Re: [PATCH v2 3/7] mm: introduce vma_count_remaining()

[Kalesh Singh <kaleshsingh@google.com>]

On Thu, Sep 18, 2025 at 7:32 AM Lorenzo Stoakes
<lorenzo.stoakes@oracle.com> wrote:
>
> On Mon, Sep 15, 2025 at 09:36:34AM -0700, Kalesh Singh wrote:
> > The checks against sysctl_max_map_count are open-coded in multiple
> > places. While simple checks are manageable, the logic in places like
> > mremap.c involves arithmetic with magic numbers that can be difficult
> > to reason about. e.g. ... >= sysctl_max_map_count - 3
> >
> > To improve readability and centralize the logic, introduce a new helper,
> > vma_count_remaining(). This function returns the VMA count headroom
> > available for a givine process.
> >
> > The most common case of checking for a single new VMA can be done with
> > the convenience helper has_vma_count_remaining():
> >
> >     if (!vma_count_remaining(mm))
> >
> > And the complex checks in mremap.c become clearer by expressing the
> > required capacity directly:
> >
> >     if (vma_count_remaining(mm) <  4)
>
> Double space after 4.

Will fix in the resend.

>
> >
> > While a capacity-based function could be misused (e.g., with an
> > incorrect '<' vs '<=' comparison), the improved readability at the call
> > sites makes such errors less likely than with the previous open-coded
> > arithmetic.
> >
> > As part of this change, sysctl_max_map_count is made static to
> > mm/mmap.c to improve encapsulation.
> >
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: David Hildenbrand <david@redhat.com>
> > Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
> > Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> > Cc: Mike Rapoport <rppt@kernel.org>
> > Cc: Minchan Kim <minchan@kernel.org>
> > Cc: Pedro Falcato <pfalcato@suse.de>
> > Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
>
> Generally logic looks ok, with various stuff fixed below + in commit msg we
> should be good to go :)
>
> > ---
> >
> > Changes in v2:
> >   - Fix documentation comment for vma_count_remaining(), per Mike
> >   - Remove extern in header, per Mike and Pedro
> >   - Move declaration to mm/internal.h, per Mike
> >   - Replace exceeds_max_map_count() with capacity-based vma_count_remaining(),
> >        per Lorenzo.
> >   - Fix tools/testing/vma, per Lorenzo.
> >
> >  include/linux/mm.h               |  2 --
> >  mm/internal.h                    |  2 ++
> >  mm/mmap.c                        | 21 ++++++++++++++++++++-
> >  mm/mremap.c                      |  7 ++++---
> >  mm/nommu.c                       |  2 +-
> >  mm/util.c                        |  1 -
> >  mm/vma.c                         | 10 +++++-----
> >  tools/testing/vma/vma_internal.h |  9 +++++++++
> >  8 files changed, 41 insertions(+), 13 deletions(-)
> >
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 1ae97a0b8ec7..138bab2988f8 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -192,8 +192,6 @@ static inline void __mm_zero_struct_page(struct page *page)
> >  #define MAPCOUNT_ELF_CORE_MARGIN     (5)
> >  #define DEFAULT_MAX_MAP_COUNT        (USHRT_MAX - MAPCOUNT_ELF_CORE_MARGIN)
> >
> > -extern int sysctl_max_map_count;
> > -
>
> Nice to get rid of this as a global! :)
>
> >  extern unsigned long sysctl_user_reserve_kbytes;
> >  extern unsigned long sysctl_admin_reserve_kbytes;
> >
> > diff --git a/mm/internal.h b/mm/internal.h
> > index 45b725c3dc03..39f1c9535ae5 100644
> > --- a/mm/internal.h
> > +++ b/mm/internal.h
> > @@ -1661,4 +1661,6 @@ static inline bool reclaim_pt_is_enabled(unsigned long start, unsigned long end,
> >  void dup_mm_exe_file(struct mm_struct *mm, struct mm_struct *oldmm);
> >  int dup_mmap(struct mm_struct *mm, struct mm_struct *oldmm);
> >
> > +int vma_count_remaining(const struct mm_struct *mm);
> > +
> >  #endif       /* __MM_INTERNAL_H */
> > diff --git a/mm/mmap.c b/mm/mmap.c
> > index e5370e7fcd8f..af88ce1fbb5f 100644
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -374,7 +374,7 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
> >               return -EOVERFLOW;
> >
> >       /* Too many mappings? */
> > -     if (mm->map_count >= sysctl_max_map_count)
> > +     if (!vma_count_remaining(mm))
> >               return -ENOMEM;
> >
> >       /*
> > @@ -1504,6 +1504,25 @@ struct vm_area_struct *_install_special_mapping(
> >  int sysctl_legacy_va_layout;
> >  #endif
> >
> > +static int sysctl_max_map_count __read_mostly = DEFAULT_MAX_MAP_COUNT;
> > +
> > +/**
> > + * vma_count_remaining - Determine available VMA slots
> > + * @mm: The memory descriptor for the process.
> > + *
> > + * Check how many more VMAs can be created for the given @mm
> > + * before hitting the sysctl_max_map_count limit.
> > + *
> > + * Return: The number of new VMAs the process can accommodate.
> > + */
> > +int vma_count_remaining(const struct mm_struct *mm)
> > +{
> > +     const int map_count = mm->map_count;
> > +     const int max_count = sysctl_max_map_count;
>
> David already commented on the READ_ONCE() here, seems wise.
>
> > +
> > +     return (max_count > map_count) ? (max_count - map_count) : 0;
>
> Not a big deal but would prefer:
>
>         if (map_count >= map_count)
>                 return 0;
>
>         return max_count - map_count;
>
> As the ternary here is a bit less clear, and it puts the 'failure' case first
> and the 'success' case afterwards.
>
> > +}
> > +
>
> As discussed in reply to the kernel bot, you've accidentally placed this in a
> CONFIG_SYSCTL block, so need to move it :>)

