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 294D25FB80; Tue, 23 Jan 2024 14:28:00 +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=1706020083; cv=none; b=lV1lcfYP/W4Udz/G8FXih6G4faawEESov2w1FJOyUsB504HbjQG6C/unQWEHUj204rT6dnMW42mVaE2m+UzZnULDrwHKy1PAmZ4uLkGAMoZ/X/XUnMkx/IdZj7jqTFJ1ynNZ8BI0PeyIQ3rcmaESTPEYhsC2hTO4E+BjJVHX+wo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1706020083; c=relaxed/simple; bh=n5Sxb3m5EKoKEa7DWkAuvTEXkevzHLo3B9dfQYIGHNQ=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=WUhj+LKDhhLRDM7wH7L9NLgSZU6R9sutaSEy3KsUhGEv6X4UAj+ipc5aCnJkA7+HX3qH97OOgmZfux5MpfcJaMwMpyL78MaoTo0ppg3Dnb/Ykl6Wg76D3Kmd9L99ZWWd1g0LafDjUpDCq2Ga7TNVYOSXhgGTuZaYL6bKpKkmuvE= 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 B78CB1FB; Tue, 23 Jan 2024 06:28:45 -0800 (PST) Received: from [10.57.77.165] (unknown [10.57.77.165]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 707653F762; Tue, 23 Jan 2024 06:27:56 -0800 (PST) Message-ID: <7db13538-20b9-4c12-b333-d197c4b2846d@arm.com> Date: Tue, 23 Jan 2024 14:27:55 +0000 Precedence: bulk X-Mailing-List: linux-s390@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v1 10/11] mm/memory: ignore dirty/accessed/soft-dirty bits in folio_pte_batch() Content-Language: en-GB To: David Hildenbrand , linux-kernel@vger.kernel.org Cc: linux-mm@kvack.org, Andrew Morton , Matthew Wilcox , Russell King , Catalin Marinas , Will Deacon , Dinh Nguyen , Michael Ellerman , Nicholas Piggin , Christophe Leroy , "Aneesh Kumar K.V" , "Naveen N. Rao" , Paul Walmsley , Palmer Dabbelt , Albert Ou , Alexander Gordeev , Gerald Schaefer , Heiko Carstens , Vasily Gorbik , Christian Borntraeger , Sven Schnelle , "David S. Miller" , linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org, linux-s390@vger.kernel.org, sparclinux@vger.kernel.org References: <20240122194200.381241-1-david@redhat.com> <20240122194200.381241-11-david@redhat.com> <59592b50-fe89-4b32-8490-2e6c296f972f@arm.com> <76740e33-9b52-4e23-b407-8ae38bac15ec@redhat.com> <94d33a07-c59a-4315-9c64-8b4d959ca1f4@arm.com> <8eb5db8e-33cc-4cbf-a1bf-0da7af230fab@redhat.com> From: Ryan Roberts In-Reply-To: <8eb5db8e-33cc-4cbf-a1bf-0da7af230fab@redhat.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 23/01/2024 14:13, David Hildenbrand wrote: >>> Although now I'm wondering if there is a race here... What happens if a page in >>> the parent becomes dirty after you have checked it but before you write protect >>> it? Isn't that already a problem with the current non-batched version? Why do we >>> even to preserve dirty in the child for private mappings? >> >> I suspect, because the parent could zap the anon folio. If the folio is >> clean, but the PTE dirty, I suspect that we could lose data of the child >> if we were to evict that clean folio (swapout). >> >> So I assume we simply copy the dirty PTE bit, so the system knows that >> that folio is actually dirty, because one PTE is dirty. > > Oh, and regarding your race concern: it's undefined which page state > would see if some write is racing with fork, so it also doesn't matter > if we would copy the PTE dirty bit or not, if it gets set in a racy fashion. Ahh that makes sense. Thanks. > > I'll not experiment with: Looks good as long as its still performant. > > From 14e83ff2a422a96ce5701f9c8454a49f9ed947e3 Mon Sep 17 00:00:00 2001 > From: David Hildenbrand > Date: Sat, 30 Dec 2023 12:54:35 +0100 > Subject: [PATCH] mm/memory: ignore dirty/accessed/soft-dirty bits in >  folio_pte_batch() > > Let's always ignore the accessed/young bit: we'll always mark the