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([2a09:80c0:192:0:5dac:bf3d:c41:c3e7]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3c70eb7eed5sm16180834f8f.18.2025.08.26.08.24.23 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 26 Aug 2025 08:24:24 -0700 (PDT) Message-ID: Date: Tue, 26 Aug 2025 17:24:22 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 08/10] mm: update fork mm->flags initialisation to use bitmap To: Lorenzo Stoakes Cc: Andrew Morton , Alexander Gordeev , Gerald Schaefer , Heiko Carstens , Vasily Gorbik , Christian Borntraeger , Sven Schnelle , "David S . Miller" , Andreas Larsson , Dave Hansen , Andy Lutomirski , Peter Zijlstra , Thomas Gleixner , Ingo Molnar , Borislav Petkov , "H . Peter Anvin" , Alexander Viro , Christian Brauner , Jan Kara , Kees Cook , Zi Yan , Baolin Wang , "Liam R . Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Xu Xin , Chengming Zhou , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , David Rientjes , Shakeel Butt , Arnaldo Carvalho de Melo , Namhyung Kim , Mark Rutland , Alexander Shishkin , Jiri Olsa , Ian Rogers , Adrian Hunter , Kan Liang , Masami Hiramatsu , Oleg Nesterov , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Valentin Schneider , Jason Gunthorpe , John Hubbard , Peter Xu , Jann Horn , Pedro Falcato , Matthew Wilcox , Mateusz Guzik , linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org, sparclinux@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-trace-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org References: <9fb8954a7a0f0184f012a8e66f8565bcbab014ba.1755012943.git.lorenzo.stoakes@oracle.com> <73a39f45-e10c-42f8-819b-7289733eae03@redhat.com> <1743164d-c2d2-44a1-a2a9-aeeed8c13bc8@lucifer.local> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 2BCBE4000B X-Rspamd-Server: rspam04 X-Rspam-User: X-Stat-Signature: pd7br3to6otatg3ics9sj7tu4so5whke X-HE-Tag: 1756221869-465137 X-HE-Meta: U2FsdGVkX1/o0M7n/5cOSwOaGTVKYgR67HPvCd0XETaa1rlrtEzpsyoUqMpZKJXvsPL1eA+0Jjniw1cyc4WjVwCMFVWCxDwewkq8H3tTDW2alTww/0HZ9J81L1ZpwtoOISlvWvLOCBc8TkB+GGt1KNZ2DPm2NdHEWeLSx/lLFh9Wajc8McN7FN3cJeYmLhLjLR1MM0NJ1SfyF++cddWBKopSWzzozg9M4bD3uW7nMcR03Ck7zr7Eo4MKbKkUrubdCMeKfPF+UzKDDmluxoFPurG2W4UpFA6xSwe4TsO/2aTA01hUkvub+j6DmXAWBNYQoMh/L/52j+q/bUg8HtWIdPYDQ0d4rrxoSInyGYrFQWHMXo69LVAMUi3GEFI6RF36G2F7lTeQW4eUYEIM7+kMsxEsbo2y10TGyT1LYui3e216h9pb5JmJyZdo8nc/VLhR089cPieZePN35s3wGYN2KdZxepnlNsJMSxXFX+nYUW5WTHpac3EsRlb762X6YuvzGrl5xUxohCcDeaFpPPOPIEq931RoNvffxBefdkKZNiKV7vCS98rHvtmbCGpkUjupphM9NoirESXKNGUEx62yFpqzgayxjqToNwSYj+hDDBU+Xy5F5Y16rOnK3i7qmLJFBbdKpDUZgZoHTJjUHRaCf7adpDY1SZ7S97PvjLT7vWEPf2KO639BMA06ItHuGZehTkQtlIXeJ85NBn3NrAY6Wq3Bqy9m4WTBigJmwDlgmR3YTlS0ntkLcfd9ZhVy2cRwy9cVtfe7ng7mlntW4aHuhgnmBZph3CoLQzWmsqJanyi0gKtiZBw7bn1/KE5+JT+ki4JqFtfdQJe836slR9rZ/Q/5D+UIMhtIl7yEhCFmU71og2HTDIbZzu6jtfY9N1FFCyweWF6pnvMVE7ie4B7LPEqUktVCLO8OedW4DeIF0lvCHuukPFsl89izK0ZKVmnzJ+3rAJoqcuPYtPM+c4W pldsSQu1 aYzdn3lCDrjQG3DamZMnm/QZpjVxZ4uaxgu1v5LVBnGtMa5My2a2CUi9a1di3avC4+CCPE/L7zN7t5hVRkiTrYykk57irUQaudtZyALei1+q0s2BynSWq/qt4aqqFNCpJ1FPOU3Cj47/dbQ84zUDb7R0Y8EfEhJOjj5SojIvYdQ3JLxV0p18D0v8/MmBNl9ivIOCbdSJhlW8iCcoXMBQ3ZOnDsHaa2z8uFM2O366baRG+2BlB0ot5eRayMmtkMejGTngKzV0CCSZL2tSSr/xtT87IxzSm6Y9m/BVVLAtZ5D4qTBSp+4S3hUnJ8sLDdKq5u04CLBzVZO+zxHGxuMoiXKZ7y3U1FJdnQtA+68HDUniSQavJECb52ej3+3RpooahWv6NgmKNNqjIfSPnUlZMHT2cVFtmP21pm+Sz2GBdTCkJc75XDs/iCwhyjuy5D1RSX6GAlIVU8Di/Iqw= 