linux-kernel - Re: [PATCH v2 2/3] x86/mm: Fix LAM inconsistency during context switch

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Date: Wed, 13 Mar 2024 15:38:46 +0200
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Yosry Ahmed <yosryahmed@...gle.com>
Cc: x86@...nel.org, Thomas Gleixner <tglx@...utronix.de>, 
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, "H. Peter Anvin" <hpa@...or.com>, 
	Andy Lutomirski <luto@...nel.org>, Peter Zijlstra <peterz@...radead.org>, 
	Rick Edgecombe <rick.p.edgecombe@...el.com>, Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/3] x86/mm: Fix LAM inconsistency during context
 switch

On Tue, Mar 12, 2024 at 03:56:40PM +0000, Yosry Ahmed wrote:
> LAM can only be enabled when a process is single-threaded.  But _kernel_
> threads can temporarily use a single-threaded process's mm.  That means
> that a context-switching kernel thread can race and observe the mm's LAM
> metadata (mm->context.lam_cr3_mask) change.
> 
> The context switch code does two logical things with that metadata:
> populate CR3 and populate 'cpu_tlbstate.lam'.  If it hits this race,
> 'cpu_tlbstate.lam' and CR3 can end up out of sync.
> 
> This de-synchronization is currently harmless.  But it is confusing and
> might lead to warnings or real bugs.
> 
> Update set_tlbstate_lam_mode() to take in the LAM mask and untag mask
> instead of an mm_struct pointer, and while we are at it, rename it to
> cpu_tlbstate_update_lam(). This should also make it clearer that we are
> updating cpu_tlbstate. In switch_mm_irqs_off(), read the LAM mask once
> and use it for both the cpu_tlbstate update and the CR3 update.
> 
> Signed-off-by: Yosry Ahmed <yosryahmed@...gle.com>

Reviewed-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>

-- 
  Kiryl Shutsemau / Kirill A. Shutemov