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[2003:d8:2f18:f900:e0ae:65d5:8bf8:8cfd]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-45e0372aea2sm172099325e9.7.2025.09.15.01.26.55 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 15 Sep 2025 01:26:56 -0700 (PDT) Message-ID: Date: Mon, 15 Sep 2025 10:26:55 +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: Lance Yang , Dev Jain , Wei Yang Cc: 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> <62680957-e9aa-40ad-893d-885bd01e1117@redhat.com> 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: X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: AO-03RQq8b-qGArOqmfCc6um6k6Bx23G5wrtPzeixcg_1757924817 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Stat-Signature: 4q9h54nnr8pq1j4rbyhi6u66neu6oqda X-Rspam-User: X-Rspamd-Queue-Id: B054240003 X-Rspamd-Server: rspam10 X-HE-Tag: 1757924820-842926 X-HE-Meta: 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 aIeiIfaw 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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 10:08, Lance Yang wrote: > > > On 2025/9/15 15:56, David Hildenbrand wrote: >> On 14.09.25 09:29, 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 :) >> >> I agree. If mm_slot_lookup() returns NULL we just just handle that >> cleanly like >> >> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >> index 6b40bdfd224c3..70a32d59d7d2f 100644 >> --- a/mm/khugepaged.c >> +++ b/mm/khugepaged.c >> @@ -488,11 +488,13 @@ void __khugepaged_exit(struct mm_struct *mm) >> >>         spin_lock(&khugepaged_mm_lock); >>         slot = mm_slot_lookup(mm_slots_hash, mm); >> -       mm_slot = mm_slot_entry(slot, struct khugepaged_mm_slot, slot); >> -       if (mm_slot && khugepaged_scan.mm_slot != mm_slot) { >> -               hash_del(&slot->hash); >> -               list_del(&slot->mm_node); >> -               free = 1; >> +       if (slot) { >> +               mm_slot = mm_slot_entry(slot, struct khugepaged_mm_slot, >> slot); >> +               if (mm_slot && khugepaged_scan.mm_slot != mm_slot) { >> +                       hash_del(&slot->hash); >> +                       list_del(&slot->mm_node); >> +                       free = 1; >> +               } >>         } >>         spin_unlock(&khugepaged_mm_lock); >> >> >> If mm_slot_lookup() is not expected to ever return NULL, then a >> VM_WARN_ON_ONCE >> might be sufficient to document that this is guaranteed. >> >> IIUC, MMF_VM_HUGEPAGE might be set in __khugepaged_enter() in case >> >> (a) test_and_set_bit() succeeds >> >> but >> >> (b) mm_slot_alloc() fails >> >> In that case we could get NULL. > > Ah, good catch! We could indeed get NULL in that case ;) > >> >> >> It is rather weird to leave the flag set in case mm_slot_alloc() failed ... > > Perhaps the MMF_VM_HUGEPAGE flag should be cleared if mm_slot_alloc() fails? Clearing might also be nasty with concurrent readers that expect that it's setup. It's all rather nasty here, because we simply swallow allocation errors essentially and silently end up not scanning that MM through khugepaged... -- Cheers David / dhildenb