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[209.132.183.28]) by mx.google.com with ESMTPS id l27si1679735qtl.125.2019.04.18.13.51.24 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 18 Apr 2019 13:51:24 -0700 (PDT) Received-SPF: pass (google.com: domain of jglisse@redhat.com designates 209.132.183.28 as permitted sender) client-ip=209.132.183.28; Authentication-Results: mx.google.com; spf=pass (google.com: domain of jglisse@redhat.com designates 209.132.183.28 as permitted sender) smtp.mailfrom=jglisse@redhat.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=redhat.com Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 78C61C04AC4B; Thu, 18 Apr 2019 20:51:23 +0000 (UTC) Received: from redhat.com (unknown [10.20.6.236]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 1926419C58; Thu, 18 Apr 2019 20:51:16 +0000 (UTC) Date: Thu, 18 Apr 2019 16:51:15 -0400 From: Jerome Glisse To: Peter Xu Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, David Hildenbrand , Hugh Dickins , Maya Gokhale , Pavel Emelyanov , Johannes Weiner , Martin Cracauer , Shaohua Li , Andrea Arcangeli , Mike Kravetz , Denis Plotnikov , Mike Rapoport , Marty McFadden , Mel Gorman , "Kirill A . Shutemov" , "Dr . David Alan Gilbert" Subject: Re: [PATCH v3 14/28] userfaultfd: wp: handle COW properly for uffd-wp Message-ID: <20190418202558.GK3288@redhat.com> References: <20190320020642.4000-1-peterx@redhat.com> <20190320020642.4000-15-peterx@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190320020642.4000-15-peterx@redhat.com> User-Agent: Mutt/1.11.3 (2019-02-01) X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.31]); Thu, 18 Apr 2019 20:51:23 +0000 (UTC) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, Mar 20, 2019 at 10:06:28AM +0800, Peter Xu wrote: > This allows uffd-wp to support write-protected pages for COW. > > For example, the uffd write-protected PTE could also be write-protected > by other usages like COW or zero pages. When that happens, we can't > simply set the write bit in the PTE since otherwise it'll change the > content of every single reference to the page. Instead, we should do > the COW first if necessary, then handle the uffd-wp fault. > > To correctly copy the page, we'll also need to carry over the > _PAGE_UFFD_WP bit if it was set in the original PTE. > > For huge PMDs, we just simply split the huge PMDs where we want to > resolve an uffd-wp page fault always. That matches what we do with > general huge PMD write protections. In that way, we resolved the huge > PMD copy-on-write issue into PTE copy-on-write. > > Signed-off-by: Peter Xu This one has a bug see below. > --- > mm/memory.c | 5 +++- > mm/mprotect.c | 64 ++++++++++++++++++++++++++++++++++++++++++++++++--- > 2 files changed, 65 insertions(+), 4 deletions(-) > > diff --git a/mm/memory.c b/mm/memory.c > index e7a4b9650225..b8a4c0bab461 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -2291,7 +2291,10 @@ vm_fault_t wp_page_copy(struct vm_fault *vmf) > } > flush_cache_page(vma, vmf->address, pte_pfn(vmf->orig_pte)); > entry = mk_pte(new_page, vma->vm_page_prot); > - entry = maybe_mkwrite(pte_mkdirty(entry), vma); > + if (pte_uffd_wp(vmf->orig_pte)) > + entry = pte_mkuffd_wp(entry); > + else > + entry = maybe_mkwrite(pte_mkdirty(entry), vma); > /* > * Clear the pte entry and flush it first, before updating the > * pte with the new entry. This will avoid a race condition > diff --git a/mm/mprotect.c b/mm/mprotect.c > index 9d4433044c21..855dddb07ff2 100644 > --- a/mm/mprotect.c > +++ b/mm/mprotect.c > @@ -73,18 +73,18 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd, > flush_tlb_batched_pending(vma->vm_mm); > arch_enter_lazy_mmu_mode(); > do { > +retry_pte: > oldpte = *pte; > if (pte_present(oldpte)) { > pte_t ptent; > bool preserve_write = prot_numa && pte_write(oldpte); > + struct page *page; > > /* > * Avoid trapping faults against the zero or KSM > * pages. See similar comment in change_huge_pmd. > */ > if (prot_numa) { > - struct page *page; > - > page = vm_normal_page(vma, addr, oldpte); > if (!page || PageKsm(page)) > continue; > @@ -114,6 +114,54 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd, > continue; > } > > + /* > + * Detect whether we'll need to COW before > + * resolving an uffd-wp fault. Note that this > + * includes detection of the zero page (where > + * page==NULL) > + */ > + if (uffd_wp_resolve) { > + /* If the fault is resolved already, skip */ > + if (!pte_uffd_wp(*pte)) > + continue; > + page = vm_normal_page(vma, addr, oldpte); > + if (!page || page_mapcount(page) > 1) { > + struct vm_fault vmf = { > + .vma = vma, > + .address = addr & PAGE_MASK, > + .page = page, > + .orig_pte = oldpte, > + .pmd = pmd, > + /* pte and ptl not needed */ > + }; > + vm_fault_t ret; > + > + if (page) > + get_page(page); > + arch_leave_lazy_mmu_mode(); > + pte_unmap_unlock(pte, ptl); > + ret = wp_page_copy(&vmf); > + /* PTE is changed, or OOM */ > + if (ret == 0) > + /* It's done by others */ > + continue; This is wrong if ret == 0 you still need to remap the pte before continuing as otherwise you will go to next pte without the page table lock for the directory. So 0 case must be handled after arch_enter_lazy_mmu_mode() below. Sorry i should have catch that in previous review. > + else if (WARN_ON(ret != VM_FAULT_WRITE)) > + return pages; > + pte = pte_offset_map_lock(vma->vm_mm, > + pmd, addr, > + &ptl); > + arch_enter_lazy_mmu_mode(); > + if (!pte_present(*pte)) > + /* > + * This PTE could have been > + * modified after COW > + * before we have taken the > + * lock; retry this PTE > + */ > + goto retry_pte; > + } > + } > + > ptent = ptep_modify_prot_start(mm, addr, pte); > ptent = pte_modify(ptent, newprot); > if (preserve_write) > unsigned long pages = 0; > unsigned long nr_huge_updates = 0; > struct mmu_notifier_range range; > + bool uffd_wp_resolve = cp_flags & MM_CP_UFFD_WP_RESOLVE; > > range.start = 0; > > @@ -202,7 +251,16 @@ static inline unsigned long change_pmd_range(struct vm_area_struct *vma, > } > > if (is_swap_pmd(*pmd) || pmd_trans_huge(*pmd) || pmd_devmap(*pmd)) { > - if (next - addr != HPAGE_PMD_SIZE) { > + /* > + * When resolving an userfaultfd write > + * protection fault, it's not easy to identify > + * whether a THP is shared with others and > + * whether we'll need to do copy-on-write, so > + * just split it always for now to simply the > + * procedure. And that's the policy too for > + * general THP write-protect in af9e4d5f2de2. > + */ > + if (next - addr != HPAGE_PMD_SIZE || uffd_wp_resolve) { Just a nit pick can you please add () to next - addr ie: if ((next - addr) != HPAGE_PMD_SIZE || uffd_wp_resolve) { I know it is not needed but each time i bump into this i have to scratch my head for second to remember the operator rules :) > __split_huge_pmd(vma, pmd, addr, false, NULL); > } else { > int nr_ptes = change_huge_pmd(vma, pmd, addr, > -- > 2.17.1 >