From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-f198.google.com (mail-pf0-f198.google.com [209.85.192.198]) by kanga.kvack.org (Postfix) with ESMTP id 5B4576B0069 for ; Tue, 18 Oct 2016 23:22:17 -0400 (EDT) Received: by mail-pf0-f198.google.com with SMTP id r16so6222057pfg.4 for ; Tue, 18 Oct 2016 20:22:17 -0700 (PDT) Received: from out4440.biz.mail.alibaba.com (out4440.biz.mail.alibaba.com. [47.88.44.40]) by mx.google.com with ESMTP id l184si35018926pgd.113.2016.10.18.20.22.14 for ; Tue, 18 Oct 2016 20:22:16 -0700 (PDT) Reply-To: "Hillf Danton" From: "Hillf Danton" References: <20161018154245.18023-1-aneesh.kumar@linux.vnet.ibm.com> In-Reply-To: <20161018154245.18023-1-aneesh.kumar@linux.vnet.ibm.com> Subject: Re: [PATCH] mm/hugetlb: Use the right pte val for compare in hugetlb_cow Date: Wed, 19 Oct 2016 11:22:01 +0800 Message-ID: <027301d229b7$f543f030$dfcbd090$@alibaba-inc.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Content-Language: zh-cn Sender: owner-linux-mm@kvack.org List-ID: To: "'Aneesh Kumar K.V'" , akpm@linux-foundation.org, 'Jan Stancek' , 'Mike Kravetz' Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org On Tuesday, October 18, 2016 11:43 PM Aneesh Kumar K.V wrote: > > We cannot use the pte value used in set_pte_at for pte_same comparison, > because archs like ppc64, filter/add new pte flag in set_pte_at. Instead > fetch the pte value inside hugetlb_cow. We are comparing pte value to > make sure the pte didn't change since we dropped the page table lock. > hugetlb_cow get called with page table lock held, and we can take a copy > of the pte value before we drop the page table lock. > > With hugetlbfs, we optimize the MAP_PRIVATE write fault path with no > previous mapping (huge_pte_none entries), by forcing a cow in the fault > path. This avoid take an addition fault to covert a read-only mapping > to read/write. Here we were comparing a recently instantiated pte (via > set_pte_at) to the pte values from linux page table. As explained above > on ppc64 such pte_same check returned wrong result, resulting in us > taking an additional fault on ppc64. > > Fixes: 6a119eae942c ("powerpc/mm: Add a _PAGE_PTE bit") > > Reported-by: Jan Stancek > Signed-off-by: Aneesh Kumar K.V > --- Acked-by: Hillf Danton > mm/hugetlb.c | 12 +++++++----- > 1 file changed, 7 insertions(+), 5 deletions(-) > > diff --git a/mm/hugetlb.c b/mm/hugetlb.c > index ec49d9ef1eef..da8fbd02b92e 100644 > --- a/mm/hugetlb.c > +++ b/mm/hugetlb.c > @@ -3386,15 +3386,17 @@ static void unmap_ref_private(struct mm_struct *mm, struct vm_area_struct *vma, > * Keep the pte_same checks anyway to make transition from the mutex easier. > */ > static int hugetlb_cow(struct mm_struct *mm, struct vm_area_struct *vma, > - unsigned long address, pte_t *ptep, pte_t pte, > - struct page *pagecache_page, spinlock_t *ptl) > + unsigned long address, pte_t *ptep, > + struct page *pagecache_page, spinlock_t *ptl) > { > + pte_t pte; > struct hstate *h = hstate_vma(vma); > struct page *old_page, *new_page; > int ret = 0, outside_reserve = 0; > unsigned long mmun_start; /* For mmu_notifiers */ > unsigned long mmun_end; /* For mmu_notifiers */ > > + pte = huge_ptep_get(ptep); > old_page = pte_page(pte); > > retry_avoidcopy: > @@ -3668,7 +3670,7 @@ static int hugetlb_no_page(struct mm_struct *mm, struct vm_area_struct *vma, > hugetlb_count_add(pages_per_huge_page(h), mm); > if ((flags & FAULT_FLAG_WRITE) && !(vma->vm_flags & VM_SHARED)) { > /* Optimization, do the COW without a second fault */ > - ret = hugetlb_cow(mm, vma, address, ptep, new_pte, page, ptl); > + ret = hugetlb_cow(mm, vma, address, ptep, page, ptl); > } > > spin_unlock(ptl); > @@ -3822,8 +3824,8 @@ int hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma, > > if (flags & FAULT_FLAG_WRITE) { > if (!huge_pte_write(entry)) { > - ret = hugetlb_cow(mm, vma, address, ptep, entry, > - pagecache_page, ptl); > + ret = hugetlb_cow(mm, vma, address, ptep, > + pagecache_page, ptl); > goto out_put_page; > } > entry = huge_pte_mkdirty(entry); > -- > 2.10.1 -- 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