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 bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 41174C5DF7D for ; Tue, 18 Aug 2026 14:10:52 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:In-Reply-To:Content-Type: MIME-Version:References:Message-ID:Subject:Cc:To:From:Date:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=l03Q5ptSYhWuGPtHN3uRfvI7ji1htrQDlyLsSY8uXvw=; b=FYXnTPqD4dk6dHvfIadOX3OE9q zalucHjQ4lc6esPgcks20oUjcYwKGxTTQUmG0irljPHBW6ZhYyfBvVaz0FbkBpA7CXhAJBlcFD9dt MKPhFJZjQJv3hM7ABBTjlV38z+8wVE9Cww30Z4GXZnxZxbG/2tnnh0plvFewlIaF9xkOMXIIpAbda vYadkk9/OdmJgUF6kN5qzvXW0Va/0Czrq4rF7+jcZWe1y/RCbQYe8+sPppIVhWrEtrClIoYwZu2an ndOJA3aDIyEVcmAN5LIRp8mCtKSlmwalQdISNEEyGmj/ohr0WWyTbqpOqZ+sHUDUyy5WyZgdpAkUy baeT5DvQ==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.99.1 #2 (Red Hat Linux)) id 1wwKWb-000000086LJ-3cQm; Tue, 18 Aug 2026 14:10:45 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.99.1 #2 (Red Hat Linux)) id 1wwKWZ-000000086KQ-0oui for linux-arm-kernel@lists.infradead.org; Tue, 18 Aug 2026 14:10:45 +0000 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 93860152B; Tue, 18 Aug 2026 07:10:37 -0700 (PDT) Received: from arm.com (usa-sjc-mx-foss1.foss.arm.com [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 6DB1A3F85F; Tue, 18 Aug 2026 07:10:39 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1787062241; bh=ViG9p523Gtj965obqgoqvstIweJ3Dnp0nqQZuKx6Weo=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=t5EewfnfFIPzdq/m5PX24TI4FOzPov6Iap7L97itk/zFF2JCkMgIjuBE6my9umw9P tFtho4uCZpiIO+IPA9jO61MjT2OuD6svwQBMCmxLRn+dHYIAz6+6ydfZvkZsFQTtnN DMrNb1prYHYEqMTQiMrKfD8+D5z9qlbLSjIWJKDU= Date: Tue, 18 Aug 2026 15:10:36 +0100 From: Catalin Marinas To: James Houghton Cc: Will Deacon , Muchun Song , Oscar Salvador , Nikos Nikoleris , Linu Cherian , Mark Rutland , David Hildenbrand , Andrew Morton , Ryan Roberts , Nanyong Sun , Yu Zhao , Frank van der Linden , David Rientjes , linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org Subject: Re: [PATCH 08/18] arm64: Implement try_update_vmemmap_pte using the AF trick Message-ID: References: <20260708031129.3503195-1-jthoughton@google.com> <20260708031129.3503195-9-jthoughton@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260708031129.3503195-9-jthoughton@google.com> X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.9.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260818_071043_322839_D565597C X-CRM114-Status: GOOD ( 25.47 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Wed, Jul 08, 2026 at 03:11:18AM +0000, James Houghton wrote: > diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h > index 5f21d3a738ee..7b11aa41d0a0 100644 > --- a/arch/arm64/include/asm/pgtable.h > +++ b/arch/arm64/include/asm/pgtable.h > @@ -1302,8 +1302,7 @@ static inline void __pte_clear(struct mm_struct *mm, > __set_pte(ptep, __pte(0)); > } > > -static inline bool __ptep_test_and_clear_young(struct vm_area_struct *vma, > - unsigned long address, pte_t *ptep) > +static inline pte_t __ptep_clear_young(pte_t *ptep) > { > pte_t old_pte, pte; > > @@ -1315,7 +1314,13 @@ static inline bool __ptep_test_and_clear_young(struct vm_area_struct *vma, > pte_val(old_pte), pte_val(pte)); > } while (pte_val(pte) != pte_val(old_pte)); > > - return pte_young(pte); > + return pte; > +} At some point, we should use LSE atomics directly here if supported rather than a CAS loop (well, this is LSE as well if supported but it doesn't guarantee forward progress of the loop). I reckon we can replace it with some test_and_clear_bit_relaxed() call. > +static inline bool __ptep_test_and_clear_young(struct vm_area_struct *vma, > + unsigned long address, pte_t *ptep) > +{ > + return pte_young(__ptep_clear_young(ptep)); > } > > static inline bool __ptep_clear_flush_young(struct vm_area_struct *vma, > @@ -1793,6 +1798,48 @@ static inline void pte_clear(struct mm_struct *mm, > __pte_clear(mm, addr, ptep); > } > > +#define __HAVE_ARCH_TRY_UPDATE_VMEMMAP_PTE > +static inline int try_update_vmemmap_pte(unsigned long addr, pte_t *ptep, > + const pte_t pte) > +{ > + const int max_attempts = 16; > + int attempts = 0; > + pte_t old_pte; > + > + if (!system_supports_hvo()) > + return -EOPNOTSUPP; > + > + /* This routine is only to be used for valid-to-valid transitions. */ > + if (WARN_ON_ONCE(!pte_valid(pte))) > + return -EINVAL; > + > + old_pte = __ptep_get(ptep); > + > + do { > + if (WARN_ON_ONCE(!pte_valid(old_pte))) > + return -EINVAL; > + > + /* We should never get a contiguous PTE here. */ > + if (WARN_ON_ONCE(pte_valid_cont(old_pte))) > + return -EINVAL; > + > + if (pte_young(old_pte)) { > + /* __ptep_clear_young() returns the overwritten PTE */ > + old_pte = pte_mkold(__ptep_clear_young(ptep)); > + > + flush_tlb_kernel_range(addr, addr + PAGE_SIZE); > + } I think this is going to do a lot of TLBIs given that the default kernel prot has PTE_AF. It somewhat defeats the VMEMMAP_REMAP_NO_TLB_FLUSH flag but I haven't figured exactly how this optimisation works. If it becomes a problem, we could do a first pass to clear AF as an optimisation or later via vmemmap_split_pmd(), only map with AF=0. We still have some page copying that touches the vmemmap, bringing AF back. Of course, you'd still need the above flush, just wondering whether we can reduce/coalesce it. > + /* > + * Translations without AF cannot be cached, so we can replace > + * them without BBM. > + */ > + } while (!try_cmpxchg_relaxed(&pte_val(*ptep), &pte_val(old_pte), > + pte_val(pte)) && > + ++attempts < max_attempts); > + > + return attempts == max_attempts ? -EAGAIN : 0; > +} Here, indeed, we do need this bounded, otherwise some pathological cases may set the AF continuously. -- Catalin