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Message-ID: <sukrlmrtkx4cc6jgwqgysjekl266hgkwvxxknt2lykcfd2ifgb@p27adyukistx>
Date: Tue, 12 Aug 2025 12:20:15 -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 01/10] mm: add bitmap mm->flags field
* Lorenzo Stoakes <lorenzo.stoakes@...cle.com> [250812 11:47]:
> We are currently in the bizarre situation where we are constrained on the
> number of flags we can set in an mm_struct based on whether this is a
> 32-bit or 64-bit kernel.
>
> This is because mm->flags is an unsigned long field, which is 32-bits on a
> 32-bit system and 64-bits on a 64-bit system.
>
> In order to keep things functional across both architectures, we do not
> permit mm flag bits to be set above flag 31 (i.e. the 32nd bit).
>
> This is a silly situation, especially given how profligate we are in
> storing metadata in mm_struct, so let's convert mm->flags into a bitmap and
> allow ourselves as many bits as we like.
>
> To keep things manageable, firstly we introduce the bitmap at a system word
> system as a new field mm->_flags, in union.
>
> This means the new bitmap mm->_flags is bitwise exactly identical to the
> existing mm->flags field.
>
> We have an opportunity to also introduce some type safety here, so let's
> wrap the mm flags field as a struct and declare it as an mm_flags_t typedef
> to keep it consistent with vm_flags_t for VMAs.
>
> We make the internal field privately accessible, in order to force the use
> of helper functions so we can enforce that accesses are bitwise as
> required.
>
> We therefore introduce accessors prefixed with mm_flags_*() for callers to
> use. We place the bit parameter first so as to match the parameter ordering
> of the *_bit() functions.
>
> Having this temporary union arrangement allows us to incrementally swap
> over users of mm->flags patch-by-patch rather than having to do everything
> in one fell swoop.
>
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@...cle.com>
> ---
> include/linux/mm.h | 32 ++++++++++++++++++++++++++++++++
> include/linux/mm_types.h | 39 ++++++++++++++++++++++++++++++++++++++-
> 2 files changed, 70 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 3868ca1a25f9..4ed4a0b9dad6 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -34,6 +34,8 @@
> #include <linux/slab.h>
> #include <linux/cacheinfo.h>
> #include <linux/rcuwait.h>
> +#include <linux/bitmap.h>
> +#include <linux/bitops.h>
>
> struct mempolicy;
> struct anon_vma;
> @@ -720,6 +722,36 @@ static inline void assert_fault_locked(struct vm_fault *vmf)
> }
> #endif /* CONFIG_PER_VMA_LOCK */
>
> +static inline bool mm_flags_test(int flag, const struct mm_struct *mm)
> +{
> + return test_bit(flag, ACCESS_PRIVATE(&mm->_flags, __mm_flags));
> +}
> +
> +static inline bool mm_flags_test_and_set(int flag, struct mm_struct *mm)
> +{
> + return test_and_set_bit(flag, ACCESS_PRIVATE(&mm->_flags, __mm_flags));
> +}
> +
> +static inline bool mm_flags_test_and_clear(int flag, struct mm_struct *mm)
> +{
> + return test_and_clear_bit(flag, ACCESS_PRIVATE(&mm->_flags, __mm_flags));
> +}
> +
> +static inline void mm_flags_set(int flag, struct mm_struct *mm)
> +{
> + set_bit(flag, ACCESS_PRIVATE(&mm->_flags, __mm_flags));
> +}
> +
> +static inline void mm_flags_clear(int flag, struct mm_struct *mm)
> +{
> + clear_bit(flag, ACCESS_PRIVATE(&mm->_flags, __mm_flags));
> +}
> +
> +static inline void mm_flags_clear_all(struct mm_struct *mm)
> +{
> + bitmap_zero(ACCESS_PRIVATE(&mm->_flags, __mm_flags), NUM_MM_FLAG_BITS);
> +}
> +
> extern const struct vm_operations_struct vma_dummy_vm_ops;
>
> static inline void vma_init(struct vm_area_struct *vma, struct mm_struct *mm)
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index cf94df4955c7..46d3fb8935c7 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -20,6 +20,7 @@
> #include <linux/seqlock.h>
> #include <linux/percpu_counter.h>
> #include <linux/types.h>
> +#include <linux/bitmap.h>
>
> #include <asm/mmu.h>
>
> @@ -927,6 +928,15 @@ struct mm_cid {
> };
> #endif
>
> +/*
> + * Opaque type representing current mm_struct flag state. Must be accessed via
> + * mm_flags_xxx() helper functions.
> + */
> +#define NUM_MM_FLAG_BITS BITS_PER_LONG
> +typedef struct {
> + __private DECLARE_BITMAP(__mm_flags, NUM_MM_FLAG_BITS);
> +} mm_flags_t;
> +
> struct kioctx_table;
> struct iommu_mm_data;
> struct mm_struct {
> @@ -1109,7 +1119,11 @@ struct mm_struct {
> /* Architecture-specific MM context */
> mm_context_t context;
>
> - unsigned long flags; /* Must use atomic bitops to access */
> + /* Temporary union while we convert users to mm_flags_t. */
> + union {
> + unsigned long flags; /* Must use atomic bitops to access */
> + mm_flags_t _flags; /* Must use mm_flags_* helpers to access */
> + };
>
> #ifdef CONFIG_AIO
> spinlock_t ioctx_lock;
> @@ -1219,6 +1233,29 @@ struct mm_struct {
> unsigned long cpu_bitmap[];
> };
>
> +/* Read the first system word of mm flags, non-atomically. */
> +static inline unsigned long __mm_flags_get_word(struct mm_struct *mm)
> +{
> + unsigned long *bitmap = ACCESS_PRIVATE(&mm->_flags, __mm_flags);
> +
> + return bitmap_read(bitmap, 0, BITS_PER_LONG);
> +}
> +
> +/* Set the first system word of mm flags, non-atomically. */
> +static inline void __mm_flags_set_word(struct mm_struct *mm,
> + unsigned long value)
> +{
> + unsigned long *bitmap = ACCESS_PRIVATE(&mm->_flags, __mm_flags);
> +
> + bitmap_copy(bitmap, &value, BITS_PER_LONG);
> +}
> +
> +/* Obtain a read-only view of the bitmap. */
> +static inline const unsigned long *__mm_flags_get_bitmap(const struct mm_struct *mm)
> +{
> + return (const unsigned long *)ACCESS_PRIVATE(&mm->_flags, __mm_flags);
> +}
> +
> #define MM_MT_FLAGS (MT_FLAGS_ALLOC_RANGE | MT_FLAGS_LOCK_EXTERN | \
> MT_FLAGS_USE_RCU)
> extern struct mm_struct init_mm;
> --
> 2.50.1
>
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