linux-kernel - Re: [PATCH v2 0/4] initial work on making VMA flags a bitmap

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Message-ID: <39922fb9-d9e7-458d-9ebe-149ebbbcf1ef@lucifer.local>
Date: Fri, 21 Nov 2025 17:20:47 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        Muchun Song <muchun.song@...ux.dev>,
        Oscar Salvador <osalvador@...e.de>,
        David Hildenbrand <david@...hat.com>,
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Subject: Re: [PATCH v2 0/4] initial work on making VMA flags a bitmap

On Fri, Nov 21, 2025 at 03:50:53PM +0100, Vlastimil Babka wrote:
> On 11/14/25 14:26, Lorenzo Stoakes wrote:
> > We are in the rather silly situation that we are running out of VMA flags
> > as they are currently limited to a system word in size.
> >
> > This leads to absurd situations where we limit features to 64-bit
> > architectures only because we simply do not have the ability to add a flag
> > for 32-bit ones.
> >
> > This is very constraining and leads to hacks or, in the worst case, simply
> > an inability to implement features we want for entirely arbitrary reasons.
> >
> > This also of course gives us something of a Y2K type situation in mm where
> > we might eventually exhaust all of the VMA flags even on 64-bit systems.
> >
> > This series lays the groundwork for getting away from this limitation by
> > establishing VMA flags as a bitmap whose size we can increase in future
> > beyond 64 bits if required.
> >
> > This is necessarily a highly iterative process given the extensive use of
> > VMA flags throughout the kernel, so we start by performing basic steps.
> >
> > Firstly, we declare VMA flags by bit number rather than by value, retaining
> > the VM_xxx fields but in terms of these newly introduced VMA_xxx_BIT
> > fields.
> >
> > While we are here, we use sparse annotations to ensure that, when dealing
> > with VMA bit number parameters, we cannot be passed values which are not
> > declared as such - providing some useful type safety.
> >
> > We then introduce an opaque VMA flag type, much like the opaque mm_struct
> > flag type introduced in commit bb6525f2f8c4 ("mm: add bitmap mm->flags
> > field"), which we establish in union with vma->vm_flags (but still set at
> > system word size meaning there is no functional or data type size change).
> >
> > We update the vm_flags_xxx() helpers to use this new bitmap, introducing
> > sensible helpers to do so.
> >
> > This series lays the foundation for further work to expand the use of
> > bitmap VMA flags and eventually eliminate these arbitrary restrictions.
> >
> >
> > v2:
> > * Corrected kdoc for vma_flag_t.
> > * Introduced DECLARE_VMA_BIT() as per Jason. We can't also declare the VMA
> >   flags in the enum as this breaks assumptions in the kernel, resulting in
> >   errors like 'enum constant in boolean context
> >   [-Werror=int-in-bool-context]'.
> > * Dropped the conversion patch - To make life simpler this cycle, let's just
> >   fixup the flag declarations and introduce the new field type and introduce
> >   vm_flags_*() changes. We can do more later.
> > * Split out VMA testing vma->__vm_flags change.
> > * Fixed vma_flag_*_atomic() helper functions for sparse purposes to work
> >   with vma_flag_t.
> > * Fixed rust breakages as reported by Nico and help provided by Alice. For
> >   now we are doing a minimal fix, we can do a more substantial one once the
> >   VMA flag helper functions are introduced in an upcoming series.
> >
> > v1:
> > https://lore.kernel.org/all/cover.1761757731.git.lorenzo.stoakes@oracle.com/
> >
> > Lorenzo Stoakes (4):
> >   mm: declare VMA flags by bit
> >   mm: simplify and rename mm flags function for clarity
> >   tools/testing/vma: eliminate dependency on vma->__vm_flags
> >   mm: introduce VMA flags bitmap type
>
> Acked-by: Vlastimil Babka <vbabka@...e.cz>

Thanks!

>
> However something has happened to patch 4/4 in git, it has a very different
> tools/testing/vma/vma_internal.h:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git/commit/?h=mm-stable&id=c3f7c506e8f122a31b9cc01d234e7fcda46b0eca

Yeah something has gone very wrong here :)

3/4 also has an issue annoyingly, I tested it locally with no issues but
now it seems to have issues... but also it seems like it should _always_
have.

Anyway let me address each at a time I guess... :)

>
> >
> >  fs/proc/task_mmu.c               |   4 +-
> >  include/linux/mm.h               | 400 +++++++++++++++------------
> >  include/linux/mm_types.h         |  78 +++++-
> >  kernel/fork.c                    |   4 +-
> >  mm/khugepaged.c                  |   2 +-
> >  mm/madvise.c                     |   2 +-
> >  rust/bindings/bindings_helper.h  |  25 ++
> >  rust/kernel/mm/virt.rs           |   2 +-
> >  tools/testing/vma/vma.c          |  20 +-
> >  tools/testing/vma/vma_internal.h | 446 ++++++++++++++++++++++++++-----
> >  10 files changed, 716 insertions(+), 267 deletions(-)
> >
> > --
> > 2.51.0
>