Message-ID: <d01001a5-03b5-4662-8955-bbade1e2f023@suse.cz>
Date: Fri, 21 Nov 2025 15:50:53 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Andrew Morton <akpm@...ux-foundation.org>
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Subject: Re: [PATCH v2 0/4] initial work on making VMA flags a bitmap

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>

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

> 
>  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

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