From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756251AbaHAV6x (ORCPT ); Fri, 1 Aug 2014 17:58:53 -0400 Received: from mx1.redhat.com ([209.132.183.28]:17483 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751460AbaHAV6w (ORCPT ); Fri, 1 Aug 2014 17:58:52 -0400 Date: Fri, 1 Aug 2014 17:58:45 -0400 From: Naoya Horiguchi To: Andrew Morton Cc: Hugh Dickins , David Rientjes , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Naoya Horiguchi Subject: Re: [PATCH v2 1/3] mm/hugetlb: take refcount under page table lock in follow_huge_pmd() Message-ID: <20140801215845.GA18622@nhori.bos.redhat.com> References: <1406914663-8631-1-git-send-email-n-horiguchi@ah.jp.nec.com> <20140801145358.0d673fc05235d941ca9dec0e@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140801145358.0d673fc05235d941ca9dec0e@linux-foundation.org> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Aug 01, 2014 at 02:53:58PM -0700, Andrew Morton wrote: > On Fri, 1 Aug 2014 13:37:41 -0400 Naoya Horiguchi wrote: > > > We have a race condition between move_pages() and freeing hugepages, > > where move_pages() calls follow_page(FOLL_GET) for hugepages internally > > and tries to get its refcount without preventing concurrent freeing. > > This race crashes the kernel, so this patch fixes it by moving FOLL_GET > > code for hugepages into follow_huge_pmd() with taking the page table lock. > > > > This patch passes the following test. And libhugetlbfs test shows no > > regression. > > > > ... > > How were these bugs discovered? Are we missing some Reported-by's? Hugh pointed out this a few months ago, so I should have added his Reported-by tag. > > --- mmotm-2014-07-22-15-58.orig/include/linux/hugetlb.h > > +++ mmotm-2014-07-22-15-58/include/linux/hugetlb.h > > @@ -101,6 +101,8 @@ struct page *follow_huge_pmd(struct mm_struct *mm, unsigned long address, > > pmd_t *pmd, int write); > > struct page *follow_huge_pud(struct mm_struct *mm, unsigned long address, > > pud_t *pud, int write); > > +struct page *follow_huge_pmd_lock(struct vm_area_struct *vma, > > + unsigned long address, pmd_t *pmd, int flags); > > int pmd_huge(pmd_t pmd); > > int pud_huge(pud_t pmd); > > unsigned long hugetlb_change_protection(struct vm_area_struct *vma, > > > > ... > > > > --- mmotm-2014-07-22-15-58.orig/mm/hugetlb.c > > +++ mmotm-2014-07-22-15-58/mm/hugetlb.c > > @@ -3687,6 +3687,33 @@ follow_huge_pud(struct mm_struct *mm, unsigned long address, > > > > #endif /* CONFIG_ARCH_WANT_GENERAL_HUGETLB */ > > > > +struct page *follow_huge_pmd_lock(struct vm_area_struct *vma, > > + unsigned long address, pmd_t *pmd, int flags) > > Some documentation here wouldn't hurt. Why it exists, what it does. > And especially: any preconditions to calling it (ie: locking). > > > +{ > > + struct page *page; > > + spinlock_t *ptl; > > + > > + if (flags & FOLL_GET) > > + ptl = huge_pte_lock(hstate_vma(vma), vma->vm_mm, (pte_t *)pmd); > > + > > + page = follow_huge_pmd(vma->vm_mm, address, pmd, flags & FOLL_WRITE); > > + > > + if (flags & FOLL_GET) { > > + /* > > + * Refcount on tail pages are not well-defined and > > + * shouldn't be taken. The caller should handle a NULL > > + * return when trying to follow tail pages. > > + */ > > + if (PageHead(page)) > > + get_page(page); > > + else > > + page = NULL; > > + spin_unlock(ptl); > > + } > > + > > + return page; > > +} > > + > > #ifdef CONFIG_MEMORY_FAILURE > > I can't find an implementation of follow_huge_pmd() which actually uses > the fourth argument "write". Zap? OK, I'll post it later. Thanks, Naoya Horiguchi > > Ditto for follow_huge_pud().