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 7BA4A477E2A for ; Tue, 18 Aug 2026 14:10:42 +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=1787062244; cv=none; b=cxOSezEHFL1xOXqVaeryqmHHjH116uEE7MRyKtCck5ELafLslxmYVpurn6ohjGNCYHUOL0cWuq5ARMwH3bEkuvWURS2bzgcNheoOcqw4LQHdsZ39n7Nx2hiEKWeeHZjzcWuZroMDsuyzno6aVmg5hmHIrsEWNpLAHne074AJvME= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787062244; c=relaxed/simple; bh=ViG9p523Gtj965obqgoqvstIweJ3Dnp0nqQZuKx6Weo=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=IzrU3GAGxQz/RcQouZkrXxcV9jbQuuSl7cWUumpW0UfrxMD06XArup5/22At5Pdfn+ZqzPIQZeGtYJ/w3R73BvkpE7x009hV5ry7KeVvbOjUD6tfG3wb2CMjBiULUQu3BrRNQ5VElg1xNd92KCuauzG36Dcl8sivCkUlxqHOOww= 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; dkim=pass (1024-bit key) header.d=arm.com header.i=@arm.com header.b=t5Eewfnf; 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 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=arm.com header.i=@arm.com header.b="t5Eewfnf" 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> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260708031129.3503195-9-jthoughton@google.com> 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