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[2003:d8:2f18:f900:e0ae:65d5:8bf8:8cfd]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3e976b5dfdasm6277318f8f.4.2025.09.15.00.57.02 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 15 Sep 2025 00:57:03 -0700 (PDT) Message-ID: Date: Mon, 15 Sep 2025 09:57:02 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/mm_slot: make sure slot is the first element of its wrapper structure To: Wei Yang , Dev Jain Cc: Lance Yang , linux-mm@kvack.org, lorenzo.stoakes@oracle.com, akpm@linux-foundation.org, chengming.zhou@linux.dev, npache@redhat.com, ryan.roberts@arm.com, xu.xin16@zte.com.cn, baohua@kernel.org, Liam.Howlett@oracle.com, ziy@nvidia.com, baolin.wang@linux.alibaba.com References: <20250914000026.17986-1-richard.weiyang@gmail.com> <463716f5-be12-4d78-9432-779e3f11460c@arm.com> <20250915015722.z4w23d6ralc2y7mk@master> <20250915074614.tsezftgt2uks3l5z@master> From: David Hildenbrand 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: <20250915074614.tsezftgt2uks3l5z@master> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: AvSZc-ztXJ2veB1Q3D3BVdt8b-VdY-ffYj2aA883yJE_1757923024 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 385B7180002 X-Stat-Signature: 14ahtyfu5toijdjcpo8dgibdcydjwnxf X-Rspam-User: X-Rspamd-Server: rspam01 X-HE-Tag: 1757923029-695767 X-HE-Meta: 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 aohW5Ll4 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 15.09.25 09:46, Wei Yang wrote: > On Mon, Sep 15, 2025 at 09:19:55AM +0530, Dev Jain wrote: >> >> On 15/09/25 7:27 am, Wei Yang wrote: >>> On Sun, Sep 14, 2025 at 12:59:33PM +0530, Dev Jain wrote: >>>> On 14/09/25 12:33 pm, Lance Yang wrote: >>>>> >>>>> On 2025/9/14 14:21, Dev Jain wrote: >>>>>> On 14/09/25 5:30 am, Wei Yang wrote: >>>>>>> When using mm_slot in ksm/khugepaged, there is code snip like: >>>>>>> >>>>>>> slot = mm_slot_lookup(mm_slots_hash, mm); >>>>>>> mm_slot = mm_slot_entry(slot, struct ksm_mm_slot, slot); >>>>>>> if (mm_slot && ..) { >>>>>>> } >>>>>>> >>>>>>> This is only valid when mm_slot is the first element of its wrapper >>>>>>> structure, otherwise a NULL slot would converted to a mm_slot with >>>>>>> negative value. And current code thinks it is valid and continue. >>>>>> Shouldn't you fix the code for the case when you can't find the slot >>>>>> in the hashtable, i.e slot == NULL? Like, if (!slot) return. >>>>> Right. For khugepaged specifically, the slot == NULL case in >>>>> __khugepaged_exit() (only user of mm_slot_lookup) should probably >>>>> be treated as a kernel BUG for new. >>>>> >>>>> But I'm not sure if the same logic applies to KSM ;) >>>> I haven't seen the KSM analogue, but restricting the position of an element >>>> >>>> in a struct to make the code work should imply that the code is wrong in >>>> >>>> the first place :) >>>> >>> Ok, if so, I think this should be a patch with Fixes tag and cc stable? And it >>> supposed to be two patches, since it fixes two different commit. Am I right? >> >> I don't think this is a fix which needs to be backported, the current code works. >> What we need is a VM_BUG_ON() or a WARN_ON_ONCE() when slot == NULL, because that >> just shouldn't happen, and then safely exit. >> > > You mean sth like this? > > @@ -2940,7 +2940,7 @@ void __ksm_exit(struct mm_struct *mm) > spin_lock(&ksm_mmlist_lock); > slot = mm_slot_lookup(mm_slots_hash, mm); > mm_slot = mm_slot_entry(slot, struct ksm_mm_slot, slot); > - if (mm_slot && ksm_scan.mm_slot != mm_slot) { > + if (WARN_ON_ONCE(slot) && ksm_scan.mm_slot != mm_slot) { > if (!mm_slot->rmap_list) { > hash_del(&slot->hash); > list_del(&slot->mm_node); See my other reply, I think this can happen if mm_slot_alloc() fails. -- Cheers David / dhildenb