linux-kernel - Re: [PATCH] Xen: Fix pte unpin BUG when !CONFIG

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Date:	Tue, 09 Sep 2008 11:05:52 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Alex Nixon <alex.nixon@...rix.com>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] Xen: Fix pte unpin BUG when !CONFIG_SMP

Alex Nixon wrote:
> We still need to pin PTEs, even if there are no PTE locks.  Otherwise we'll BUG whenever there aren't PTE locks (i.e. whenever NR_CPUS < CONFIG_SPLIT_PTLOCK_CPUS), as we try to unpin PTEs which were never pinned in the first place.
>   

Where does the unpin happen?  xen_unpin_page() also checks to see if it
took the lock before trying to unpin, symmetric with xen_pin_page().

    J
> Signed-off-by: Alex Nixon <alex.nixon@...rix.com>
> Cc: Jeremy Fitzhardinge <jeremy@...p.org>
> Cc: Ingo Molnar <mingo@...e.hu>
> ---
>  arch/x86/xen/mmu.c |    7 +++----
>  1 files changed, 3 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
> index f5af913..1239bda 100644
> --- a/arch/x86/xen/mmu.c
> +++ b/arch/x86/xen/mmu.c
> @@ -819,9 +819,10 @@ static int xen_pin_page(struct page *page, enum pt_level level)
>  					pfn_pte(pfn, PAGE_KERNEL_RO),
>  					level == PT_PGD ? UVMF_TLB_FLUSH : 0);
>  
> -		if (ptl) {
> +		if (level == PT_PTE)
>  			xen_do_pin(MMUEXT_PIN_L1_TABLE, pfn);
>  
> +		if (ptl) {
>  			/* Queue a deferred unlock for when this batch
>  			   is completed. */
>  			xen_mc_callback(xen_pte_unlock, ptl);
> @@ -924,9 +925,7 @@ static int xen_unpin_page(struct page *page, enum pt_level level)
>  		 */
>  		if (level == PT_PTE) {
>  			ptl = xen_pte_lock(page);
> -
> -			if (ptl)
> -				xen_do_pin(MMUEXT_UNPIN_TABLE, pfn);
> +			xen_do_pin(MMUEXT_UNPIN_TABLE, pfn);
>  		}
>  
>  		mcs = __xen_mc_entry(0);
>   

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