Yeah I unintentionally placed it there, I've moved this out in a newer
version. Thanks.

>
> >  static const struct ctl_table mmap_table[] = {
> >               {
> >                               .procname       = "max_map_count",
> > diff --git a/mm/mremap.c b/mm/mremap.c
> > index 35de0a7b910e..14d35d87e89b 100644
> > --- a/mm/mremap.c
> > +++ b/mm/mremap.c
> > @@ -1040,7 +1040,7 @@ static unsigned long prep_move_vma(struct vma_remap_struct *vrm)
> >        * We'd prefer to avoid failure later on in do_munmap:
> >        * which may split one vma into three before unmapping.
> >        */
> > -     if (current->mm->map_count >= sysctl_max_map_count - 3)
> > +     if (vma_count_remaining(current->mm) < 4)
> >               return -ENOMEM;
>
> This is much clearer.
>
> >
> >       if (vma->vm_ops && vma->vm_ops->may_split) {
> > @@ -1814,9 +1814,10 @@ static unsigned long check_mremap_params(struct vma_remap_struct *vrm)
> >        * split in 3 before unmapping it.
> >        * That means 2 more maps (1 for each) to the ones we already hold.
> >        * Check whether current map count plus 2 still leads us to 4 maps below
> > -      * the threshold, otherwise return -ENOMEM here to be more safe.
> > +      * the threshold. In other words, is the current map count + 6 at or
> > +      * below the threshold? Otherwise return -ENOMEM here to be more safe.
> >        */
> > -     if ((current->mm->map_count + 2) >= sysctl_max_map_count - 3)
> > +     if (vma_count_remaining(current->mm) < 6)
> >               return -ENOMEM;
>
> I hate that we do this silly check here, but the time to revisit it is another
> series...

Agreed, I also wondered if this check is more conservative than it
needs to be in all cases.

