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 Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 447B4C47258 for ; Tue, 23 Jan 2024 14:28:04 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id B5F5B6B0081; Tue, 23 Jan 2024 09:28:03 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id AE7A06B0082; Tue, 23 Jan 2024 09:28:03 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 987F96B0083; Tue, 23 Jan 2024 09:28:03 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0015.hostedemail.com [216.40.44.15]) by kanga.kvack.org (Postfix) with ESMTP id 8393D6B0081 for ; Tue, 23 Jan 2024 09:28:03 -0500 (EST) Received: from smtpin30.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay04.hostedemail.com (Postfix) with ESMTP id 3517D1A0AEA for ; Tue, 23 Jan 2024 14:28:03 +0000 (UTC) X-FDA: 81710805246.30.8D65A1F Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by imf16.hostedemail.com (Postfix) with ESMTP id 553FB180018 for ; Tue, 23 Jan 2024 14:28:01 +0000 (UTC) Authentication-Results: imf16.hostedemail.com; dkim=none; spf=pass (imf16.hostedemail.com: domain of ryan.roberts@arm.com designates 217.140.110.172 as permitted sender) smtp.mailfrom=ryan.roberts@arm.com; dmarc=pass (policy=none) header.from=arm.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1706020081; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=bGpblSKd13u+uTzxfd0ykuIoC5Jx/5ugY1jYPg6QuO4=; b=R2D+t7h3em8TgbLcKiTHVSZE/UxHukZ5P4fDi+Skpug7Zxxta6aNWIizsxgsUFjjlDpV9m Nkt0jxe2Y+7Olrb7pFXlSWNBmUVhWLHBuqM++QXzgj0DfoaRHiY8JTSAxZbj4pEzGjrQB1 vENYmZlFsXuR+kiZg6yPSv9fBEX8xqE= ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1706020081; a=rsa-sha256; cv=none; b=KN9H5dG5zqzVS2Sby6SMEjUmzp4lrwk18VfrVEgMxHdki3w4Z/rTAgT7ctNdvpDKz/NSD9 hS691dfccJR+h+fFO3TqGaGEg9eHIm98Jt+3wB//uGmSYLRQV4AQiUWKqncvOVjouk30y6 PKDOXp9w2kuZ25P8XXf9CdP/7xR9H6Y= ARC-Authentication-Results: i=1; imf16.hostedemail.com; dkim=none; spf=pass (imf16.hostedemail.com: domain of ryan.roberts@arm.com designates 217.140.110.172 as permitted sender) smtp.mailfrom=ryan.roberts@arm.com; dmarc=pass (policy=none) header.from=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 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 X-Rspamd-Queue-Id: 553FB180018 X-Rspam-User: X-Stat-Signature: z5oenbugixf5ypwb1pcpjibgji3yrjo5 X-Rspamd-Server: rspam03 X-HE-Tag: 1706020081-597612 X-HE-Meta: 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 fDp+fTKd FjYXYaqRAowsQjthFdtE4H05TuF+ENFQMPRxuzPVeQdiQ26D4eJHtGVe99Pzno+W23uzWG5VFJ6mRbhHpiAMvws9A2yh0CEVv7xTxk/MZdSzxqsrXcdk1xJqUWPS1/9RvxQJI7/HhbuKBwgLgcrDcBz+/Db4NhTnkiildTEwRG5tYiontci92DfXbNp32lyj0uXNZZmCkDeWGUyAQ1nE6TceI4gwpOn6vFkdE 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: List-Subscribe: List-Unsubscribe: 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,