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

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Message-ID: <CAC_TJvdkVPcw+aKEjUOmTjwYfe8BevR51X_JPOo69hWSQ1MGcw@mail.gmail.com>
Date: Tue, 16 Sep 2025 10:36:57 -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 8:52 AM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Mon, 15 Sep 2025 18:19:53 -0700
> Kalesh Singh <kaleshsingh@...gle.com> wrote:
>
>
> > Hi Steve,
> >
> > Thanks for the comments and suggestion you are right we can use bpf to
> > get the comm. There is nothing special about this trace event.  I will
> > drop comm in the next revision.
> >
> > The reason I did the task_struct parameter (current): I believe there
> > is a limitation that we must  specify at least 1 parameter to the
> > TRACE_EVENT()  PROTO and ARGS macros.
>
> OK, then this is another issue. We don't want tracepoint "markers".
> Each tracepoint can take up to 5K in memory due to the code it
> generates and the meta data to control it.
>
> For something like that, we highly recommend dynamic probes (fprobes,
> kprobes, etc).
>
> The only purpose of a static tracepoint is to get data within a
> function that is too difficult to get via a probe. It should never be
> used as a trigger where its purpose is "we hit this path".

Hi Steve,

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).

Thanks,
Kalesh

>
> -- Steve