From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 94875C4363D for ; Wed, 23 Sep 2020 14:20:07 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 4972221D92 for ; Wed, 23 Sep 2020 14:20:07 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726638AbgIWOUG (ORCPT ); Wed, 23 Sep 2020 10:20:06 -0400 Received: from mx2.suse.de ([195.135.220.15]:53916 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726151AbgIWOUF (ORCPT ); Wed, 23 Sep 2020 10:20:05 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id E3CB7B23E; Wed, 23 Sep 2020 14:20:40 +0000 (UTC) Received: by quack2.suse.cz (Postfix, from userid 1000) id 3C2BC1E12E3; Wed, 23 Sep 2020 16:20:03 +0200 (CEST) Date: Wed, 23 Sep 2020 16:20:03 +0200 From: Jan Kara To: Peter Xu Cc: Jason Gunthorpe , John Hubbard , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton , Jan Kara , Michal Hocko , Kirill Tkhai , Kirill Shutemov , Hugh Dickins , Christoph Hellwig , Andrea Arcangeli , Oleg Nesterov , Leon Romanovsky , Linus Torvalds , Jann Horn Subject: Re: [PATCH 1/5] mm: Introduce mm_struct.has_pinned Message-ID: <20200923142003.GB15875@quack2.suse.cz> References: <20200921211744.24758-1-peterx@redhat.com> <20200921211744.24758-2-peterx@redhat.com> <224908c1-5d0f-8e01-baa9-94ec2374971f@nvidia.com> <20200922151736.GD19098@xz-x1> <20200922161046.GB731578@ziepe.ca> <20200922175415.GI19098@xz-x1> <20200922191116.GK8409@ziepe.ca> <20200923002735.GN19098@xz-x1> <20200923131043.GA59978@xz-x1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200923131043.GA59978@xz-x1> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed 23-09-20 09:10:43, Peter Xu wrote: > On Tue, Sep 22, 2020 at 08:27:35PM -0400, Peter Xu wrote: > > On Tue, Sep 22, 2020 at 04:11:16PM -0300, Jason Gunthorpe wrote: > > > On Tue, Sep 22, 2020 at 01:54:15PM -0400, Peter Xu wrote: > > > > diff --git a/mm/memory.c b/mm/memory.c > > > > index 8f3521be80ca..6591f3f33299 100644 > > > > +++ b/mm/memory.c > > > > @@ -888,8 +888,8 @@ copy_one_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm, > > > > * Because we'll need to release the locks before doing cow, > > > > * pass this work to upper layer. > > > > */ > > > > - if (READ_ONCE(src_mm->has_pinned) && wp && > > > > - page_maybe_dma_pinned(page)) { > > > > + if (wp && page_maybe_dma_pinned(page) && > > > > + READ_ONCE(src_mm->has_pinned)) { > > > > /* We've got the page already; we're safe */ > > > > data->cow_old_page = page; > > > > data->cow_oldpte = *src_pte; > > > > > > > > I can also add some more comment to emphasize this. > > > > > > It is not just that, but the ptep_set_wrprotect() has to be done > > > earlier. > > > > Now I understand your point, I think.. So I guess it's not only about > > has_pinned, but it should be a race between the fast-gup and the fork() code, > > even if has_pinned is always set. > > > > > > > > Otherwise it races like: > > > > > > pin_user_pages_fast() fork() > > > atomic_set(has_pinned, 1); > > > [..] > > > atomic_read(page->_refcount) //false > > > // skipped atomic_read(has_pinned) > > > atomic_add(page->_refcount) > > > ordered check write protect() > > > ordered set write protect() > > > > > > And now have a write protect on a DMA pinned page, which is the > > > invarient we are trying to create. > > > > > > The best algorithm I've thought of is something like: > > > > > > pte_map_lock() > > > if (page) { > > > if (wp) { > > > ptep_set_wrprotect() > > > /* Order with try_grab_compound_head(), either we see > > > * page_maybe_dma_pinned(), or they see the wrprotect */ > > > get_page(); > > > > Is this get_page() a must to be after ptep_set_wrprotect() explicitly? IIUC > > what we need is to order ptep_set_wrprotect() and page_maybe_dma_pinned() here. > > E.g., would a "mb()" work? > > > > Another thing is, do we need similar thing for e.g. gup_pte_range(), so that > > to guarantee ordering of try_grab_compound_head() and the pte change check? > > > > > > > > if (page_maybe_dma_pinned() && READ_ONCE(src_mm->has_pinned)) { > > > put_page(); > > > ptep_clear_wrprotect() > > > > > > // do copy > > > return > > > } > > > } else { > > > get_page(); > > > } > > > page_dup_rmap() > > > pte_unmap_lock() > > > > > > Then the do_wp_page() path would have to detect that the page is not > > > write protected under the pte lock inside the fault handler and just > > > do nothing. > > > > Yes, iiuc do_wp_page() should be able to handle spurious write page faults like > > this already, as below: > > > > vmf->ptl = pte_lockptr(vmf->vma->vm_mm, vmf->pmd); > > spin_lock(vmf->ptl); > > ... > > if (vmf->flags & FAULT_FLAG_WRITE) { > > if (!pte_write(entry)) > > return do_wp_page(vmf); > > entry = pte_mkdirty(entry); > > } > > > > So when spin_lock() returns: > > > > - When it's a real cow (not pinned pages; we write-protected it and it keeps > > write-protected), we should do cow here as usual. > > > > - When it's a fake cow (pinned pages), the write bit should have been > > recovered before the page table lock released, and we'll skip do_wp_page() > > and retry the page fault immediately. > > > > > Ie the set/clear could be visible to the CPU and trigger a > > > spurious fault, but never trigger a COW. > > > > > > Thus 'wp' becomes a 'lock' that prevents GUP from returning this page. > > > > Another question is, how about read fast-gup for pinning? Because we can't use > > the write-protect mechanism to block a read gup. I remember we've discussed > > similar things and iirc your point is "pinned pages should always be with > > WRITE". However now I still doubt it... Because I feel like read gup is still > > legal (as I mentioned previously - when device purely writes to the page and > > the processor only reads from it). > > > > > > > > Very tricky, deserves a huge comment near the ptep_clear_wrprotect() > > > > > > Consider the above algorithm beside the gup_fast() algorithm: > > > > > > if (!pte_access_permitted(pte, flags & FOLL_WRITE)) > > > goto pte_unmap; > > > [..] > > > head = try_grab_compound_head(page, 1, flags); > > > if (!head) > > > goto pte_unmap; > > > if (unlikely(pte_val(pte) != pte_val(*ptep))) { > > > put_compound_head(head, 1, flags); > > > goto pte_unmap; > > > > > > That last *ptep will check that the WP is not set after making > > > page_maybe_dma_pinned() true. > > > > > > It still looks reasonable, the extra work is still just the additional > > > atomic in page_maybe_dma_pinned(), just everything else has to be very > > > carefully sequenced due to unlocked page table accessors. > > > > Tricky! I'm still thinking about some easier way but no much clue so far. > > Hopefully we'll figure out something solid soon. > > Hmm, how about something like below? Would this be acceptable? > > ------8<-------- > diff --git a/mm/gup.c b/mm/gup.c > index 2d9019bf1773..698bc2b520ac 100644 > --- a/mm/gup.c > +++ b/mm/gup.c > @@ -2136,6 +2136,18 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end, > struct dev_pagemap *pgmap = NULL; > int nr_start = *nr, ret = 0; > pte_t *ptep, *ptem; > + spinlock_t *ptl = NULL; > + > + /* > + * More strict with FOLL_PIN, otherwise it could race with fork(). The > + * page table lock guarantees that fork() will capture all the pinned > + * pages when dup_mm() and do proper page copy on them. > + */ > + if (flags & FOLL_PIN) { > + ptl = pte_lockptr(mm, pmd); > + if (!spin_trylock(ptl)) > + return 0; > + } I'd hate to take spinlock in the GUP-fast path. Also I don't think this is quite correct because GUP-fast-only can be called from interrupt context and page table locks are not interrupt safe. That being said I don't see what's wrong with the solution Jason proposed of first setting writeprotect and then checking page_may_be_dma_pinned() during fork(). That should work just fine AFAICT... BTW note that GUP-fast code is (and this is deliberated because e.g. DAX depends on this) first updating page->_refcount and then rechecking PTE didn't change and the page->_refcount update is actually done using atomic_add_unless() so that it cannot be reordered wrt the PTE check. So the fork() code only needs to add barriers to pair with this. Honza -- Jan Kara SUSE Labs, CR