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

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Message-ID: <88cb9996-0e9b-49ec-bc94-f816a5b64870@intel.com>
Date: Thu, 22 Feb 2024 08:48:17 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: Yosry Ahmed <yosryahmed@...gle.com>, Ingo Molnar <mingo@...hat.com>
Cc: 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 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.
 */

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:

> 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);
> }