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Message-ID: <1743164d-c2d2-44a1-a2a9-aeeed8c13bc8@lucifer.local>
Date: Tue, 26 Aug 2025 15:32:07 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.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>, Zi Yan <ziy@...dia.com>,
Baolin Wang <baolin.wang@...ux.alibaba.com>,
"Liam R . Howlett" <Liam.Howlett@...cle.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
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 <lorenzo.stoakes@...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);
> > >
> > > 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), 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.
My solution works (unless you can see a reason it shouldn't) and I don't find
this approach any simpler.
>
> --
> Cheers
>
> David / dhildenb
>
Cheers, Lorenzo
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