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Howlett" , Vlastimil Babka , Suren Baghdasaryan , Michal Hocko , Jann Horn , Pedro Falcato , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm: centralize and fix max map count limit checking Message-ID: References: <20250903232437.1454293-1-kaleshsingh@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250903232437.1454293-1-kaleshsingh@google.com> X-Rspamd-Queue-Id: B3F69A000D X-Rspamd-Server: rspam04 X-Rspam-User: X-Stat-Signature: u5zppr3m9aizspjab654fent3imn4uzr X-HE-Tag: 1756970967-231272 X-HE-Meta: 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 +l8sulT8 n0frBaD4pr7TbYhGUmLfT6ycXSltn4v+LZNNEDTNduRl1iCJWQeax6x+pLJVLHbSBYL7KaLCIK8PzAMxnpvD/LREnOPiUby4wlhuFXo/hyXDNyW+e/OwkJ7ANEIWPfZvRXpYlJdAFC9SGFUuZCEY4KbFxRwuBJ5Ya6ust5xHXcoQNmryZKbm49RFPFeDGsNcnWJNU5xjS806ZB5trDAXNHJ5GMtbyESnmCPl1nb8dl17LQrhOAzpvT/84KxurrdOJ7XuOEzqqhAt7jecWAGVGq2E0A5rhAp2c9CWjskItI4EP/tee6aUwHZk0xAbjtMb/t7vctfGudWdg9cv/aU8x8YkF2JxEjIyymggjgfJYbFBqYQPm9zlrY1qVw0Ud8l2QNWqUo9K5/rzfBuWO9fq379rghnUJkjI8tN/2 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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 > Cc: Minchan Kim > Cc: Lorenzo Stoakes > Signed-off-by: Kalesh Singh > --- > 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:/ > + */ > +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.