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([2a09:80c0:192:0:5dac:bf3d:c41:c3e7]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3c711211cc8sm16703127f8f.36.2025.08.26.08.53.37 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 26 Aug 2025 08:53:39 -0700 (PDT) Message-ID: <49df1a2a-7c5b-4ada-98b8-8d7871fd7228@redhat.com> Date: Tue, 26 Aug 2025 17:53:36 +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> <9e7f5149-afb7-4e94-b231-78876c41a438@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: C5A0D2000B X-Rspamd-Server: rspam04 X-Rspam-User: X-Stat-Signature: y4z3xudxq778yq5p3j738gbm8uwq7mqj X-HE-Tag: 1756223623-103537 X-HE-Meta: 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 veWAYzvH DDCVY87+X48i1YTZwLNEAQY8dSMw2TEw0E7Ko0E1+frPnC6X2COxtwxN9nV9NIIaN51B76I11k3yJ/Eg+XLliwTpFyj6gU4dT+vjx4RxP/Eyk6Ndf4Rd1Fqt8xzVuGFUSdksi6bvcCAUpmiizo1tZYYHVyLXn1Qg/7WCfPuRIo4Xb3lKDIOhkN7Jtw0NxvKweonG90fg13AUCFbcQNxUQ8dTEBU86P1hcTSixuWfxsWzJt2/3jsYCBJJdSUNB5YXCCV6/oiqeOLkpZ9wB07iP2BNJ2mmG1sJ9LzzB4Cn/ro/d/d6kGRaj0h3ygUSKm1w+m6BuLXSEo1u9H1N5kI9R6ps+UH7hGklEaRbTozuAUUU4D9rz/In3uOnt7Xw4aGDU9GjATCvSiontUP6ot4GOVupjq+Rmpnimb2thKwrZW5ZjYjsghBjWnpCq2hdeZAmZLMubppgbLQUOFq4= 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 17:39, Lorenzo Stoakes wrote: > On Tue, Aug 26, 2025 at 05:24:22PM +0200, David Hildenbrand wrote: >> 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 > > Fine, but it's not a static assert and they can be delayed. > >> >>> 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? > > It's not a static assert. > >> >>> >>> 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 > > OK right I see, in both cases BIT(32) is going to cause a warning on 32-bit. > > I was wrong in thinking (u64)(1UL << 32) would get fixed up because of the > outer cast I guess. > > This was the mistake here, so fine, we could do it this way. > > I guess I'll have to respin the series at this point. Let me think again: assuming someone would mess up the BIT() thing and convert back to 1 << NR, your variant would catch it on 32bit I guess. So given I was primarily confused by the "u64" when talking about 32it, no need for a full resend of this series just to clean this up. -- Cheers David / dhildenb