH v3 3/5] mm/hugetlb: fix getting refcount 0 page in hugetlb_fault() Date: Mon, 15 Sep 2014 18:39:57 -0400 Message-Id: <1410820799-27278-4-git-send-email-n-horiguchi@ah.jp.nec.com> In-Reply-To: <1410820799-27278-1-git-send-email-n-horiguchi@ah.jp.nec.com> References: <1410820799-27278-1-git-send-email-n-horiguchi@ah.jp.nec.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org When running the test which causes the race as shown in the previous patch, we can hit the BUG "get_page() on refcount 0 page" in hugetlb_fault(). This race happens when pte turns into migration entry just after the first check of is_hugetlb_entry_migration() in hugetlb_fault() passed with false. To fix this, we need to check pte_present() again after huge_ptep_get(). This patch also reorders taking ptl and doing pte_page(), because pte_page() should be done in ptl. Due to this reordering, we need use trylock_page() in page != pagecache_page case to respect locking order. Fixes: 66aebce747ea ("hugetlb: fix race condition in hugetlb_fault()") Signed-off-by: Naoya Horiguchi Cc: # [3.2+] --- ChangeLog v4: - move !pte_present(entry) (i.e. migration/hwpoison) check before taking page table lock - call wait_on_page_locked() if trylock_page() returns false - remove unused label out_unlock_page - fix the order of put_page() and unlock_page() after out_put_page label - move changelog under '---' Hugh advised me for ver.3 that we can call migration_entry_wait_huge() when the !pte_present(entry) check returns true to avoid busy faulting. But it seems that in that case only one additional page fault happens instead of busy faulting, because is_hugetlb_entry_migration() in the second call of hugetlb_fault() should return true and then migration_entry_wait_huge() is called. We could avoid this additional page fault by adding another migration_entry_wait_huge(), but then we should separate pte_present() check into is_hugetlb_entry_migration() path and is_hugetlb_entry_hwpoisoned() path, which makes code complicated. So let me take the simpler approach for sending stable tree. And it's also advised that we can clean up is_hugetlb_entry_migration() and is_hugetlb_entry_hwpoisoned() things. This will be done in another work, and the above migration_entry_wait_huge problem will be revisited there. ChangeLog v3: - doing pte_page() and taking refcount under page table lock - check pte_present after taking ptl, which makes it unnecessary to use get_page_unless_zero() - use trylock_page in page != pagecache_page case - fixed target stable version --- mm/hugetlb.c | 42 ++++++++++++++++++++++++++++-------------- 1 file changed, 28 insertions(+), 14 deletions(-) diff --git mmotm-2014-09-09-14-42.orig/mm/hugetlb.c mmotm-2014-09-09-14-42/mm/hugetlb.c index 941832ee3d5a..52f001943169 100644 --- mmotm-2014-09-09-14-42.orig/mm/hugetlb.c +++ mmotm-2014-09-09-14-42/mm/hugetlb.c @@ -3128,6 +3128,7 @@ int hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma, struct page *pagecache_page = NULL; struct hstate *h = hstate_vma(vma); struct address_space *mapping; + int need_wait_lock = 0; address &= huge_page_mask(h); @@ -3164,6 +3165,15 @@ int hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma, } ret = 0; + /* + * entry could be a migration/hwpoison entry at this point, so this + * check prevents the kernel from going below assuming that we have + * a active hugepage in pagecache. This goto expects the 2nd page fault, + * and is_hugetlb_entry_(migration|hwpoisoned) check will properly + * handle it. + */ + if (!pte_present(entry)) + goto out_mutex; /* * If we are going to COW the mapping later, we examine the pending @@ -3184,6 +3194,12 @@ int hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma, vma, address); } + ptl = huge_pte_lock(h, mm, ptep); + + /* Check for a racing update before calling hugetlb_cow */ + if (unlikely(!pte_same(entry, huge_ptep_get(ptep)))) + goto out_ptl; + /* * hugetlb_cow() requires page locks of pte_page(entry) and * pagecache_page, so here we need take the former one @@ -3192,22 +3208,19 @@ int hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma, * so no worry about deadlock. */ page = pte_page(entry); - get_page(page); if (page != pagecache_page) - lock_page(page); - - ptl = huge_pte_lockptr(h, mm, ptep); - spin_lock(ptl); - /* Check for a racing update before calling hugetlb_cow */ - if (unlikely(!pte_same(entry, huge_ptep_get(ptep)))) - goto out_ptl; + if (!trylock_page(page)) { + need_wait_lock = 1; + goto out_ptl; + } + get_page(page); if (flags & FAULT_FLAG_WRITE) { if (!huge_pte_write(entry)) { ret = hugetlb_cow(mm, vma, address, ptep, entry, pagecache_page, ptl); - goto out_ptl; + goto out_put_page; } entry = huge_pte_mkdirty(entry); } @@ -3215,7 +3228,10 @@ int hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma, if (huge_ptep_set_access_flags(vma, address, ptep, entry, flags & FAULT_FLAG_WRITE)) update_mmu_cache(vma, address, ptep); - +out_put_page: + if (page != pagecache_page) + unlock_page(page); + put_page(page); out_ptl: spin_unlock(ptl); @@ -3223,12 +3239,10 @@ int hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma, unlock_page(pagecache_page); put_page(pagecache_page); } - if (page != pagecache_page) - unlock_page(page); - put_page(page); - out_mutex: mutex_unlock(&htlb_fault_mutex_table[hash]); + if (need_wait_lock) + wait_on_page_locked(page); return ret; } -- 1.9.3