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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 Subject: Re: [PATCH 08/10] mm: update fork mm->flags initialisation to use bitmap Message-ID: <9e7f5149-afb7-4e94-b231-78876c41a438@lucifer.local> References: 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a=e1sVV491RgrpLwSTMOnk8w==:17 a=6eWqkTHjU83fiwn7nKZWdM+Sl24=:19 a=z/mQ4Ysz8XfWz/Q5cLBRGdckG28=:19 a=lCpzRmAYbLLaTzLvsPZ7Mbvzbb8=:19 a=wKuvFiaSGQ0qltdbU6+NXLB8nM8=:19 a=Ol13hO9ccFRV9qXi2t6ftBPywas=:19 a=xqWC_Br6kY4A:10 a=kj9zAlcOel0A:10 a=2OwXVqhp2XgA:10 a=GoEa3M9JfhUA:10 a=yPCof4ZbAAAA:8 a=n3WvvtQ5RJhdEwuKYYcA:9 a=CjuIK1q_8ugA:10 cc=ntf awl=host:13602 X-Proofpoint-Spam-Details-Enc: AW1haW4tMjUwODIzMDAzMCBTYWx0ZWRfXx/GsylzUA83g J1CY3JYEUg+OnA+jD6dfLVojWm1of9NydJE43n7cr3I4tOtlEmXwt3gtQpOr13TEQu1KsrmZxk9 npAarpPUm4EeJxMGFr/85s1RvtSD46vbCM14OHmlScil0zJRtkMuRBoz79DsKyF1jEXlr95kCko o+zAwbu6pleRSf2mYupK2SiQ7tAJJ1PQsHzjsRLLwfJf/3B4xLVD7OuKseN992PKFaodt4hliJb Ip/v+oj8XPuwayTC+rBpKfNa4k6TP/fjwSbD1jcPQyrbV5aBDmjxWF0cxgO7GpBrEQYbRKFJl58 O0NqD2UCcZ541ai6OkGD4c+sRHjR4VcH+H52Skicuk6bpCWcIv2rRmQgxunhExfDIe/fBDVNUfq wzGPFolntODPi67ARceZyestqktV9w== 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.