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[2003:d8:2f18:f900:e0ae:65d5:8bf8:8cfd]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3e8f4d644adsm7899581f8f.52.2025.09.15.01.25.45 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 15 Sep 2025 01:25:46 -0700 (PDT) Message-ID: Date: Mon, 15 Sep 2025 10:25:45 +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: Dev Jain , Lance Yang , 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> <220cb3d6-d095-4656-bede-9fb8624f4e12@arm.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: <220cb3d6-d095-4656-bede-9fb8624f4e12@arm.com> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: HSz5g-BpYBKGjvKy6LotleAHBQTJ5qBG8BxedIhAMsQ_1757924747 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 5E2B71A000D X-Stat-Signature: rsbjbf8dcr48zdmxr88s9kmiqaf4h3uh X-Rspam-User: X-HE-Tag: 1757924750-118600 X-HE-Meta: U2FsdGVkX19nOc0+p3RKV6FX4cv2zWpx9mTY0bpUxtOTAFTw9/LoS1iatm8BCWKW60A8SnSiNdrIYAjmAcaPndsjF9Z6AA9Omhe47Bizcz1WQvhKESgzowFQzbv4pkrrmZL5PAu/ccgVG2WTIjNiAaqZaigjqubKm18fFANXTpEG37AEaahFUdg/cri3BMHXR6ap1lVLD+Pge62dTYXFb6d3QIc/6XBX/BSDuDRruIPYUSgerNLsgYQ+YujuDL1BUAxhCiz/gheEatKKLPdoUV//6+WPb7okk136gyfWrCk6ocF9OMO0+9G80P6vj7ymkd4fjbQoT56MfSnzSYJB0THjEL1qo1K8Z/xoPDH4Opsg64Vyo661gd5C1IY7g1Wpd2/WGZG4QtU1ba6C9MB+86sbmvgNniELOSt78mWIkecSjzTTj+pNoMdCCFzNELqGmMazUrkbuSvsPZMzyy3sCyEBAwaHGQZxVCQf4CKWo/rpBdctb4zTzgLr4sLOZcym0V445hDrVbtE705AEtQKQBw6vtw4rBh8S7R9VxAP91OlOava35Vqn0hnhWXTDJZf/8qSmK6faBbyfPa9tGuVyymhnbeb3cLKHWDJWs/HD06NHxLcvA/kXstkUxYDHpDRw1lwz8qMSqS8eC4R5dMBbunf7rEvL/MYblzVNH0SgwusSst2cOKWhDECUADH+Hl+rhEnwcEZToWHSYV1ofDm5e12f0ZYeE6uTmEYNjHcgNR78TRiE4kv3vZ7oDQaLMErXo1VK9Ohs/EZ/nn8JAB+udk1B7OVwwFBM/7Un8wRSB+nHOACAg9JxLuWwVOwTYZpKG7NVSwiIO6b5wazm9dtWf729hN2ZGN2rqjwJecbvWXAUMpuUc/zfXtYj2NNIbwBcRn8PTYoYKE11lreRL3g93YlheiLYPSh+jQ/Y4QwCToj2KSqth+5itp9LeEl2xefT6bSfuNVxgjx8lSyE0k JWW58x2f 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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 10:11, Dev Jain wrote: > > On 15/09/25 1:26 pm, 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. >> >> >> It is rather weird to leave the flag set in case mm_slot_alloc() >> failed ... > > Good spot! We should move the slot allocation line before the test_and_set. You'll have to handle races differently then, and free the allocated mm_slot if the mm_flags_test_and_set fails I guess. Maybe you'd want an early check if the flag is already set first. -- Cheers David / dhildenb