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[209.132.183.28]) by mx.google.com with ESMTPS id j6si36396537qkc.61.2019.08.01.07.50.35 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 01 Aug 2019 07:50:36 -0700 (PDT) Received-SPF: pass (google.com: domain of oleg@redhat.com designates 209.132.183.28 as permitted sender) client-ip=209.132.183.28; Authentication-Results: mx.google.com; spf=pass (google.com: domain of oleg@redhat.com designates 209.132.183.28 as permitted sender) smtp.mailfrom=oleg@redhat.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=redhat.com Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 2F8822D0FC7; Thu, 1 Aug 2019 14:50:35 +0000 (UTC) Received: from dhcp-27-174.brq.redhat.com (unknown [10.43.17.136]) by smtp.corp.redhat.com (Postfix) with SMTP id 5531B5C207; Thu, 1 Aug 2019 14:50:33 +0000 (UTC) Received: by dhcp-27-174.brq.redhat.com (nbSMTP-1.00) for uid 1000 oleg@redhat.com; Thu, 1 Aug 2019 16:50:34 +0200 (CEST) Date: Thu, 1 Aug 2019 16:50:32 +0200 From: Oleg Nesterov To: Song Liu Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, akpm@linux-foundation.org, matthew.wilcox@oracle.com, kirill.shutemov@linux.intel.com, kernel-team@fb.com, william.kucharski@oracle.com, srikar@linux.vnet.ibm.com Subject: Re: [PATCH v2 1/2] khugepaged: enable collapse pmd for pte-mapped THP Message-ID: <20190801145032.GB31538@redhat.com> References: <20190731183331.2565608-1-songliubraving@fb.com> <20190731183331.2565608-2-songliubraving@fb.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190731183331.2565608-2-songliubraving@fb.com> User-Agent: Mutt/1.5.24 (2015-08-30) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.29]); Thu, 01 Aug 2019 14:50:35 +0000 (UTC) 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: On 07/31, Song Liu wrote: > > +static int khugepaged_add_pte_mapped_thp(struct mm_struct *mm, > + unsigned long addr) > +{ > + struct mm_slot *mm_slot; > + int ret = 0; > + > + /* hold mmap_sem for khugepaged_test_exit() */ > + VM_BUG_ON_MM(!rwsem_is_locked(&mm->mmap_sem), mm); > + VM_BUG_ON(addr & ~HPAGE_PMD_MASK); > + > + if (unlikely(khugepaged_test_exit(mm))) > + return 0; > + > + if (!test_bit(MMF_VM_HUGEPAGE, &mm->flags) && > + !test_bit(MMF_DISABLE_THP, &mm->flags)) { > + ret = __khugepaged_enter(mm); > + if (ret) > + return ret; > + } could you explain why do we need mm->mmap_sem, khugepaged_test_exit() check and __khugepaged_enter() ? > + spin_lock(&khugepaged_mm_lock); > + mm_slot = get_mm_slot(mm); > + if (likely(mm_slot && mm_slot->nr_pte_mapped_thp < MAX_PTE_MAPPED_THP)) > + mm_slot->pte_mapped_thp[mm_slot->nr_pte_mapped_thp++] = addr; if get_mm_slot() returns mm_slot != NULL we can safely modify ->pte_mapped_thp. We do not care even if this task has already passed __mmput/__khugepaged_exit, this slot can't go away. No? Oleg.