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

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Message-ID: <CAC_TJvcXzrxhWFWyE7QEgPVmEJbnfT1W=s4TdgPGGAgCpn=8Og@mail.gmail.com>
Date: Wed, 17 Sep 2025 10:10:24 -0700
From: Kalesh Singh <kaleshsingh@...gle.com>
To: David Hildenbrand <david@...hat.com>
Cc: akpm@...ux-foundation.org, minchan@...nel.org, lorenzo.stoakes@...cle.com, 
	Liam.Howlett@...cle.com, rppt@...nel.org, pfalcato@...e.de, 
	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 Wed, Sep 17, 2025 at 6:38 AM David Hildenbrand <david@...hat.com> 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. ... >= 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.

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
>