linux-kernel - Re: [PATCH v2 3/7] mm: introduce vma_count

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Message-ID: <c6eacb69-86f5-4bdb-9c6b-04e3f7ef7c29@redhat.com>
Date: Wed, 17 Sep 2025 15:38:35 +0200
From: David Hildenbrand <david@...hat.com>
To: Kalesh Singh <kaleshsingh@...gle.com>, akpm@...ux-foundation.org,
 minchan@...nel.org, lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com,
 rppt@...nel.org, pfalcato@...e.de
Cc: kernel-team@...roid.com, android-mm@...gle.com,
 Alexander Viro <viro@...iv.linux.org.uk>,
 Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
 Kees Cook <kees@...nel.org>, Vlastimil Babka <vbabka@...e.cz>,
 Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
 Steven Rostedt <rostedt@...dmis.org>, Masami Hiramatsu
 <mhiramat@...nel.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
 Juri Lelli <juri.lelli@...hat.com>,
 Vincent Guittot <vincent.guittot@...aro.org>,
 Dietmar Eggemann <dietmar.eggemann@....com>, Ben Segall
 <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
 Valentin Schneider <vschneid@...hat.com>, Jann Horn <jannh@...gle.com>,
 Shuah Khan <shuah@...nel.org>, linux-kernel@...r.kernel.org,
 linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
 linux-trace-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v2 3/7] mm: introduce vma_count_remaining()

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 <akpm@...ux-foundation.org>
> Cc: David Hildenbrand <david@...hat.com>
> Cc: "Liam R. Howlett" <Liam.Howlett@...cle.com>
> Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> Cc: Mike Rapoport <rppt@...nel.org>
> Cc: Minchan Kim <minchan@...nel.org>
> Cc: Pedro Falcato <pfalcato@...e.de>
> Signed-off-by: Kalesh Singh <kaleshsingh@...gle.com>
> ---

[...]

>   	/*
> @@ -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