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[2003:d8:2f27:6d00:7b96:afc9:83d0:5bd]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-46139122cb5sm36519695e9.8.2025.09.17.06.38.36 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 17 Sep 2025 06:38:38 -0700 (PDT) Message-ID: Date: Wed, 17 Sep 2025 15:38:35 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 3/7] mm: introduce vma_count_remaining() To: Kalesh Singh , akpm@linux-foundation.org, minchan@kernel.org, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, rppt@kernel.org, pfalcato@suse.de Cc: kernel-team@android.com, android-mm@google.com, Alexander Viro , Christian Brauner , Jan Kara , Kees Cook , Vlastimil Babka , Suren Baghdasaryan , Michal Hocko , Steven Rostedt , Masami Hiramatsu , Mathieu Desnoyers , Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Ben Segall , Mel Gorman , Valentin Schneider , Jann Horn , Shuah Khan , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-trace-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org References: <20250915163838.631445-1-kaleshsingh@google.com> <20250915163838.631445-4-kaleshsingh@google.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 158C480006 X-Rspam-User: X-Rspamd-Server: rspam07 X-Stat-Signature: 9tsxmokacexigr3yhdon1q8139mxtzpn X-HE-Tag: 1758116326-112778 X-HE-Meta: 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 jFzrRwY/ 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 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 15.09.25 18:36, 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. s/givine/given/ s/process/mm/ > > 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) > > 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 > Cc: David Hildenbrand > Cc: "Liam R. Howlett" > Cc: Lorenzo Stoakes > Cc: Mike Rapoport > Cc: Minchan Kim > Cc: Pedro Falcato > Signed-off-by: Kalesh Singh > --- [...] > /* > @@ -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; If we worry about rare races (sysctl_max_map_count changing?) we should probably force a single read through READ_ONCE()? Otherwise one might trick vma_count_remaining() into returning a negative number I assume. > + > + return (max_count > map_count) ? (max_count - map_count) : 0; > +} Nothing else jumped at me. Not sure what the buildbot complains about but I'm sure you'll figure it out :) -- Cheers David / dhildenb