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[209.132.183.28]) by mx.google.com with ESMTPS id f42si3006842qta.250.2019.04.18.23.27.05 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 18 Apr 2019 23:27:05 -0700 (PDT) Received-SPF: pass (google.com: domain of peterx@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 peterx@redhat.com designates 209.132.183.28 as permitted sender) smtp.mailfrom=peterx@redhat.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=redhat.com Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 4C65830BA369; Fri, 19 Apr 2019 06:27:04 +0000 (UTC) Received: from xz-x1 (ovpn-12-224.pek2.redhat.com [10.72.12.224]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 673C41001DCF; Fri, 19 Apr 2019 06:26:55 +0000 (UTC) Date: Fri, 19 Apr 2019 14:26:50 +0800 From: Peter Xu To: Jerome Glisse 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: <20190419062650.GF13323@xz-x1> References: <20190320020642.4000-1-peterx@redhat.com> <20190320020642.4000-15-peterx@redhat.com> <20190418202558.GK3288@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20190418202558.GK3288@redhat.com> User-Agent: Mutt/1.10.1 (2018-07-13) X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.49]); Fri, 19 Apr 2019 06:27:04 +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 Thu, Apr 18, 2019 at 04:51:15PM -0400, Jerome Glisse wrote: > 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. My fault to not have noticed it since the very beginning... thanks for spotting that. I'm squashing below changes into the patch: diff --git a/mm/mprotect.c b/mm/mprotect.c index 3cddfd6627b8..13d493b836bb 100644 --- a/mm/mprotect.c +++ b/mm/mprotect.c @@ -141,22 +141,19 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd, 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; - else if (WARN_ON(ret != VM_FAULT_WRITE)) + if (ret != VM_FAULT_WRITE && ret != 0) + /* Probably OOM */ return pages; pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl); arch_enter_lazy_mmu_mode(); - if (!pte_present(*pte)) + if (ret == 0 || !pte_present(*pte)) /* * This PTE could have been - * modified after COW - * before we have taken the - * lock; retry this PTE + * modified during or after + * COW before take the lock; + * retry. */ goto retry_pte; } [...] > > 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 :) Sure, as usual. :) And I tend to agree it's a good habit. It's just me that always forgot about it. Thanks, -- Peter Xu