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Message-ID: <zvoxt7bjp2niffmnxjki7tcfspvwb4mcxkw5ab5huwnnsh3hev@qbwtyxdg53uf>
Date: Tue, 12 Aug 2025 13:31:50 -0400
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Alexander Gordeev <agordeev@...ux.ibm.com>,
Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>,
Christian Borntraeger <borntraeger@...ux.ibm.com>,
Sven Schnelle <svens@...ux.ibm.com>,
"David S . Miller" <davem@...emloft.net>,
Andreas Larsson <andreas@...sler.com>,
Dave Hansen <dave.hansen@...ux.intel.com>,
Andy Lutomirski <luto@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
Borislav Petkov <bp@...en8.de>, "H . Peter Anvin" <hpa@...or.com>,
Alexander Viro <viro@...iv.linux.org.uk>,
Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
Kees Cook <kees@...nel.org>, David Hildenbrand <david@...hat.com>,
Zi Yan <ziy@...dia.com>, Baolin Wang <baolin.wang@...ux.alibaba.com>,
Nico Pache <npache@...hat.com>, Ryan Roberts <ryan.roberts@....com>,
Dev Jain <dev.jain@....com>, Barry Song <baohua@...nel.org>,
Xu Xin <xu.xin16@....com.cn>,
Chengming Zhou <chengming.zhou@...ux.dev>,
Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
David Rientjes <rientjes@...gle.com>,
Shakeel Butt <shakeel.butt@...ux.dev>,
Arnaldo Carvalho de Melo <acme@...nel.org>,
Namhyung Kim <namhyung@...nel.org>,
Mark Rutland <mark.rutland@....com>,
Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
Adrian Hunter <adrian.hunter@...el.com>,
Kan Liang <kan.liang@...ux.intel.com>,
Masami Hiramatsu <mhiramat@...nel.org>,
Oleg Nesterov <oleg@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>,
Mel Gorman <mgorman@...e.de>, Valentin Schneider <vschneid@...hat.com>,
Jason Gunthorpe <jgg@...pe.ca>, John Hubbard <jhubbard@...dia.com>,
Peter Xu <peterx@...hat.com>, Jann Horn <jannh@...gle.com>,
Pedro Falcato <pfalcato@...e.de>, Matthew Wilcox <willy@...radead.org>,
Mateusz Guzik <mjguzik@...il.com>, linux-s390@...r.kernel.org,
linux-kernel@...r.kernel.org, sparclinux@...r.kernel.org,
linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
linux-trace-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org
Subject: Re: [PATCH 08/10] mm: update fork mm->flags initialisation to use
bitmap
* Lorenzo Stoakes <lorenzo.stoakes@...cle.com> [250812 11:47]:
> 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 <lorenzo.stoakes@...cle.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@...cle.com>
> ---
> 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);
> +
> +/*
> + * Initialise legacy flags according to masks, propagating selected flags on
> + * fork. Further flag manipulation can be performed by the caller.
> + */
> +static inline unsigned long mmf_init_legacy_flags(unsigned long flags)
> {
> if (flags & (1UL << MMF_HAS_MDWE_NO_INHERIT))
> flags &= ~((1UL << MMF_HAS_MDWE) |
> (1UL << MMF_HAS_MDWE_NO_INHERIT));
> - return flags & MMF_INIT_MASK;
> + return flags & MMF_INIT_LEGACY_MASK;
> }
>
> #endif /* _LINUX_MM_TYPES_H */
> diff --git a/kernel/fork.c b/kernel/fork.c
> index c4ada32598bd..b311caec6419 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1056,11 +1056,14 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
> mm_init_uprobes_state(mm);
> hugetlb_count_init(mm);
>
> + mm_flags_clear_all(mm);
> if (current->mm) {
> - mm->flags = mmf_init_flags(current->mm->flags);
> + unsigned long flags = __mm_flags_get_word(current->mm);
> +
> + __mm_flags_set_word(mm, mmf_init_legacy_flags(flags));
> mm->def_flags = current->mm->def_flags & VM_INIT_DEF_MASK;
> } else {
> - mm->flags = default_dump_filter;
> + __mm_flags_set_word(mm, default_dump_filter);
> mm->def_flags = 0;
> }
>
> --
> 2.50.1
>
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