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

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Message-ID: <3997965c-4cc6-405c-b6fe-a70b4e75afa6@redhat.com>
Date: Thu, 4 Sep 2025 12:14:43 +0200
From: David Hildenbrand <david@...hat.com>
To: Kalesh Singh <kaleshsingh@...gle.com>, akpm@...ux-foundation.org,
 minchan@...nel.org, lorenzo.stoakes@...cle.com
Cc: kernel-team@...roid.com, android-mm@...gle.com,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>, Vlastimil Babka
 <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
 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 04.09.25 01:24, 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.

It's not always a "will" right? At least I remember that this was the 
worst case scenario in some ("may split").

"The number of new VMAs the operation may create in the worst case.


-- 
Cheers

David / dhildenb