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=-13.5 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,NICE_REPLY_A,SIGNED_OFF_BY,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 761C2C4363A for ; Tue, 27 Oct 2020 07:32:49 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id C29F92071A for ; Tue, 27 Oct 2020 07:32:48 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=nvidia.com header.i=@nvidia.com header.b="ZdjEY4QR" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org C29F92071A Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=nvidia.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 087DD6B006E; Tue, 27 Oct 2020 03:32:48 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 060E86B0070; Tue, 27 Oct 2020 03:32:48 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id E42C56B0071; Tue, 27 Oct 2020 03:32:47 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0089.hostedemail.com [216.40.44.89]) by kanga.kvack.org (Postfix) with ESMTP id B8DB56B006E for ; Tue, 27 Oct 2020 03:32:47 -0400 (EDT) Received: from smtpin30.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay01.hostedemail.com (Postfix) with ESMTP id 62949180AD807 for ; Tue, 27 Oct 2020 07:32:47 +0000 (UTC) X-FDA: 77416888374.30.cord77_550f6122727a Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin30.hostedemail.com (Postfix) with ESMTP id 41D80180B3C83 for ; Tue, 27 Oct 2020 07:32:47 +0000 (UTC) X-HE-Tag: cord77_550f6122727a X-Filterd-Recvd-Size: 9538 Received: from hqnvemgate26.nvidia.com (hqnvemgate26.nvidia.com [216.228.121.65]) by imf33.hostedemail.com (Postfix) with ESMTP for ; Tue, 27 Oct 2020 07:32:46 +0000 (UTC) Received: from hqmail.nvidia.com (Not Verified[216.228.121.13]) by hqnvemgate26.nvidia.com (using TLS: TLSv1.2, AES256-SHA) id ; Tue, 27 Oct 2020 00:32:24 -0700 Received: from [10.2.58.85] (10.124.1.5) by HQMAIL107.nvidia.com (172.20.187.13) with Microsoft SMTP Server (TLS) id 15.0.1473.3; Tue, 27 Oct 2020 07:32:44 +0000 Subject: Re: [PATCH 2/2] mm: prevent gup_fast from racing with COW during fork To: Jason Gunthorpe , CC: Andrea Arcangeli , Andrew Morton , Aneesh Kumar K.V , Christoph Hellwig , Hugh Dickins , Jan Kara , Jann Horn , Kirill Shutemov , Kirill Tkhai , Leon Romanovsky , Linux-MM , Michal Hocko , Oleg Nesterov , Peter Xu , Linus Torvalds References: <2-v1-281e425c752f+2df-gup_fork_jgg@nvidia.com> From: John Hubbard Message-ID: <46ae9b97-2d5d-548b-9968-17e08746f2a6@nvidia.com> Date: Tue, 27 Oct 2020 00:32:44 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.12.0 MIME-Version: 1.0 In-Reply-To: <2-v1-281e425c752f+2df-gup_fork_jgg@nvidia.com> Content-Type: text/plain; charset="utf-8"; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.124.1.5] X-ClientProxiedBy: HQMAIL101.nvidia.com (172.20.187.10) To HQMAIL107.nvidia.com (172.20.187.13) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=nvidia.com; s=n1; t=1603783944; bh=tdwV8dzEFI87Va0ff8upkVEmpndx48qm1/gy960ZFFk=; h=Subject:To:CC:References:From:Message-ID:Date:User-Agent: MIME-Version:In-Reply-To:Content-Type:Content-Language: Content-Transfer-Encoding:X-Originating-IP:X-ClientProxiedBy; b=ZdjEY4QRblb3mcVYUFqVEniuqiVdDgKYnN9vZa/zZcBuYoBk3/sXCbjjd0Iz0azQA 0JOxmkj18mKfSUo7deeNQNP4XXpzKovlFA5BB6Ggupb3uyBGjH9Gm0XXN4fw97ktBL ivXVJMX62IoOat1Q+4+3+re2K/gFj82wNFSSzvkaCjhkRAcyQGnFknwxH0J++M4ftS 1FZ2jsVXb1GD5j+OIO/6oiYWbOj91WtS/V9PIjclQqB1Qji4pd4qQoZGjvysfSLE3O H/zeSA1vH1kcKCqLHCqMLEZ01z4t6HR+uKHW567Yab5rb3ygp9fMPzSUN3gIklabry ZK3of4x3/joJg== 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 10/23/20 5:19 PM, Jason Gunthorpe wrote: > Since commit 70e806e4e645 ("mm: Do early cow for pinned pages during > fork() for ptes") pages under a FOLL_PIN will not be write protected > during COW for fork. This means that pages returned from > pin_user_pages(FOLL_WRITE) should not become write protected while the pin > is active. > > However, there is a small race where get_user_pages_fast(FOLL_PIN) can > establish a FOLL_PIN at the same time copy_present_page() is write > protecting it: > > CPU 0 CPU 1 > get_user_pages_fast() > internal_get_user_pages_fast() > copy_page_range() > pte_alloc_map_lock() > copy_present_page() > atomic_read(has_pinned) == 0 > page_maybe_dma_pinned() == false > atomic_set(has_pinned, 1); > gup_pgd_range() > gup_pte_range() > pte_t pte = gup_get_pte(ptep) > pte_access_permitted(pte) > try_grab_compound_head() > pte = maybe_mkwrite() Are you sure you didn't reverse this part? That makes it writeable, not write-protected. And it doesn't actually happen as shown, either: if page_maybe_dma_pinned() returns false, then copy_present_page() returns early and does *not* do the maybe_mkwrite(), but instead does: pte = pte_wrprotect(pte); ...so probably you