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[184.145.4.219]) by smtp.gmail.com with ESMTPSA id m128sm2183277qkf.134.2021.05.27.14.41.20 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 27 May 2021 14:41:21 -0700 (PDT) Date: Thu, 27 May 2021 17:41:19 -0400 From: Peter Xu To: Peter Collingbourne Cc: Andrew Morton , Kostya Kortchinsky , Evgenii Stepanov , Andrea Arcangeli , linux-mm@kvack.org Subject: Re: [PATCH v4] mm: improve mprotect(R|W) efficiency on pages referenced once Message-ID: References: <20210527190453.1259020-1-pcc@google.com> MIME-Version: 1.0 In-Reply-To: <20210527190453.1259020-1-pcc@google.com> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Authentication-Results: imf29.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=CmsUBaZZ; spf=none (imf29.hostedemail.com: domain of peterx@redhat.com has no SPF policy when checking 216.205.24.124) smtp.mailfrom=peterx@redhat.com; dmarc=pass (policy=none) header.from=redhat.com X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 015B77B7 X-Stat-Signature: ikn4cdurc8put5anoqsjujijo461y6nu X-HE-Tag: 1622151675-905275 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: Peter, On Thu, May 27, 2021 at 12:04:53PM -0700, Peter Collingbourne wrote: [...] > +static bool may_avoid_write_fault(pte_t pte, struct vm_area_struct *vma, > + unsigned long cp_flags) > +{ > + if (!(cp_flags & MM_CP_DIRTY_ACCT)) { > + if (!(vma_is_anonymous(vma) && (vma->vm_flags & VM_WRITE))) > + return false; > + > + if (page_count(pte_page(pte)) != 1) > + return false; > + } Can we make MM_CP_DIRTY_ACCT still in charge? IIUC that won't affect your use case, something like: /* Never apply trick if MM_CP_DIRTY_ACCT not set */ if (!(cp_flags & MM_CP_DIRTY_ACCT)) return false; The thing is that's really what MM_CP_DIRTY_ACCT is about, imho (as its name shows). Say, we should start to count on the dirty bit for applying the write bit only if that flag set. With above, I think we can drop the pte_uffd_wp() check below because uffd_wp never applies MM_CP_DIRTY_ACCT when do change_protection(). Thanks, > + > + if (!pte_dirty(pte)) > + return false; > + > + if (!pte_soft_dirty(pte) && (vma->vm_flags & VM_SOFTDIRTY)) > + return false; > + > + if (pte_uffd_wp(pte)) > + return false; > + > + return true; > +} > + > static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd, > unsigned long addr, unsigned long end, pgprot_t newprot, > unsigned long cp_flags) > @@ -43,7 +66,6 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd, > spinlock_t *ptl; > unsigned long pages = 0; > int target_node = NUMA_NO_NODE; > - bool dirty_accountable = cp_flags & MM_CP_DIRTY_ACCT; > bool prot_numa = cp_flags & MM_CP_PROT_NUMA; > bool uffd_wp = cp_flags & MM_CP_UFFD_WP; > bool uffd_wp_resolve = cp_flags & MM_CP_UFFD_WP_RESOLVE; > @@ -132,11 +154,8 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd, > } > > /* Avoid taking write faults for known dirty pages */ > - if (dirty_accountable && pte_dirty(ptent) && > - (pte_soft_dirty(ptent) || > - !(vma->vm_flags & VM_SOFTDIRTY))) { > + if (may_avoid_write_fault(ptent, vma, cp_flags)) > ptent = pte_mkwrite(ptent); > - } > ptep_modify_prot_commit(vma, addr, pte, oldpte, ptent); > pages++; > } else if (is_swap_pte(oldpte)) { > -- > 2.32.0.rc0.204.g9fa02ecfa5-goog > -- Peter Xu