PTE > as old in our child process during fork, and upcoming users will > similarly not care. > > Ignore the dirty bit only if we don't want to duplicate the dirty bit > into the child process during fork. Maybe, we could just set all PTEs > in the child dirty if any PTE is dirty. For now, let's keep the behavior > unchanged. > > Ignore the soft-dirty bit only if the bit doesn't have any meaning in > the src vma. > > Signed-off-by: David Hildenbrand > --- >  mm/memory.c | 34 ++++++++++++++++++++++++++++++---- >  1 file changed, 30 insertions(+), 4 deletions(-) > > diff --git a/mm/memory.c b/mm/memory.c > index 7690994929d26..9aba1b0e871ca 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -953,24 +953,44 @@ static __always_inline void __copy_present_ptes(struct > vm_area_struct *dst_vma, >      set_ptes(dst_vma->vm_mm, addr, dst_pte, pte, nr); >  } >   > +/* Flags for folio_pte_batch(). */ > +typedef int __bitwise fpb_t; > + > +/* Compare PTEs after pte_mkclean(), ignoring the dirty bit. */ > +#define FPB_IGNORE_DIRTY        ((__force fpb_t)BIT(0)) > + > +/* Compare PTEs after pte_clear_soft_dirty(), ignoring the soft-dirty bit. */ > +#define FPB_IGNORE_SOFT_DIRTY        ((__force fpb_t)BIT(1)) > + > +static inline pte_t __pte_batch_clear_ignored(pte_t pte, fpb_t flags) > +{ > +    if (flags & FPB_IGNORE_DIRTY) > +        pte = pte_mkclean(pte); > +    if (likely(flags & FPB_IGNORE_SOFT_DIRTY)) > +        pte = pte_clear_soft_dirty(pte); > +    return pte_mkold(pte); > +} > + >  /* >   * Detect a PTE batch: consecutive (present) PTEs that map consecutive >   * pages of the same folio. >   * >   * All PTEs inside a PTE batch have the same PTE bits set, excluding the PFN. > + * the accessed bit, dirty bit (with FPB_IGNORE_DIRTY) and soft-dirty bit > + * (with FPB_IGNORE_SOFT_DIRTY). >   */ >  static inline int folio_pte_batch(struct folio *folio, unsigned long addr, > -        pte_t *start_ptep, pte_t pte, int max_nr) > +        pte_t *start_ptep, pte_t pte, int max_nr, fpb_t flags) >  { >      unsigned long folio_end_pfn = folio_pfn(folio) + folio_nr_pages(folio); >      const pte_t *end_ptep = start_ptep + max_nr; > -    pte_t expected_pte = pte_next_pfn(pte); > +    pte_t expected_pte = __pte_batch_clear_ignored(pte_next_pfn(pte), flags); >      pte_t *ptep = start_ptep + 1; >   >      VM_WARN_ON_FOLIO(!pte_present(pte), folio); >   >      while (ptep != end_ptep) { > -        pte = ptep_get(ptep); > +        pte = __pte_batch_clear_ignored(ptep_get(ptep), flags); >   >          if (!pte_same(pte, expected_pte)) >              break; > @@ -1004,6 +1024,7 @@ copy_present_ptes(struct vm_area_struct *dst_vma, struct > vm_area_struct *src_vma >  { >      struct page *page; >      struct folio *folio; > +    fpb_t flags = 0; >      int err, nr; >   >      page = vm_normal_page(src_vma, addr, pte); > @@ -1018,7 +1039,12 @@ copy_present_ptes(struct vm_area_struct *dst_vma, struct > vm_area_struct *src_vma >       * by keeping the batching logic separate. >       */ >      if (unlikely(!*prealloc && folio_test_large(folio) && max_nr != 1)) { > -        nr = folio_pte_batch(folio, addr, src_pte, pte, max_nr); > +        if (src_vma->vm_flags & VM_SHARED) > +            flags |= FPB_IGNORE_DIRTY; > +        if (!vma_soft_dirty_enabled(src_vma)) > +            flags |= FPB_IGNORE_SOFT_DIRTY; > + > +        nr = folio_pte_batch(folio, addr, src_pte, pte, max_nr, flags); >          folio_ref_add(folio, nr); >          if (folio_test_anon(folio)) { >              if (unlikely(folio_try_dup_anon_rmap_ptes(folio, page,