linux-kernel - Re: [PATCH 2/2] x86/mm: clarify "prev" usage in switch_mm_irqs

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Date: Thu, 22 Feb 2024 18:43:30 +0000
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Dave Hansen <dave.hansen@...el.com>
Cc: Ingo Molnar <mingo@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, Andy Lutomirski <luto@...nel.org>, 
	Peter Zijlstra <peterz@...radead.org>, Andrew Morton <akpm@...ux-foundation.org>, x86@...nel.org, 
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] x86/mm: clarify "prev" usage in switch_mm_irqs_off()

On Thu, Feb 22, 2024 at 08:48:17AM -0800, Dave Hansen wrote:
> On 1/26/24 00:06, Yosry Ahmed wrote:
> > +/*
> > + * The "prev" argument passed by the caller does not always match CR3. For
> > + * example, the scheduler passes in active_mm when switching from lazy TLB mode
> > + * to normal mode, but switch_mm_irqs_off() can be called from x86 code without
> > + * updating active_mm. Use cpu_tlbstate.loaded_mm instead.
> > + */
> > +void switch_mm_irqs_off(struct mm_struct *unused, struct mm_struct *next,
> >  			struct task_struct *tsk)
> 
> One nit here: It's not obvious that "unused" is 'the "prev" argument'.
> 
> Would something like this be more clear?
> 
> /*
>  * This optimizes when not actually switching mm's.  Some architectures
>  * use the 'unused' argument for this optimization, but x86 must use
>  * 'cpu_tlbstate.loaded_mm' instead because it does not always keep
>  * ->active_mm up to date.
>  */

Yes, this is more clear, thanks! However, Andrew already merged that
patch into mm-stable, so it cannot be amended. I can send a separate
patch to rewrite the comment tho if you'd like, WDYT?

> 
> Also, I think it might be useful to have the rule that arch/x86 code
> _always_ calls switch_mm_irqs_off() with the first argument (the
> newly-named 'unused') set to NULL.  I think there's only one site:

Agreed. I can also send a separate patch for this. Thanks!

> 
> > void switch_mm(struct mm_struct *prev, struct mm_struct *next,
> >                struct task_struct *tsk)
> > {
> >         unsigned long flags;
> > 
> >         local_irq_save(flags);
> >         switch_mm_irqs_off(prev, next, tsk);
> >         local_irq_restore(flags);
> > }
>