linux-kernel - Re: [PATCH] mm: centralize and fix max map count limit checking

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Message-ID: <CAC_TJvciKbEOyBv_V01s8nJF8MMKznKq-Gw6+Shv9AAppJ46+w@mail.gmail.com>
Date: Thu, 4 Sep 2025 09:20:34 -0700
From: Kalesh Singh <kaleshsingh@...gle.com>
To: Mike Rapoport <rppt@...nel.org>
Cc: akpm@...ux-foundation.org, minchan@...nel.org, lorenzo.stoakes@...cle.com, 
	kernel-team@...roid.com, android-mm@...gle.com, 
	David Hildenbrand <david@...hat.com>, "Liam R. Howlett" <Liam.Howlett@...cle.com>, 
	Vlastimil Babka <vbabka@...e.cz>, Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>, 
	Jann Horn <jannh@...gle.com>, Pedro Falcato <pfalcato@...e.de>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: centralize and fix max map count limit checking

On Thu, Sep 4, 2025 at 12:29 AM Mike Rapoport <rppt@...nel.org> wrote:
>
> On Wed, Sep 03, 2025 at 04:24:35PM -0700, Kalesh Singh wrote:
> > The check against the max map count (sysctl_max_map_count) was
> > open-coded in several places. This led to inconsistent enforcement
> > and subtle bugs where the limit could be exceeded.
> >
> > For example, some paths would check map_count > sysctl_max_map_count
> > before allocating a new VMA and incrementing the count, allowing the
> > process to reach sysctl_max_map_count + 1:
> >
> >     int do_brk_flags(...)
> >     {
> >         if (mm->map_count > sysctl_max_map_count)
> >             return -ENOMEM;
> >
> >         /* We can get here with mm->map_count == sysctl_max_map_count */
> >
> >         vma = vm_area_alloc(mm);
> >         ...
> >         mm->map_count++   /* We've now exceeded the threshold. */
> >     }
> >
> > To fix this and unify the logic, introduce a new function,
> > exceeds_max_map_count(), to consolidate the check. All open-coded
> > checks are replaced with calls to this new function, ensuring the
> > limit is applied uniformly and correctly.
> >
> > To improve encapsulation, sysctl_max_map_count is now static to
> > mm/mmap.c. The new helper also adds a rate-limited warning to make
> > debugging applications that exhaust their VMA limit easier.
> >
> > Cc: Andrew Morton <akpm@...ux-foundation.org>
> > Cc: Minchan Kim <minchan@...nel.org>
> > Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> > Signed-off-by: Kalesh Singh <kaleshsingh@...gle.com>
> > ---
> >  include/linux/mm.h | 11 ++++++++++-
> >  mm/mmap.c          | 15 ++++++++++++++-
> >  mm/mremap.c        |  7 ++++---
> >  mm/nommu.c         |  2 +-
> >  mm/util.c          |  1 -
> >  mm/vma.c           |  6 +++---
> >  6 files changed, 32 insertions(+), 10 deletions(-)
> >
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 1ae97a0b8ec7..d4e64e6a9814 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -192,7 +192,16 @@ 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;
> > +/**
> > + * exceeds_max_map_count - check if a VMA operation would exceed max_map_count
> > + * @mm: The memory descriptor for the process.
> > + * @new_vmas: The number of new VMAs the operation will create.
> > + *
> > + * Returns true if the operation would cause the number of VMAs to exceed
> > + * the sysctl_max_map_count limit, false otherwise. A rate-limited warning
> > + * is logged if the limit is exceeded.
>
> kernel-doc will be unhappy because of missing Return: description. Please
> s/Returns/Return:/

Hi Mike, thanks for catching this. I'll fix it in the next version.

-- Kalesh

>
> > + */
> > +extern bool exceeds_max_map_count(struct mm_struct *mm, unsigned int new_vmas);
>
> No need for 'extern' here. And the declaration should go to mm/internal.h
>
> >  extern unsigned long sysctl_user_reserve_kbytes;
> >  extern unsigned long sysctl_admin_reserve_kbytes;
>
> --
> Sincerely yours,
> Mike.