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AJvYcCVInGDTvzqMKjntavHnmEpjeQnhTrCJPcIqG0zwNUYVPJ8+rFe40IdZiNpAZGVcXjUNlE+WAAUnEQ==@kvack.org X-Gm-Message-State: AOJu0YzHz8TaVBarttdF1og2DX6Q/ahXgFqHAwAVhFJtQrYM7mQq950t aWjPhFomleNfSe2YuriraZNUspeCVZg2zKc5OsSeQ5olB5fb1V2wp0cz1+S3cj55IrG5AOTpSP1 UCDo076In+/tUwOJgzlhhtLsD+vy/Uj7D1uuf+hU5 X-Gm-Gg: ASbGncuuNUIBCb1McEvSA5KE4VIajhAwZQyd3DoJAZav1tSrpmwY/XRw8CDHyM23BuU +14SLlg7WRxmEv+HtYb+lJ1QfcI4yWtqZRO4+34DNrkil4ppAGmJlPJJSR9hyQ9mXXcuXu/wRJt JI4zjiT842hGI898scjK9fVGLNOfGZOigVAlFB/e6Y9jIMRuIQyVBMonHcfOxqTeuqUlxZL1dR7 zC5a2hYABj9mvUYtPd/39nzEJ2RKbsLiwDOBwr5tVNe X-Google-Smtp-Source: AGHT+IFd4BAxZT/ufICRk+rEAu4Df/3LlVy7dl8wb8UYvf1Vkm7Wii7RkWbyYEbSSxpnLkv8kfjaHMhL5z1X210qqcg= X-Received: by 2002:a17:902:d2c9:b0:266:b8a2:f605 with SMTP id d9443c01a7336-26808a2fc00mr4248005ad.3.1758129035969; Wed, 17 Sep 2025 10:10:35 -0700 (PDT) MIME-Version: 1.0 References: <20250915163838.631445-1-kaleshsingh@google.com> <20250915163838.631445-4-kaleshsingh@google.com> In-Reply-To: From: Kalesh Singh Date: Wed, 17 Sep 2025 10:10:24 -0700 X-Gm-Features: AS18NWB7v1zd5wRtV4XjcNUI2Kl7h_0nJ3bBXH5VRcSHiB0MqTNwu-jJe9fGDsw Message-ID: Subject: Re: [PATCH v2 3/7] mm: introduce vma_count_remaining() To: David Hildenbrand Cc: akpm@linux-foundation.org, minchan@kernel.org, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, rppt@kernel.org, pfalcato@suse.de, 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 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Queue-Id: 87429120007 X-Stat-Signature: on1petzgn1579eyr8k5bqyrf7wfbqnn9 X-Rspam-User: X-Rspamd-Server: rspam01 X-HE-Tag: 1758129037-565166 X-HE-Meta: U2FsdGVkX18sMOKRrjFcc+rhPi+kg9djTdrYxuJfTAXPVdo/g3BuE3JzcVqehtUzgtaN3NuRJDdcXCLRupTIYQFKpLCm0lvuBJl6uuhnLIyL/WCCRCY0vP+xm2Pc1dxsTCNDqc0fid9Avham6pKPoEpjXssV1SyQRLHNIZ5Jw1lpKDlM4Unt3dQa1/POhUE58oiXkJrauEqfdi6frZqo3npY6D8mUIMDd1KMWXa32n+SQF8gyIkMztFo/5YI2c3oGcsrmvgDcnhBYOZHFE1EbyI0tedNXmOd2je3x6CUuaLm8kQwdt5MIw0DaQ+GTnmlyn9HEU8tEAVwUXZpv24mXGOeM0Yl9iIRn+q0bdC32Ybl5z6Nd/eRa82UylKIM5NvrsmDjp+3tajeiInxfgumtRjW3FrkcqzMlvXT53Wewgvo/8ZGBtKwrncvNVaAv070ZQXuH62uhTDYuOAFKGfyGVAEaJXhJoZ88QEncOIkWN0NfRHw6yvhanZFGtCq8HRk8D/z6CIWcscMBpvpItRfnX+jph9eYU1oNyfTTwkgZoNfJHzMxd0I2uw4kvdsVwt26TzSuOzq4UaU17LARZYcj3v08kQDszwGOjIvrYP0gy2GBqIGNrWqISmUnZMKmi1EMYRkPrBQAx27/pc13gJj7/mHcIK0jGYQrHdZR/v84mHwji40JGIAP/uI60JjuLrVtJ0D1JKrpE6TffJz39tcl6n6EucPlDhi2vfNgcdBU8koqmd5cL5zx9YzeARfIyg8VSVw/iRMH1JGrekCPmkN4j1O21vAt8VXn7nEJpYtUiIfplwJcZ9E4UInhyZ3Ny3RQ0JN0/Ukggee3qjIwOW2wJWI33Zk1J0jZl1z9hvaBXe+woUyjzu8HauLDJYVIrwQF1AEqX6xlE7JU3tlUtxo9yhQEE4l+jc5AdPWM4ROllIha4EnAj/Y/7+vf4RmuhS7qUNm7R5OZg9NCMYKNjn SYvcQMtR +b0N88SCQ+8IMlQXkGa6Yvq/d+8vvIiQveVQ6YnCeTGQhmC9p3C73d/xXKF4rRtL/Ih+4dTb2dKloSrQqroH6Zwwhsk55s2WwIbVbHWyse+n1LXqAZ1J/JkoelTYbeiMMw7HTXHOaYI3xH6tz+3KKeSwmUpR7IYeV/1uy3/FrFi2nTUne5kaePFO3XigmB96kzuHWeTy/ExuwGX5b378b5SqAzy8XWXbsTUN4P6n5YgD8nLlmpL8vpedx2CY0wHJbItZEl+Xwjkcp27UcUdFZV8jsEQEtE3xwuMsUdul+OvKbWnFMKOK2TJd9+gbvtT6z8GYWTEkHR5p1jzzOZVhkZ8vvixocVnAaB6Ti42un3TNW7PeinB6HZBX572CnM4uGcJHA/RuWNbw7tDA= 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 17, 2025 at 6:38=E2=80=AFAM David Hildenbrand wrote: > > 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. ... >=3D 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 '<=3D' comparison), the improved readability at the ca= ll > > 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 =3D DEFAULT_MAX_MAP_COUN= T; > > + > > +/** > > + * 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 =3D mm->map_count; > > + const int max_count =3D 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. Agreed, I'll add the READ_ONCE when resending. Thanks, Kalesh > > > + > > + 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 >