linux-kernel - Re: [PATCH v2 7/7] mm/tracing: introduce max_vma_count

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Message-ID: <CAC_TJvc6aqjBRZ05wyGb49AU+-aKpSph=ZSk3fdV2xraXi-_nQ@mail.gmail.com>
Date: Tue, 16 Sep 2025 10:57:43 -0700
From: Kalesh Singh <kaleshsingh@...gle.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: akpm@...ux-foundation.org, minchan@...nel.org, lorenzo.stoakes@...cle.com, 
	david@...hat.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>, 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 7/7] mm/tracing: introduce max_vma_count_exceeded trace event

On Tue, Sep 16, 2025 at 10:47 AM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Tue, 16 Sep 2025 10:36:57 -0700
> Kalesh Singh <kaleshsingh@...gle.com> wrote:
>
> > I completely agree with the principle that static tracepoints
> > shouldn't be used as markers if a dynamic probe will suffice. The
> > intent here is to avoid introducing overhead in the common case to
> > avoid regressing mmap, munmap, and other syscall latencies; while
> > still providing observability for the max vma_count exceeded failure
> > condition.
> >
> > The original centralized check (before previous review rounds) was
> > indeed in a dedicated function, exceeds_max_map_count(), where a
> > kprobe/fprobe could have been easily attached without impacting the
> > common path. This was changed due to previous review feedback to the
> > capacity based vma_count_remaining() which necessitated the check to
> > be done externally by the callers:
> >
> > https://lore.kernel.org/r/20250903232437.1454293-1-kaleshsingh@google.com/
> >
> > Would you be ok with something like:
> >
> > trace_max_vma_count_exceeded(mm);
> >
> > TP_STRUCT__entry(
> > __field(unsigned int, mm_id)
> > __field(unsigned int vma_count)
> > )
> >
> > mm_id would be the hash of the mm_struct ptr similar to rss_stat and
> > the vma_count is the current vma count (some syscalls have different
> > requirements on the capacity remaining: mremap requires 6 available
> > slots, other syscalls require 1).
> >
>
> BTW, why the hash of the mm pointer and not the pointer itself? We save
> pointers in lots of places, and if it is the pointer, you could use an
> eprobe to attache to the trace event to dereference its fields.

In Android we try to avoid exposing raw kernel pointers to userspace
for security reasons: raising /proc/sys/kernel/kptr_restrict to 2
immediately after symbols are resolved for necessary telemetry tooling
during early boot. I believe this is also why rss_stat uses the hash
and not the raw pointer.

Thanks,
Kalesh

>
> -- Steve
>
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