From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id B2A182A1B8 for ; Fri, 26 Apr 2024 04:20:00 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1714105202; cv=none; b=LYqYN6fZPArT2V5YLrrn2OkVcRc5rMTuCW7tbGon/KXdXaMVCnk5CxeMMWXWKKUjokAY9/KPPFyEU70oJQyf/KRu98MZNEXQIGlcRDydAGytg9gwMSIMcC2ID6c2+Wn2hPhluV3RMV/75tyKWGFLfJmiQbSk2vIsuS3Y9YSFmug= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1714105202; c=relaxed/simple; bh=KBYDDni2ds5f5iEZVUCZEFfFYg3cu4lWDGAJMTJ7cPg=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=dj0ocKEaR1Qnu5iq/Hgow73TIz4D7TmhmieOW1eKTewoan4CPhD11/jYP1G23+nUv24jwKjFXabvuiP4YO6SqiZgE0bV4Ph8tM5Zqdy7xjU/b86XTIsUyf45lES8/RszKV71TnYl2eG4xnHH0KIvrZW8LvYww5JDagWn/i1ZwGg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id C14C11007; Thu, 25 Apr 2024 21:20:27 -0700 (PDT) Received: from [10.163.32.46] (unknown [10.163.32.46]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 67D2A3F64C; Thu, 25 Apr 2024 21:19:57 -0700 (PDT) Message-ID: <39d0236c-4297-4001-bc7e-f516bc674f7b@arm.com> Date: Fri, 26 Apr 2024 09:49:53 +0530 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v1] mm: Fix race between __split_huge_pmd_locked() and GUP-fast Content-Language: en-US To: Ryan Roberts , Andrew Morton , Zi Yan , "Aneesh Kumar K.V" Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20240425170704.3379492-1-ryan.roberts@arm.com> From: Anshuman Khandual In-Reply-To: <20240425170704.3379492-1-ryan.roberts@arm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 4/25/24 22:37, Ryan Roberts wrote: > __split_huge_pmd_locked() can be called for a present THP, devmap or > (non-present) migration entry. It calls pmdp_invalidate() > unconditionally on the pmdp and only determines if it is present or not > based on the returned old pmd. This is a problem for the migration entry > case because pmd_mkinvalid(), called by pmdp_invalidate() must only be > called for a present pmd. pmdp_invalidate() must be called only for present PMD - is this expected by core MM ? Does this cause any problem otherwise ? > > On arm64 at least, pmd_mkinvalid() will mark the pmd such that any > future call to pmd_present() will return true. And therefore any IIRC the following semantics needs to be followed as expected by core MM. ------------------------------------------------------------------------- | PMD states | pmd_present | pmd_trans_huge | ------------------------------------------------------------------------- | Mapped | Yes | Yes | ------------------------------------------------------------------------- | Splitting | Yes | Yes | ------------------------------------------------------------------------- | Migration/Swap | No | No | ------------------------------------------------------------------------- > lockless pgtable walker could see the migration entry pmd in this state > and start interpretting the fields as if it were present, leading to > BadThings (TM). GUP-fast appears to be one such lockless pgtable walker. Could you please explain how bad things might happen ? > I suspect the same is possible on other architectures. > > Fix this by only calling pmdp_invalidate() for a present pmd. And for > good measure let's add a warning to the generic implementation of > pmdp_invalidate(). I've manually reviewed all other > pmdp_invalidate[_ad]() call sites and believe all others to be > conformant. > > This is a theoretical bug found during code review. I don't have any > test case to trigger it in practice. > > Fixes: 84c3fc4e9c56 ("mm: thp: check pmd migration entry in common path") > Signed-off-by: Ryan Roberts > --- > > Applies on top of v6.9-rc5. Passes all the mm selftests on arm64. > > Thanks, > Ryan > > > mm/huge_memory.c | 5 +++-- > mm/pgtable-generic.c | 2 ++ > 2 files changed, 5 insertions(+), 2 deletions(-) > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index 89f58c7603b2..80939ad00718 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -2513,12 +2513,12 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd, > * for this pmd), then we flush the SMP TLB and finally we write the > * non-huge version of the pmd entry with pmd_populate. > */ > - old_pmd = pmdp_invalidate(vma, haddr, pmd); > > - pmd_migration = is_pmd_migration_entry(old_pmd); > + pmd_migration = is_pmd_migration_entry(*pmd); > if (unlikely(pmd_migration)) { > swp_entry_t entry; > > + old_pmd = *pmd; > entry = pmd_to_swp_entry(old_pmd); > page = pfn_swap_entry_to_page(entry); > write = is_writable_migration_entry(entry); > @@ -2529,6 +2529,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd, > soft_dirty = pmd_swp_soft_dirty(old_pmd); > uffd_wp = pmd_swp_uffd_wp(old_pmd); > } else { > + old_pmd = pmdp_invalidate(vma, haddr, pmd); > page = pmd_page(old_pmd); > folio = page_folio(page); > if (pmd_dirty(old_pmd)) { > diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c > index 4fcd959dcc4d..74e34ea90656 100644 > --- a/mm/pgtable-generic.c > +++ b/mm/pgtable-generic.c > @@ -198,6 +198,7 @@ pgtable_t pgtable_trans_huge_withdraw(struct mm_struct *mm, pmd_t *pmdp) > pmd_t pmdp_invalidate(struct vm_area_struct *vma, unsigned long address, > pmd_t *pmdp) > { > + VM_WARN_ON(!pmd_present(*pmdp)); > pmd_t old = pmdp_establish(vma, address, pmdp, pmd_mkinvalid(*pmdp)); > flush_pmd_tlb_range(vma, address, address + HPAGE_PMD_SIZE); > return old; > @@ -208,6 +209,7 @@ pmd_t pmdp_invalidate(struct vm_area_struct *vma, unsigned long address, > pmd_t pmdp_invalidate_ad(struct vm_area_struct *vma, unsigned long address, > pmd_t *pmdp) > { > + VM_WARN_ON(!pmd_present(*pmdp)); > return pmdp_invalidate(vma, address, pmdp); > } > #endif > -- > 2.25.1 > >