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[2003:d8:2f0c:c200:fa4a:c4ff:1b32:21ce]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-42708bcea1bsm1267844f8f.14.2025.10.17.01.43.52 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 17 Oct 2025 01:43:53 -0700 (PDT) Message-ID: <4a7a7fbb-e33e-4033-91e7-efce7915cf7f@redhat.com> Date: Fri, 17 Oct 2025 10:43:52 +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-new 1/1] mm/khugepaged: guard is_zero_pfn() calls with pte_present() To: Lance Yang , Wei Yang , Dev Jain Cc: akpm@linux-foundation.org, lorenzo.stoakes@oracle.com, ziy@nvidia.com, baolin.wang@linux.alibaba.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, baohua@kernel.org, ioworker0@gmail.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <20251016033643.10848-1-lance.yang@linux.dev> <17c4c5f9-6ac8-4914-838f-f511dfbf948f@arm.com> <20251017012724.4bo5oj2g6tdmp2fv@master> <1674efca-6d4e-4247-8b1c-b6816360d8bb@redhat.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: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 17.10.25 10:37, Lance Yang wrote: > > > On 2025/10/17 16:11, David Hildenbrand wrote: >> On 17.10.25 03:27, Wei Yang wrote: >>> On Thu, Oct 16, 2025 at 11:47:06AM +0530, Dev Jain wrote: >>>> >>>> On 16/10/25 9:06 am, Lance Yang wrote: >>>>> From: Lance Yang >>>>> >>>>> A non-present entry, like a swap PTE, contains completely different >>>>> data >>>>> (swap type and offset). pte_pfn() doesn't know this, so if we feed it a >>>>> non-present entry, it will spit out a junk PFN. >>>>> >>>>> What if that junk PFN happens to match the zeropage's PFN by sheer >>>>> chance? While really unlikely, this would be really bad if it did. >>>>> >>>>> So, let's fix this potential bug by ensuring all calls to is_zero_pfn() >>>>> in khugepaged.c are properly guarded by a pte_present() check. >>>>> >>>>> Suggested-by: Lorenzo Stoakes >>>>> Signed-off-by: Lance Yang >>>>> --- >>>>>    mm/khugepaged.c | 13 ++++++++----- >>>>>    1 file changed, 8 insertions(+), 5 deletions(-) >>>>> >>>>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >>>>> index d635d821f611..0341c3d13e9e 100644 >>>>> --- a/mm/khugepaged.c >>>>> +++ b/mm/khugepaged.c >>>>> @@ -516,7 +516,7 @@ static void release_pte_pages(pte_t *pte, pte_t >>>>> *_pte, >>>>>            pte_t pteval = ptep_get(_pte); >>>>>            unsigned long pfn; >>>>> -        if (pte_none(pteval)) >>>>> +        if (!pte_present(pteval)) >>>>>                continue; >>>>>            pfn = pte_pfn(pteval); >>>>>            if (is_zero_pfn(pfn)) >>>>> @@ -690,9 +690,10 @@ static void >>>>> __collapse_huge_page_copy_succeeded(pte_t *pte, >>>>>             address += nr_ptes * PAGE_SIZE) { >>>>>            nr_ptes = 1; >>>>>            pteval = ptep_get(_pte); >>>>> -        if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) { >>>>> +        if (pte_none(pteval) || >>>>> +            (pte_present(pteval) && is_zero_pfn(pte_pfn(pteval)))) { >>>>>                add_mm_counter(vma->vm_mm, MM_ANONPAGES, 1); >>>>> -            if (is_zero_pfn(pte_pfn(pteval))) { >>>>> +            if (!pte_none(pteval)) { >>>> >>>> Could save a level of indentation by saying >>>> if (pte_none(pteval)) >>>>     continue; >>>> >>> >>> Vote for this :-) >> >> I suspect there will be a v2, correct? > > I was hoping a v2 wouldn't be necessary for this ;p > > Of course, if we'd prefer a v2, I'm happy to send one out. I lost track of what the result will be, so a v2 would be nice at least for me :) -- Cheers David / dhildenb