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[2003:d8:2f0c:c200:fa4a:c4ff:1b32:21ce]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-427f009a993sm16658313f8f.24.2025.10.20.12.17.39 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 20 Oct 2025 12:17:39 -0700 (PDT) Message-ID: <02686adb-7575-4eb4-8d60-c5374de60752@redhat.com> Date: Mon, 20 Oct 2025 21:17:38 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] arm64, mm: avoid always making PTE dirty in pte_mkwrite() To: Anshuman Khandual , Catalin Marinas , Huang Ying Cc: Will Deacon , Ryan Roberts , Gavin Shan , Ard Biesheuvel , "Matthew Wilcox (Oracle)" , Yicong Yang , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <20251015023712.46598-1-ying.huang@linux.alibaba.com> <0e6d1f1f-a917-4e36-80de-03ba94c6d850@arm.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZoEEwEIAEQCGwMCF4ACGQEFCwkIBwICIgIG FQoJCAsCBBYCAwECHgcWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaJzangUJJlgIpAAKCRBN 3hD3AP+DWhAxD/9wcL0A+2rtaAmutaKTfxhTP0b4AAp1r/eLxjrbfbCCmh4pqzBhmSX/4z11 opn2KqcOsueRF1t2ENLOWzQu3Roiny2HOU7DajqB4dm1BVMaXQya5ae2ghzlJN9SIoopTWlR 0Af3hPj5E2PYvQhlcqeoehKlBo9rROJv/rjmr2x0yOM8qeTroH/ZzNlCtJ56AsE6Tvl+r7cW 3x7/Jq5WvWeudKrhFh7/yQ7eRvHCjd9bBrZTlgAfiHmX9AnCCPRPpNGNedV9Yty2Jnxhfmbv Pw37LA/jef8zlCDyUh2KCU1xVEOWqg15o1RtTyGV1nXV2O/mfuQJud5vIgzBvHhypc3p6VZJ lEf8YmT+Ol5P7SfCs5/uGdWUYQEMqOlg6w9R4Pe8d+mk8KGvfE9/zTwGg0nRgKqlQXrWRERv cuEwQbridlPAoQHrFWtwpgYMXx2TaZ3sihcIPo9uU5eBs0rf4mOERY75SK+Ekayv2ucTfjxr Kf014py2aoRJHuvy85ee/zIyLmve5hngZTTe3Wg3TInT9UTFzTPhItam6dZ1xqdTGHZYGU0O otRHcwLGt470grdiob6PfVTXoHlBvkWRadMhSuG4RORCDpq89vu5QralFNIf3EysNohoFy2A LYg2/D53xbU/aa4DDzBb5b1Rkg/udO1gZocVQWrDh6I2K3+cCs7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: <0e6d1f1f-a917-4e36-80de-03ba94c6d850@arm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 20.10.25 04:09, Anshuman Khandual wrote: > > > On 17/10/25 11:36 PM, Catalin Marinas wrote: >> On Wed, Oct 15, 2025 at 10:37:12AM +0800, Huang Ying wrote: >>> Current pte_mkwrite_novma() makes PTE dirty unconditionally. This may >>> mark some pages that are never written dirty wrongly. For example, >>> do_swap_page() may map the exclusive pages with writable and clean PTEs >>> if the VMA is writable and the page fault is for read access. >>> However, current pte_mkwrite_novma() implementation always dirties the >>> PTE. This may cause unnecessary disk writing if the pages are >>> never written before being reclaimed. >>> >>> So, change pte_mkwrite_novma() to clear the PTE_RDONLY bit only if the >>> PTE_DIRTY bit is set to make it possible to make the PTE writable and >>> clean. >>> >>> The current behavior was introduced in commit 73e86cb03cf2 ("arm64: >>> Move PTE_RDONLY bit handling out of set_pte_at()"). Before that, >>> pte_mkwrite() only sets the PTE_WRITE bit, while set_pte_at() only >>> clears the PTE_RDONLY bit if both the PTE_WRITE and the PTE_DIRTY bits >>> are set. >>> >>> To test the performance impact of the patch, on an arm64 server >>> machine, run 16 redis-server processes on socket 1 and 16 >>> memtier_benchmark processes on socket 0 with mostly get >>> transactions (that is, redis-server will mostly read memory only). >>> The memory footprint of redis-server is larger than the available >>> memory, so swap out/in will be triggered. Test results show that the >>> patch can avoid most swapping out because the pages are mostly clean. >>> And the benchmark throughput improves ~23.9% in the test. >>> >>> Fixes: 73e86cb03cf2 ("arm64: Move PTE_RDONLY bit handling out of set_pte_at()") >>> Signed-off-by: Huang Ying >>> Cc: Catalin Marinas >>> Cc: Will Deacon >>> Cc: Anshuman Khandual >>> Cc: Ryan Roberts >>> Cc: Gavin Shan >>> Cc: Ard Biesheuvel >>> Cc: "Matthew Wilcox (Oracle)" >>> Cc: Yicong Yang >>> Cc: linux-arm-kernel@lists.infradead.org >>> Cc: linux-kernel@vger.kernel.org >>> --- >>> arch/arm64/include/asm/pgtable.h | 3 ++- >>> 1 file changed, 2 insertions(+), 1 deletion(-) >>> >>> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h >>> index aa89c2e67ebc..0944e296dd4a 100644 >>> --- a/arch/arm64/include/asm/pgtable.h >>> +++ b/arch/arm64/include/asm/pgtable.h >>> @@ -293,7 +293,8 @@ static inline pmd_t set_pmd_bit(pmd_t pmd, pgprot_t prot) >>> static inline pte_t pte_mkwrite_novma(pte_t pte) >>> { >>> pte = set_pte_bit(pte, __pgprot(PTE_WRITE)); >>> - pte = clear_pte_bit(pte, __pgprot(PTE_RDONLY)); >>> + if (pte_sw_dirty(pte)) >>> + pte = clear_pte_bit(pte, __pgprot(PTE_RDONLY)); >>> return pte; >>> } >> >> This seems to be the right thing. I recall years ago I grep'ed >> (obviously not hard enough) and most pte_mkwrite() places had a >> pte_mkdirty(). But I missed do_swap_page() and possibly others. >> >> For this patch: >> >> Reviewed-by: Catalin Marinas >> >> I wonder whether we should also add (as a separate patch): >> >> diff --git a/mm/debug_vm_pgtable.c b/mm/debug_vm_pgtable.c >> index 830107b6dd08..df1c552ef11c 100644 >> --- a/mm/debug_vm_pgtable.c >> +++ b/mm/debug_vm_pgtable.c >> @@ -101,6 +101,7 @@ static void __init pte_basic_tests(struct pgtable_debug_args *args, int idx) >> WARN_ON(pte_dirty(pte_mkclean(pte_mkdirty(pte)))); >> WARN_ON(pte_write(pte_wrprotect(pte_mkwrite(pte, args->vma)))); >> WARN_ON(pte_dirty(pte_wrprotect(pte_mkclean(pte)))); >> + WARN_ON(pte_dirty(pte_mkwrite_novma(pte_mkclean(pte)))); >> WARN_ON(!pte_dirty(pte_wrprotect(pte_mkdirty(pte)))); >> } >> >> For completeness, also (and maybe other combinations): >> >> WARN_ON(!pte_write(pte_mkdirty(pte_mkwrite_novma(pte)))); > > Adding similar tests to pte_wrprotect(). > > diff --git a/mm/debug_vm_pgtable.c b/mm/debug_vm_pgtable.c > index 830107b6dd08..573632ebf304 100644 > --- a/mm/debug_vm_pgtable.c > +++ b/mm/debug_vm_pgtable.c > @@ -102,6 +102,11 @@ static void __init pte_basic_tests(struct pgtable_debug_args *args, int idx) > WARN_ON(pte_write(pte_wrprotect(pte_mkwrite(pte, args->vma)))); > WARN_ON(pte_dirty(pte_wrprotect(pte_mkclean(pte)))); > WARN_ON(!pte_dirty(pte_wrprotect(pte_mkdirty(pte)))); > + > + WARN_ON(pte_dirty(pte_mkwrite_novma(pte_mkclean(pte)))); > + WARN_ON(!pte_write(pte_mkdirty(pte_mkwrite_novma(pte)))); > + WARN_ON(!pte_write(pte_mkwrite_novma(pte_wrprotect(pte)))); > + WARN_ON(pte_write(pte_wrprotect(pte_mkwrite_novma(pte)))); > } > > static void __init pte_advanced_tests(struct pgtable_debug_args *args) > @@ -195,6 +200,9 @@ static void __init pmd_basic_tests(struct pgtable_debug_args *args, int idx) > WARN_ON(pmd_write(pmd_wrprotect(pmd_mkwrite(pmd, args->vma)))); > WARN_ON(pmd_dirty(pmd_wrprotect(pmd_mkclean(pmd)))); > WARN_ON(!pmd_dirty(pmd_wrprotect(pmd_mkdirty(pmd)))); > + > + WARN_ON(!pmd_write(pmd_mkwrite_novma(pmd_wrprotect(pmd)))); > + WARN_ON(pmd_write(pmd_wrprotect(pmd_mkwrite_novma(pmd)))); > /* > * A huge page does not point to next level page table > * entry. Hence this must qualify as pmd_bad(). >> >> I cc'ed linux-mm in case we missed anything. If nothing raised, I'll >> queue it next week. Can we please send patches touching mm/debug_vm_pgtable.c properly to linux-mm? I wrote tools/testing/selftests/mm/mkdirty.c a while ago. I wonder whether that could also be expressed in these tests here. But my memory comes back: ARCH_HAS_DEBUG_VM_PGTABLE is not set by all architectures (and in particluar not sparc64 which I fixed back then) -- Cheers David / dhildenb