-- Kalesh

>
> >
> >       return 0;
> > diff --git a/mm/nommu.c b/mm/nommu.c
> > index 8b819fafd57b..dd75f2334812 100644
> > --- a/mm/nommu.c
> > +++ b/mm/nommu.c
> > @@ -1316,7 +1316,7 @@ static int split_vma(struct vma_iterator *vmi, struct vm_area_struct *vma,
> >               return -ENOMEM;
> >
> >       mm = vma->vm_mm;
> > -     if (mm->map_count >= sysctl_max_map_count)
> > +     if (!vma_count_remaining(mm))
> >               return -ENOMEM;
> >
> >       region = kmem_cache_alloc(vm_region_jar, GFP_KERNEL);
> > diff --git a/mm/util.c b/mm/util.c
> > index f814e6a59ab1..b6e83922cafe 100644
> > --- a/mm/util.c
> > +++ b/mm/util.c
> > @@ -751,7 +751,6 @@ EXPORT_SYMBOL(folio_mc_copy);
> >  int sysctl_overcommit_memory __read_mostly = OVERCOMMIT_GUESS;
> >  static int sysctl_overcommit_ratio __read_mostly = 50;
> >  static unsigned long sysctl_overcommit_kbytes __read_mostly;
> > -int sysctl_max_map_count __read_mostly = DEFAULT_MAX_MAP_COUNT;
> >  unsigned long sysctl_user_reserve_kbytes __read_mostly = 1UL << 17; /* 128MB */
> >  unsigned long sysctl_admin_reserve_kbytes __read_mostly = 1UL << 13; /* 8MB */
> >
> > diff --git a/mm/vma.c b/mm/vma.c
> > index 033a388bc4b1..df0e8409f63d 100644
> > --- a/mm/vma.c
> > +++ b/mm/vma.c
> > @@ -491,8 +491,8 @@ void unmap_region(struct ma_state *mas, struct vm_area_struct *vma,
> >  }
> >
> >  /*
> > - * __split_vma() bypasses sysctl_max_map_count checking.  We use this where it
> > - * has already been checked or doesn't make sense to fail.
> > + * __split_vma() bypasses vma_count_remaining() checks. We use this where
> > + * it has already been checked or doesn't make sense to fail.
> >   * VMA Iterator will point to the original VMA.
> >   */
> >  static __must_check int
> > @@ -592,7 +592,7 @@ __split_vma(struct vma_iterator *vmi, struct vm_area_struct *vma,
> >  static int split_vma(struct vma_iterator *vmi, struct vm_area_struct *vma,
> >                    unsigned long addr, int new_below)
> >  {
> > -     if (vma->vm_mm->map_count >= sysctl_max_map_count)
> > +     if (!vma_count_remaining(vma->vm_mm))
> >               return -ENOMEM;
> >
> >       return __split_vma(vmi, vma, addr, new_below);
> > @@ -1345,7 +1345,7 @@ static int vms_gather_munmap_vmas(struct vma_munmap_struct *vms,
> >                * its limit temporarily, to help free resources as expected.
> >                */
> >               if (vms->end < vms->vma->vm_end &&
> > -                 vms->vma->vm_mm->map_count >= sysctl_max_map_count) {
> > +                 !vma_count_remaining(vms->vma->vm_mm)) {
> >                       error = -ENOMEM;
> >                       goto map_count_exceeded;
> >               }
> > @@ -2772,7 +2772,7 @@ int do_brk_flags(struct vma_iterator *vmi, struct vm_area_struct *vma,
> >       if (!may_expand_vm(mm, vm_flags, len >> PAGE_SHIFT))
> >               return -ENOMEM;
> >
> > -     if (mm->map_count >= sysctl_max_map_count)
> > +     if (!vma_count_remaining(mm))
> >               return -ENOMEM;
> >
> >       if (security_vm_enough_memory_mm(mm, len >> PAGE_SHIFT))
> > diff --git a/tools/testing/vma/vma_internal.h b/tools/testing/vma/vma_internal.h
> > index 3639aa8dd2b0..52cd7ddc73f4 100644
> > --- a/tools/testing/vma/vma_internal.h
> > +++ b/tools/testing/vma/vma_internal.h
> > @@ -1517,4 +1517,13 @@ static inline vm_flags_t ksm_vma_flags(const struct mm_struct *, const struct fi
> >       return vm_flags;
> >  }
> >
> > +/* Helper to get VMA count capacity */
> > +static int vma_count_remaining(const struct mm_struct *mm)
> > +{
> > +     const int map_count = mm->map_count;
> > +     const int max_count = sysctl_max_map_count;
> > +
> > +     return (max_count > map_count) ? (max_count - map_count) : 0;
> > +}
> > +
> >  #endif       /* __MM_VMA_INTERNAL_H */
> > --
> > 2.51.0.384.g4c02a37b29-goog
> >