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Tue, 26 Aug 2025 14:21:31 +0000 Received: from DM4PR10MB8218.namprd10.prod.outlook.com ([fe80::2650:55cf:2816:5f2]) by DM4PR10MB8218.namprd10.prod.outlook.com ([fe80::2650:55cf:2816:5f2%5]) with mapi id 15.20.9052.019; Tue, 26 Aug 2025 14:21:31 +0000 Date: Tue, 26 Aug 2025 15:21:28 +0100 From: Lorenzo Stoakes To: David Hildenbrand 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 Subject: Re: [PATCH 08/10] mm: update fork mm->flags initialisation to use bitmap Message-ID: References: <9fb8954a7a0f0184f012a8e66f8565bcbab014ba.1755012943.git.lorenzo.stoakes@oracle.com> 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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? Kinda the point here is that the issue would get picked up on either. I think with the comment above it's fine as-is! :) > > > + > > +/* > > + * Initialise legacy flags according to masks, propagating selected flags on > > + * fork. Further flag manipulation can be performed by the caller. > > It's weird not reading "initialize", but I am afraid the kernel is already > tainted :P > > t14s: ~/git/linux nth_page $ git grep "initialise" | wc -l > 1778 > t14s: ~/git/linux nth_page $ git grep "initialize" | wc -l > 22043 British English in the kernel will survive if I have anything to do with it ;) > > Besides the assert simplification > > Acked-by: David Hildenbrand Thanks! > > -- > Cheers > > David / dhildenb >