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 26.08.25 16:32, Lorenzo Stoakes wrote: > On Tue, Aug 26, 2025 at 04:28:20PM +0200, David Hildenbrand wrote: >> On 26.08.25 16:21, Lorenzo Stoakes wrote: >>> On Tue, Aug 26, 2025 at 03:12:08PM +0200, David Hildenbrand wrote: >>>> On 12.08.25 17:44, Lorenzo Stoakes wrote: >>>>> We now need to account for flag initialisation on fork. We retain the >>>>> existing logic as much as we can, but dub the existing flag mask legacy. >>>>> >>>>> These flags are therefore required to fit in the first 32-bits of the flags >>>>> field. >>>>> >>>>> However, further flag propagation upon fork can be implemented in mm_init() >>>>> on a per-flag basis. >>>>> >>>>> We ensure we clear the entire bitmap prior to setting it, and use >>>>> __mm_flags_get_word() and __mm_flags_set_word() to manipulate these legacy >>>>> fields efficiently. >>>>> >>>>> Signed-off-by: Lorenzo Stoakes >>>>> --- >>>>> include/linux/mm_types.h | 13 ++++++++++--- >>>>> kernel/fork.c | 7 +++++-- >>>>> 2 files changed, 15 insertions(+), 5 deletions(-) >>>>> >>>>> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h >>>>> index 38b3fa927997..25577ab39094 100644 >>>>> --- a/include/linux/mm_types.h >>>>> +++ b/include/linux/mm_types.h >>>>> @@ -1820,16 +1820,23 @@ enum { >>>>> #define MMF_TOPDOWN 31 /* mm searches top down by default */ >>>>> #define MMF_TOPDOWN_MASK _BITUL(MMF_TOPDOWN) >>>>> -#define MMF_INIT_MASK (MMF_DUMPABLE_MASK | MMF_DUMP_FILTER_MASK |\ >>>>> +#define MMF_INIT_LEGACY_MASK (MMF_DUMPABLE_MASK | MMF_DUMP_FILTER_MASK |\ >>>>> MMF_DISABLE_THP_MASK | MMF_HAS_MDWE_MASK |\ >>>>> MMF_VM_MERGE_ANY_MASK | MMF_TOPDOWN_MASK) >>>>> -static inline unsigned long mmf_init_flags(unsigned long flags) >>>>> +/* Legacy flags must fit within 32 bits. */ >>>>> +static_assert((u64)MMF_INIT_LEGACY_MASK <= (u64)UINT_MAX); >>>> >>>> Why not use the magic number 32 you are mentioning in the comment? :) >>> >>> Meh I mean UINT_MAX works as a good 'any bit' mask and this will work on >>> both 32-bit and 64-bit systems. >>> >>>> >>>> static_assert((u32)MMF_INIT_LEGACY_MASK != MMF_INIT_LEGACY_MASK); >>> >>> On 32-bit that'd not work would it? >> >> On 32bit, BIT(32) would exceed the shift width of unsigned long -> undefined >> behavior. >> >> The compiler should naturally complain. > > Yeah, I don't love that sorry. Firstly it's a warning, so you may well miss it > (I just tried), Upstream bots usually complain at you for warnings :P > and secondly you're making the static assert not have any > meaning except that you expect to trigger a compiler warning, it's a bit > bizarre. On 64 bit where BIT(32) *makes any sense* it triggers as expected, no? > > My solution works (unless you can see a reason it shouldn't) and I don't find > this approach any simpler. Please explain to me like I am a 5 yo how your approach works with BIT(32) on 32bit when the behavior on 32bit is undefined. :P -- Cheers David / dhildenb