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[80.187.80.144]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-40fc6921f4esm22946784f8f.44.2025.09.30.07.43.15 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 30 Sep 2025 07:43:16 -0700 (PDT) Message-ID: <25653f37-6ba3-442f-9348-d879a8ad4704@redhat.com> Date: Tue, 30 Sep 2025 16:43:13 +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] mm/ptdump: Replace READ_ONCE() with standard page table accessors To: Anshuman Khandual , Dev Jain , linux-mm@kvack.org Cc: Andrew Morton , linux-kernel@vger.kernel.org References: <20250930025246.1143340-1-anshuman.khandual@arm.com> <9736fd6a-8987-4b10-9b05-e03106463c34@arm.com> <1f3c8fe8-cff0-4e3e-bea8-285b00fc7a5d@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: <1f3c8fe8-cff0-4e3e-bea8-285b00fc7a5d@arm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 30.09.25 09:00, Anshuman Khandual wrote: > > > On 30/09/25 12:11 PM, David Hildenbrand wrote: >> On 30.09.25 06:37, Dev Jain wrote: >>> >>> On 30/09/25 8:22 am, Anshuman Khandual wrote: >>>> Replace READ_ONCE() with standard page table accessors i.e pxdp_get() which >>>> anyways default into READ_ONCE() in cases where platform does not override. >>>> >>>> Cc: Andrew Morton >>>> Cc: David Hildenbrand >>>> Cc: linux-mm@kvack.org >>>> Cc: linux-kernel@vger.kernel.org >>>> Signed-off-by: Anshuman Khandual >>>> --- >>>>    mm/ptdump.c | 8 ++++---- >>>>    1 file changed, 4 insertions(+), 4 deletions(-) >>>> >>>> diff --git a/mm/ptdump.c b/mm/ptdump.c >>>> index b600c7f864b8..18861501b533 100644 >>>> --- a/mm/ptdump.c >>>> +++ b/mm/ptdump.c >>>> @@ -31,7 +31,7 @@ static int ptdump_pgd_entry(pgd_t *pgd, unsigned long addr, >>>>                    unsigned long next, struct mm_walk *walk) >>>>    { >>>>        struct ptdump_state *st = walk->private; >>>> -    pgd_t val = READ_ONCE(*pgd); >>>> +    pgd_t val = pgdp_get(pgd); >>>>       #if CONFIG_PGTABLE_LEVELS > 4 && \ >>>>            (defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_KASAN_SW_TAGS)) >>>> @@ -54,7 +54,7 @@ static int ptdump_p4d_entry(p4d_t *p4d, unsigned long addr, >>>>                    unsigned long next, struct mm_walk *walk) >>>>    { >>>>        struct ptdump_state *st = walk->private; >>>> -    p4d_t val = READ_ONCE(*p4d); >>>> +    p4d_t val = p4dp_get(p4d); >>>>       #if CONFIG_PGTABLE_LEVELS > 3 && \ >>>>            (defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_KASAN_SW_TAGS)) >>>> @@ -77,7 +77,7 @@ static int ptdump_pud_entry(pud_t *pud, unsigned long addr, >>>>                    unsigned long next, struct mm_walk *walk) >>>>    { >>>>        struct ptdump_state *st = walk->private; >>>> -    pud_t val = READ_ONCE(*pud); >>>> +    pud_t val = pudp_get(pud); >>>>       #if CONFIG_PGTABLE_LEVELS > 2 && \ >>>>            (defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_KASAN_SW_TAGS)) >>>> @@ -100,7 +100,7 @@ static int ptdump_pmd_entry(pmd_t *pmd, unsigned long addr, >>>>                    unsigned long next, struct mm_walk *walk) >>>>    { >>>>        struct ptdump_state *st = walk->private; >>>> -    pmd_t val = READ_ONCE(*pmd); >>>> +    pmd_t val = pmdp_get(pmd); >>> >>> I believe this should go through pmdp_get_lockless(). I can see in pgtable.h that >>> some magic is required on some arches to decode the pmd correctly in case walking >>> without locks. >> >> pmdp_get_lockless() is a nasty thingy to handle selected 32bit architectures. >> >> But given that we're using ptep_get_lockless() in ptdump_pmd_entry() it probably wouldn't hurt to use pmdp_get_lockless() here. >> >> Staring at ARCH_HAS_PTDUMP, I don't think any 32bit arch would actually end up compiling ptdump.c. >> >> E.g., on x86 only X86_64 ends up selecting ARCH_HAS_PTDUMP. >> > > pxdp_get_lockless() not really required here, let's stick with pxdp_get() instead. I'd suggest that we keep it consistent. That is, also removing the ptep_get_lockless() if not really required. -- Cheers David / dhildenb