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 DFEC5288DB; Tue, 30 Sep 2025 04:50:26 +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=1759207829; cv=none; b=ngXKXd1v/yglbO8mMPmjOole/0tsMT22D3qea5kdvTAtG+v8jMm+fdpONgQqMzt05lxQ7Bq4x0CK3tt0n/MHkFxDfo0OOeRYC9JxjMKCQu3Keb1zEfSb4xUIfJWbXGLAomk9wcYEhXW2ohfEtKhkuacgAq5ImOlSpF15CKqD0iQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1759207829; c=relaxed/simple; bh=q7JKcLc5rFsue9IeSq60djpSLcwWdKNibnqwpCcjTG0=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=UxnBxFxiZyjDotR//q50SD4+CVPJVcDaCQoqPlpr3vGW7aTR+4Yz9C3Kgs3aBv5wL0f0pQq4BoQhMkCIkQldveUT4M+xCBt8CtAe6SqPZt6Si6O3k1n8yMX/yKEH410Qc1OooyzF5kmXXCDt4IrkanH/lhfLnCvBgbTpLbSJadc= 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 D70691424; Mon, 29 Sep 2025 21:50:17 -0700 (PDT) Received: from [10.164.18.53] (MacBook-Pro.blr.arm.com [10.164.18.53]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 78AB33F66E; Mon, 29 Sep 2025 21:50:17 -0700 (PDT) Message-ID: <0c498301-7434-4b2b-b7bc-73abe2057b67@arm.com> Date: Tue, 30 Sep 2025 10:20:14 +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 v2 1/1] mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage To: Lance Yang , akpm@linux-foundation.org, david@redhat.com, lorenzo.stoakes@oracle.com Cc: peterx@redhat.com, ziy@nvidia.com, baolin.wang@linux.alibaba.com, baohua@kernel.org, ryan.roberts@arm.com, npache@redhat.com, riel@surriel.com, Liam.Howlett@oracle.com, vbabka@suse.cz, harry.yoo@oracle.com, jannh@google.com, matthew.brost@intel.com, joshua.hahnjy@gmail.com, rakie.kim@sk.com, byungchul@sk.com, gourry@gourry.net, ying.huang@linux.alibaba.com, apopple@nvidia.com, usamaarif642@gmail.com, yuzhao@google.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, ioworker0@gmail.com, stable@vger.kernel.org References: <20250930043351.34927-1-lance.yang@linux.dev> Content-Language: en-US From: Dev Jain In-Reply-To: <20250930043351.34927-1-lance.yang@linux.dev> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 30/09/25 10:03 am, Lance Yang wrote: > From: Lance Yang > > When splitting an mTHP and replacing a zero-filled subpage with the shared > zeropage, try_to_map_unused_to_zeropage() currently drops several important > PTE bits. > > For userspace tools like CRIU, which rely on the soft-dirty mechanism for > incremental snapshots, losing the soft-dirty bit means modified pages are > missed, leading to inconsistent memory state after restore. > > As pointed out by David, the more critical uffd-wp bit is also dropped. > This breaks the userfaultfd write-protection mechanism, causing writes > to be silently missed by monitoring applications, which can lead to data > corruption. > > Preserve both the soft-dirty and uffd-wp bits from the old PTE when > creating the new zeropage mapping to ensure they are correctly tracked. > > Cc: > Fixes: b1f202060afe ("mm: remap unused subpages to shared zeropage when splitting isolated thp") > Suggested-by: David Hildenbrand > Suggested-by: Dev Jain > Acked-by: David Hildenbrand > Signed-off-by: Lance Yang > --- > v1 -> v2: > - Avoid calling ptep_get() multiple times (per Dev) > - Double-check the uffd-wp bit (per David) > - Collect Acked-by from David - thanks! > - https://lore.kernel.org/linux-mm/20250928044855.76359-1-lance.yang@linux.dev/ > > mm/migrate.c | 9 ++++++++- > 1 file changed, 8 insertions(+), 1 deletion(-) > > diff --git a/mm/migrate.c b/mm/migrate.c > index ce83c2c3c287..50aa91d9ab4e 100644 > --- a/mm/migrate.c > +++ b/mm/migrate.c > @@ -300,13 +300,14 @@ static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw, > unsigned long idx) > { > struct page *page = folio_page(folio, idx); > + pte_t oldpte = ptep_get(pvmw->pte); What I meant to say was, you can pass oldpte from remove_migration_pte to this function. Basically define old_pte = ptep_get(pvmw.pte) in the declarations of the start of the while block in remove_migration_pte and remove the existing one. That will ensure ptep_get() gets called only once per iteration. > pte_t newpte; > > if (PageCompound(page)) > return false; > VM_BUG_ON_PAGE(!PageAnon(page), page); > VM_BUG_ON_PAGE(!PageLocked(page), page); > - VM_BUG_ON_PAGE(pte_present(ptep_get(pvmw->pte)), page); > + VM_BUG_ON_PAGE(pte_present(oldpte), page); > > if (folio_test_mlocked(folio) || (pvmw->vma->vm_flags & VM_LOCKED) || > mm_forbids_zeropage(pvmw->vma->vm_mm)) > @@ -322,6 +323,12 @@ static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw, > > newpte = pte_mkspecial(pfn_pte(my_zero_pfn(pvmw->address), > pvmw->vma->vm_page_prot)); > + > + if (pte_swp_soft_dirty(oldpte)) > + newpte = pte_mksoft_dirty(newpte); > + if (pte_swp_uffd_wp(oldpte)) > + newpte = pte_mkuffd_wp(newpte); > + > set_pte_at(pvmw->vma->vm_mm, pvmw->address, pvmw->pte, newpte); > > dec_mm_counter(pvmw->vma->vm_mm, mm_counter(folio));