linux-kernel - Re: [PATCH 2/3] x86/mm: Sync also unmappings in vmalloc_sync

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Message-ID: <alpine.DEB.2.21.1907172337590.1778@nanos.tec.linutronix.de>
Date:   Wed, 17 Jul 2019 23:43:43 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Joerg Roedel <joro@...tes.org>
cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Joerg Roedel <jroedel@...e.de>
Subject: Re: [PATCH 2/3] x86/mm: Sync also unmappings in vmalloc_sync_one()

On Wed, 17 Jul 2019, Joerg Roedel wrote:

> From: Joerg Roedel <jroedel@...e.de>
> 
> With huge-page ioremap areas the unmappings also need to be
> synced between all page-tables. Otherwise it can cause data
> corruption when a region is unmapped and later re-used.
> 
> Make the vmalloc_sync_one() function ready to sync
> unmappings.
> 
> Fixes: 5d72b4fba40ef ('x86, mm: support huge I/O mapping capability I/F')
> Signed-off-by: Joerg Roedel <jroedel@...e.de>
> ---
>  arch/x86/mm/fault.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index 4a4049f6d458..d71e167662c3 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -194,11 +194,12 @@ static inline pmd_t *vmalloc_sync_one(pgd_t *pgd, unsigned long address)
>  
>  	pmd = pmd_offset(pud, address);
>  	pmd_k = pmd_offset(pud_k, address);
> -	if (!pmd_present(*pmd_k))
> -		return NULL;
>  
> -	if (!pmd_present(*pmd))
> +	if (pmd_present(*pmd) ^ pmd_present(*pmd_k))
>  		set_pmd(pmd, *pmd_k);
> +
> +	if (!pmd_present(*pmd_k))
> +		return NULL;
>  	else
>  		BUG_ON(pmd_pfn(*pmd) != pmd_pfn(*pmd_k));

So in case of unmap, this updates only the first entry in the pgd_list
because vmalloc_sync_all() will break out of the iteration over pgd_list
when NULL is returned from vmalloc_sync_one().

I'm surely missing something, but how is that supposed to sync _all_ page
tables on unmap as the changelog claims?

Thanks,

	tglx