From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754497AbdBFQH4 (ORCPT ); Mon, 6 Feb 2017 11:07:56 -0500 Received: from mail-wm0-f67.google.com ([74.125.82.67]:33053 "EHLO mail-wm0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754443AbdBFQHy (ORCPT ); Mon, 6 Feb 2017 11:07:54 -0500 Date: Mon, 6 Feb 2017 19:07:51 +0300 From: "Kirill A. Shutemov" To: Zi Yan , mgorman@techsingularity.net, riel@redhat.com Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, kirill.shutemov@linux.intel.com, akpm@linux-foundation.org, minchan@kernel.org, vbabka@suse.cz, n-horiguchi@ah.jp.nec.com, khandual@linux.vnet.ibm.com, zi.yan@cs.rutgers.edu, Zi Yan Subject: Re: [PATCH v3 03/14] mm: use pmd lock instead of racy checks in zap_pmd_range() Message-ID: <20170206160751.GA29962@node.shutemov.name> References: <20170205161252.85004-1-zi.yan@sent.com> <20170205161252.85004-4-zi.yan@sent.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170205161252.85004-4-zi.yan@sent.com> User-Agent: Mutt/1.5.23.1 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Feb 05, 2017 at 11:12:41AM -0500, Zi Yan wrote: > From: Zi Yan > > Originally, zap_pmd_range() checks pmd value without taking pmd lock. > This can cause pmd_protnone entry not being freed. > > Because there are two steps in changing a pmd entry to a pmd_protnone > entry. First, the pmd entry is cleared to a pmd_none entry, then, > the pmd_none entry is changed into a pmd_protnone entry. > The racy check, even with barrier, might only see the pmd_none entry > in zap_pmd_range(), thus, the mapping is neither split nor zapped. That's definately a good catch. But I don't agree with the solution. Taking pmd lock on each zap_pmd_range() is a significant hit by scalability of the code path. Yes, split ptl lock helps, but it would be nice to avoid the lock in first place. Can we fix change_huge_pmd() instead? Is there a reason why we cannot setup the pmd_protnone() atomically? Mel? Rik? > > Later, in free_pmd_range(), pmd_none_or_clear() will see the > pmd_protnone entry and clear it as a pmd_bad entry. Furthermore, > since the pmd_protnone entry is not properly freed, the corresponding > deposited pte page table is not freed either. > > This causes memory leak or kernel crashing, if VM_BUG_ON() is enabled. > > This patch relies on __split_huge_pmd_locked() and > __zap_huge_pmd_locked(). > > Signed-off-by: Zi Yan > --- > mm/memory.c | 24 +++++++++++------------- > 1 file changed, 11 insertions(+), 13 deletions(-) > > diff --git a/mm/memory.c b/mm/memory.c > index 3929b015faf7..7cfdd5208ef5 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -1233,33 +1233,31 @@ static inline unsigned long zap_pmd_range(struct mmu_gather *tlb, > struct zap_details *details) > { > pmd_t *pmd; > + spinlock_t *ptl; > unsigned long next; > > pmd = pmd_offset(pud, addr); > + ptl = pmd_lock(vma->vm_mm, pmd); > do { > next = pmd_addr_end(addr, end); > if (pmd_trans_huge(*pmd) || pmd_devmap(*pmd)) { > if (next - addr != HPAGE_PMD_SIZE) { > VM_BUG_ON_VMA(vma_is_anonymous(vma) && > !rwsem_is_locked(&tlb->mm->mmap_sem), vma); > - __split_huge_pmd(vma, pmd, addr, false, NULL); > - } else if (zap_huge_pmd(tlb, vma, pmd, addr)) > - goto next; > + __split_huge_pmd_locked(vma, pmd, addr, false); > + } else if (__zap_huge_pmd_locked(tlb, vma, pmd, addr)) > + continue; > /* fall through */ > } > - /* > - * Here there can be other concurrent MADV_DONTNEED or > - * trans huge page faults running, and if the pmd is > - * none or trans huge it can change under us. This is > - * because MADV_DONTNEED holds the mmap_sem in read > - * mode. > - */ > - if (pmd_none_or_trans_huge_or_clear_bad(pmd)) > - goto next; > + > + if (pmd_none_or_clear_bad(pmd)) > + continue; > + spin_unlock(ptl); > next = zap_pte_range(tlb, vma, pmd, addr, next, details); > -next: > cond_resched(); > + spin_lock(ptl); > } while (pmd++, addr = next, addr != end); > + spin_unlock(ptl); > > return addr; > } > -- > 2.11.0 > > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@kvack.org. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: email@kvack.org -- Kirill A. Shutemov