linux-kernel - Re: [PATCH 7/8] kernel/fork: Only cache the VMAP stack in finish_task

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Date:   Mon, 14 Feb 2022 13:10:04 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-ia64@...r.kernel.org,
        Ben Segall <bsegall@...gle.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [PATCH 7/8] kernel/fork: Only cache the VMAP stack in
 finish_task_switch().

On 2022-02-11 15:55:01 [-0800], Andy Lutomirski wrote:
> > Set the lowest bit of task_struct::stack if the stack was released via
> > put_task_stack_sched() and needs a final free in
> > delayed_put_task_struct(). If the bit is missing then a reference is
> > held and put_task_stack() will release it.
> 
> I don't understand what this bit is for or why the logic needs to be this
> complicated.  Can you set ->stack to NULL if and only if you freed it early?

What do I do if put_task_stack() is invoked from finish_task_switch()
and I can't free but have to do something?

> > +static void free_thread_stack(struct task_struct *tsk, bool cache_only)
> 
> This is messy.  Please clean it up for real:
> 
> static bool try_release_thread_stack_to_cache(struct vm_struct *vm)
> {
>   for (...) try to put it in this slot;
> }
> 
> And the callers can do things like:
> 
> if (try_release_thread_stack_to_cache(...))
>   return;
> 
> /* need to free for real */
> free it or delayed-free it.

I think I could use the first few bytes of the stack as a RCU-head. Let
me try that.

> --Andy

Sebastian