want to write that line, instead of what you have. > set_pte_at(); > pte_unmap_unlock() > // GUP now returns with a write protected page > > The first attempt to resolve this by using the write protect caused > problems (and was missing a barrrier), see commit f3c64eda3e50 ("mm: avoid > early COW write protect games during fork()") > > Instead wrap copy_p4d_range() with the write side of something like a > seqcount and check the read side around gup_pgd_range(). If there is a > collision then get_user_pages_fast() fails and falls back to slow GUP. > > Slow GUP is safe against this race because copy_page_range() is only > called while holding the write side of the mmap_lock on the src mm_struct. > > Fixes: f3c64eda3e50 ("mm: avoid early COW write protect games during fork()") > Suggested-by: Linus Torvalds > Link: https://lore.kernel.org/r/CAHk-=wi=iCnYCARbPGjkVJu9eyYeZ13N64tZYLdOB8CP5Q_PLw@mail.gmail.com > Signed-off-by: Jason Gunthorpe OK, I've *finally* convinced myself that the seqlock-like thing and the memory barriers are correct. :) And the overall locking story also looks good. Aside from the minor commit log question above, the diffs look correct, so either way, Reviewed-by: John Hubbard thanks, -- John Hubbard NVIDIA > --- > include/linux/mm_types.h | 6 ++++++ > kernel/fork.c | 1 + > mm/gup.c | 19 +++++++++++++++++++ > mm/memory.c | 16 +++++++++++++++- > 4 files changed, 41 insertions(+), 1 deletion(-) > > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h > index 5a9238f6caad97..8c7c9de476c4f8 100644 > --- a/include/linux/mm_types.h > +++ b/include/linux/mm_types.h > @@ -446,6 +446,12 @@ struct mm_struct { > */ > atomic_t has_pinned; > > + /** > + * @write_protecet_seq: Odd when any thread is write > + * protecting pages in this mm, for instance during fork(). > + */ > + unsigned long write_protect_seq; > + > #ifdef CONFIG_MMU > atomic_long_t pgtables_bytes; /* PTE page table pages */ > #endif > diff --git a/kernel/fork.c b/kernel/fork.c > index 32083db7a2a23e..342243f621c742 100644 > --- a/kernel/fork.c > +++ b/kernel/fork.c > @@ -1007,6 +1007,7 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p, > mm->vmacache_seqnum = 0; > atomic_set(&mm->mm_users, 1); > atomic_set(&mm->mm_count, 1); > + mm->write_protect_seq = 0; > mmap_init_lock(mm); > INIT_LIST_HEAD(&mm->mmlist); > mm->core_state = NULL; > diff --git a/mm/gup.c b/mm/gup.c > index ecbe1639ea2af7..2c1a1e0555479e 100644 > --- a/mm/gup.c > +++ b/mm/gup.c > @@ -2677,12 +2677,19 @@ static unsigned int lockless_pages_from_mm(unsigned long addr, > struct page **pages) > { > unsigned long flags; > + unsigned long seq; > int nr_pinned = 0; > > if (!IS_ENABLED(CONFIG_HAVE_FAST_GUP) || > !gup_fast_permitted(addr, end)) > return 0; > > + if (gup_flags & FOLL_PIN) { > + seq = smp_load_acquire(¤t->mm->write_protect_seq); > + if (seq & 1) > + return 0; > + } > + > /* > * Disable interrupts. The nested form is used, in order to allow full, > * general purpose use of this routine. > @@ -2697,6 +2704,18 @@ static unsigned int lockless_pages_from_mm(unsigned long addr, > local_irq_save(flags); > gup_pgd_range(addr, end, gup_flags, pages, &nr_pinned); > local_irq_restore(flags); > + > + /* > + * When pinning pages for DMA there could be a concurrent write protect > + * from fork() via copy_page_range(), in this case always fail fast GUP. > + */ > + if (gup_flags & FOLL_PIN) { > + smp_rmb(); > + if (READ_ONCE(current->mm->write_protect_seq) != seq) { > + unpin_user_pages(pages, nr_pinned); > + return 0; > + } > + } > return nr_pinned; > } > > diff --git a/mm/memory.c b/mm/memory.c > index c48f8df6e50268..e2f959cce8563d 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -1171,6 +1171,17 @@ copy_page_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma) > mmu_notifier_range_init(&range, MMU_NOTIFY_PROTECTION_PAGE, > 0, src_vma, src_mm, addr, end); > mmu_notifier_invalidate_range_start(&range); > + /* > + * This is like a seqcount where the mmap_lock provides > + * serialization for the write side. However, unlike seqcount > + * the read side falls back to obtaining the mmap_lock rather > + * than spinning. For this reason none of the preempt related > + * machinery in seqcount is desired here. > + */ > + mmap_assert_write_locked(src_mm); > + WRITE_ONCE(src_mm->write_protect_seq, > + src_mm->write_protect_seq + 1); > + smp_wmb(); > } > > ret = 0; > @@ -1187,8 +1198,11 @@ copy_page_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma) > } > } while (dst_pgd++, src_pgd++, addr = next, addr != end); > > - if (is_cow) > + if (is_cow) { > + smp_store_release(&src_mm->write_protect_seq, > + src_mm->write_protect_seq + 1); > mmu_notifier_invalidate_range_end(&range); > + } > return ret; > } > >