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[209.132.183.28]) by mx.google.com with ESMTPS id q14si4246355qkl.87.2019.02.24.23.13.50 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sun, 24 Feb 2019 23:13:50 -0800 (PST) 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 EA0F73099F9F; Mon, 25 Feb 2019 07:13:49 +0000 (UTC) Received: from xz-x1 (ovpn-12-105.pek2.redhat.com [10.72.12.105]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 73D6410027D9; Mon, 25 Feb 2019 07:13:39 +0000 (UTC) Date: Mon, 25 Feb 2019 15:13:36 +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 , Marty McFadden , Andrea Arcangeli , Mike Kravetz , Denis Plotnikov , Mike Rapoport , Mel Gorman , "Kirill A . Shutemov" , "Dr . David Alan Gilbert" Subject: Re: [PATCH v2 14/26] userfaultfd: wp: handle COW properly for uffd-wp Message-ID: <20190225071336.GC28121@xz-x1> References: <20190212025632.28946-1-peterx@redhat.com> <20190212025632.28946-15-peterx@redhat.com> <20190221180423.GN2813@redhat.com> <20190222084603.GK8904@xz-x1> <20190222153508.GE7783@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20190222153508.GE7783@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 [10.5.110.41]); Mon, 25 Feb 2019 07:13:50 +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 Fri, Feb 22, 2019 at 10:35:09AM -0500, Jerome Glisse wrote: > On Fri, Feb 22, 2019 at 04:46:03PM +0800, Peter Xu wrote: > > On Thu, Feb 21, 2019 at 01:04:24PM -0500, Jerome Glisse wrote: > > > On Tue, Feb 12, 2019 at 10:56:20AM +0800, Peter Xu wrote: > > > > This allows uffd-wp to support write-protected pages for COW. > > [...] > > > > > diff --git a/mm/mprotect.c b/mm/mprotect.c > > > > index 9d4433044c21..ae93721f3795 100644 > > > > --- a/mm/mprotect.c > > > > +++ b/mm/mprotect.c > > > > @@ -77,14 +77,13 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd, > > > > 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 +113,46 @@ 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) { > > > > > > This is wrong, if you allow page to be NULL then you gonna segfault > > > in wp_page_copy() down below. Are you sure you want to test for > > > special page ? For anonymous memory this should never happens ie > > > anon page always are regular page. So if you allow userfaulfd to > > > write protect only anonymous vma then there is no point in testing > > > here beside maybe a BUG_ON() just in case ... > > > > It's majorly for zero pages where page can be NULL. Would this be > > clearer: > > > > if (is_zero_pfn(pte_pfn(old_pte)) || (page && page_mapcount(page))) > > > > ? > > > > Now we treat zero pages as normal COW pages so we'll do COW here even > > for zero pages. I think maybe we can do special handling on all over > > the places for zero pages (e.g., we don't write protect a PTE if we > > detected that this is the zero PFN) but I'm uncertain on whether > > that's what we want, so I chose to start with current solution at > > least to achieve functionality first. > > You can keep the vm_normal_page() in that case but split the if > between page == NULL and page != NULL with mapcount > 1. As other- > wise you will segfault below. Could I ask what's the segfault you mentioned? My understanding is that below code has taken page==NULL into consideration already, e.g., we only do get_page() if page!=NULL, and inside wp_page_copy() it has similar considerations. > > > > > > > > > > > + 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; > > > > + else if (WARN_ON(ret != VM_FAULT_WRITE)) > > > > + return pages; > > > > + pte = pte_offset_map_lock(vma->vm_mm, > > > > + pmd, addr, > > > > + &ptl); > > > > > > Here you remap the pte locked but you are not checking if the pte is > > > the one you expect ie is it pointing to the copied page and does it > > > have expect uffd_wp flag. Another thread might have raced between the > > > time you called wp_page_copy() and the time you pte_offset_map_lock() > > > I have not check the mmap_sem so maybe you are protected by it as > > > mprotect is taking it in write mode IIRC, if so you should add a > > > comments at very least so people do not see this as a bug. > > > > Thanks for spotting this. With nornal uffd-wp page fault handling > > path we're only with read lock held (and I would suspect it's racy > > even with write lock...). I agree that there can be a race right > > after the COW has done. > > > > Here IMHO we'll be fine as long as it's still a present PTE, in other > > words, we should be able to tolerate PTE changes as long as it's still > > present otherwise we'll need to retry this single PTE (e.g., the page > > can be quickly marked as migrating swap entry, or even the page could > > be freed beneath us). Do you think below change look good to you to > > be squashed into this patch? > > Ok, but below if must be after arch_enter_lazy_mmu_mode(); not before. Oops... you are right. :) Thanks, > > > > > diff --git a/mm/mprotect.c b/mm/mprotect.c > > index 73a65f07fe41..3423f9692838 100644 > > --- a/mm/mprotect.c > > +++ b/mm/mprotect.c > > @@ -73,6 +73,7 @@ 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; > > @@ -149,6 +150,13 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd, > > pte = pte_offset_map_lock(vma->vm_mm, > > pmd, addr, > > &ptl); > > + if (!pte_present(*pte)) > > + /* > > + * This PTE could have > > + * been modified when COW; > > + * retry it > > + */ > > + goto retry_pte; > > arch_enter_lazy_mmu_mode(); > > } > > } -- Peter Xu