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 5D946288DB; Tue, 30 Sep 2025 05:29:27 +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=1759210169; cv=none; b=i3SnUO0I57EtnS9wGcSieR/DpjpqO4HkrnM+amEyy5DL/KNUmJGJRTwf9VVYHBvDnPfmbOslVnZiRtLyP4B2ZWAH78mPHMu6KiB+OAL1Mn1PDaq0WNWGQOXNBgCGy787b/I2mCxRpWN+LTpH59yf1QBLgUMdnzj+Bn3lpL7Dqvs= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1759210169; c=relaxed/simple; bh=mKoCDWmOpjr09w/2zz9LMS5AmKiUiLhdr7GQPnjWlcE=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Fc16c1DBS+4XEhrvdTfflMk1RDLD7SYDO+k1SvjixKbB+NP2t4QfTnxnFdAlX+TrHKgnaQLr1Wxd/9wB0C910AdgJER8XPWy3F1I7dV6w2lUJgnpLvaoe88nQ5zIiqxT04L/3jWGde4vqyMjPQfpv9Co19V/gwHpdsrav0KyOtw= 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 95FEA1424; Mon, 29 Sep 2025 22:29:18 -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 50DEF3F5A1; Mon, 29 Sep 2025 22:29:18 -0700 (PDT) Message-ID: <68cff704-bbdb-41a7-81d4-7195f71ba73a@arm.com> Date: Tue, 30 Sep 2025 10:59:15 +0530 Precedence: bulk X-Mailing-List: stable@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 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, lorenzo.stoakes@oracle.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, ioworker0@gmail.com, stable@vger.kernel.org, akpm@linux-foundation.org, david@redhat.com References: <20250930043351.34927-1-lance.yang@linux.dev> <0c498301-7434-4b2b-b7bc-73abe2057b67@arm.com> <668bfb74-014c-4fd5-a636-ff5ec17861c3@linux.dev> Content-Language: en-US From: Dev Jain In-Reply-To: <668bfb74-014c-4fd5-a636-ff5ec17861c3@linux.dev> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 30/09/25 10:52 am, Lance Yang wrote: > > > On 2025/9/30 12:50, Dev Jain wrote: >> >> 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. > > Ah, got it. Thanks for the clarification! > > IIUC, you mean something like this: > > ``` > diff --git a/mm/migrate.c b/mm/migrate.c > index ce83c2c3c287..bafd8cb3bebe 100644 > --- a/mm/migrate.c > +++ b/mm/migrate.c > @@ -297,6 +297,7 @@ bool isolate_folio_to_list(struct folio *folio, > struct list_head *list) > >  static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk > *pvmw, >                        struct folio *folio, > +                      pte_t old_pte, >                        unsigned long idx) >  { >      struct page *page = folio_page(folio, idx); > @@ -306,7 +307,7 @@ static bool try_to_map_unused_to_zeropage(struct > page_vma_mapped_walk *pvmw, >          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(old_pte), 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(old_pte)) > +        newpte = pte_mksoft_dirty(newpte); > +    if (pte_swp_uffd_wp(old_pte)) > +        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)); > @@ -344,7 +351,7 @@ static bool remove_migration_pte(struct folio *folio, > >      while (page_vma_mapped_walk(&pvmw)) { >          rmap_t rmap_flags = RMAP_NONE; > -        pte_t old_pte; > +        pte_t old_pte = ptep_get(pvmw.pte); >          pte_t pte; >          swp_entry_t entry; >          struct page *new; > @@ -365,12 +372,11 @@ static bool remove_migration_pte(struct folio > *folio, >          } >  #endif >          if (rmap_walk_arg->map_unused_to_zeropage && > -            try_to_map_unused_to_zeropage(&pvmw, folio, idx)) > +            try_to_map_unused_to_zeropage(&pvmw, folio, old_pte, idx)) >              continue; > >          folio_get(folio); >          pte = mk_pte(new, READ_ONCE(vma->vm_page_prot)); > -        old_pte = ptep_get(pvmw.pte); > >          entry = pte_to_swp_entry(old_pte); >          if (!is_migration_entry_young(entry)) > ``` > > ptep_get() gets called only once per iteration, right? Yup.