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Date:   Fri, 18 Nov 2022 11:32:54 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Dave Hansen <dave.hansen@intel.com>
Cc:     Marco Elver <elver@google.com>,
        Naresh Kamboju <naresh.kamboju@linaro.org>,
        kasan-dev <kasan-dev@googlegroups.com>, X86 ML <x86@kernel.org>,
        open list <linux-kernel@vger.kernel.org>,
        linux-mm <linux-mm@kvack.org>, regressions@lists.linux.dev,
        lkft-triage@lists.linaro.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexander Potapenko <glider@google.com>
Subject: Re: WARNING: CPU: 0 PID: 0 at arch/x86/include/asm/kfence.h:46
 kfence_protect
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On Thu, Nov 17, 2022 at 03:54:21PM -0800, Dave Hansen wrote:
> On 11/17/22 15:23, Marco Elver wrote:
> > Yes - it's the 'level != PG_LEVEL_4K'.
> 
> That plus the bisect made it pretty easy to find, thanks for the effort!
> 
> Could you double-check that the attached patch fixes it?  It seemed to
> for me.
> 
> The issue was that the new "No changes, easy!" check in the suspect
> commit didn't check the cpa->force_split option.  It didn't split down
> to 4k and then all hell broke loose.
> 
> Oh, and I totally misread the kfence ability to tolerate partial TLB
> flushes.  Sorry for the noise there!

> diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
> index 220361ceb997..9b4e2ad957f6 100644
> --- a/arch/x86/mm/pat/set_memory.c
> +++ b/arch/x86/mm/pat/set_memory.c
> @@ -1727,7 +1727,8 @@ static int __change_page_attr_set_clr(struct cpa_data *cpa, int primary)
>  	/*
>  	 * No changes, easy!
>  	 */
> -	if (!(pgprot_val(cpa->mask_set) | pgprot_val(cpa->mask_clr)))
> +	if (!(pgprot_val(cpa->mask_set) | pgprot_val(cpa->mask_clr))
> +	    && !cpa->force_split)

(operators go at the end of the previous line)

>  		return ret;
>  
>  	while (rempages) {

Urgh.. sorry about that.