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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-Rspamd-Queue-Id: 654F820009 X-Rspamd-Server: rspam10 X-Rspam-User: X-Stat-Signature: qckh8po9ee5ye9gf9hnixx6dramy4q7d X-HE-Tag: 1787062242-234741 X-HE-Meta: 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 Eux8YuvT gLmji7plOw7VIU3IZHkND4U0r9LlFY54tYZyoXphSmPJHLJtG9jBU45u0q8sxUgTKSM37FisBTCrjLPzhrjhSUK2/e7ZfXGxNxlcZYdDVykXZRgXurkhJw0RBg3B/2zmRRJUqXacHmEmHmCLmKZYtrEV8D2+CuxzKKZ8U2INoEWL0TaW4IK8TrWtAj4d+ulBed82puHF6QNZssdOWw+xeeOZaHCHiyTTMO8yp1zP+lOVfibLuGCNmHX6MKUL34WeiQic7nsmoo1GbWhoBAcGCw10PkQ== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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