linux-kernel - Re: [PATCH] x86/mm: Drop usage of __flush_tlb_all() in kernel_physical_mapping

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Message-ID: <20181120090336.GF2131@hirez.programming.kicks-ass.net>
Date:   Tue, 20 Nov 2018 10:03:36 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Williams, Dan J" <dan.j.williams@...el.com>
Cc:     "tglx@...utronix.de" <tglx@...utronix.de>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "bigeasy@...utronix.de" <bigeasy@...utronix.de>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "luto@...nel.org" <luto@...nel.org>, "bp@...en8.de" <bp@...en8.de>
Subject: Re: [PATCH] x86/mm: Drop usage of __flush_tlb_all() in
 kernel_physical_mapping_init()

On Tue, Nov 20, 2018 at 02:59:32AM +0000, Williams, Dan J wrote:
> On Mon, 2018-11-19 at 15:43 -0800, Dave Hansen wrote:
> > On 11/19/18 3:19 PM, Dan Williams wrote:
> > > Andy wondered why a path that can sleep was using __flush_tlb_all()
> > > [1]
> > > and Dave confirmed the expectation for TLB flush is for modifying /
> > > invalidating existing pte entries, but not initial population [2].
> > 
> > I _think_ this is OK.
> > 
> > But, could we sprinkle a few WARN_ON_ONCE(p*_present()) calls in
> > there
> > to help us sleep at night?
> 
> Well, I'm having nightmares now because my naive patch to sprinkle some
> WARN_ON_ONCE() calls is leading to my VM live locking at boot... no
> backtrace. If I revert the patch below and just go with the
> __flush_tlb_all() removal it seems fine.
> 
> I'm going to set this aside for a bit, but if anyone has any thoughts
> in the meantime I'd appreciate it.

Have you tried using early_printk ?

So kernel_physical_mapping_init() has a comment that states the virtual
and physical addresses we create mappings for should be PMD aligned,
which implies pud/p4d could have overlap between the mappings.

But in that case, I would expect the new and old values to match.

So maybe you should be checking something like:

	WARN_ON_ONCE(pud_present(*pud) && !pud_same(pud, new));

> @@ -573,6 +578,7 @@ phys_pud_init(pud_t *pud_page, unsigned long paddr, unsigned long paddr_end,
>  					     E820_TYPE_RAM) &&
>  			    !e820__mapped_any(paddr & PUD_MASK, paddr_next,
>  					     E820_TYPE_RESERVED_KERN))
> +				WARN_ON_ONCE(pud_present(*pud));
>  				set_pud(pud, __pud(0));
>  			continue;
>  		}
> @@ -610,6 +616,7 @@ phys_pud_init(pud_t *pud_page, unsigned long paddr, unsigned long paddr_end,
>  		if (page_size_mask & (1<<PG_LEVEL_1G)) {
>  			pages++;
>  			spin_lock(&init_mm.page_table_lock);
> +			WARN_ON_ONCE(pud_present(*pud));
>  			set_pte((pte_t *)pud,
>  				pfn_pte((paddr & PUD_MASK) >> PAGE_SHIFT,
>  					PAGE_KERNEL_LARGE));
> @@ -623,6 +630,7 @@ phys_pud_init(pud_t *pud_page, unsigned long paddr, unsigned long paddr_end,
>  					   page_size_mask, prot);
>  
>  		spin_lock(&init_mm.page_table_lock);
> +		WARN_ON_ONCE(pud_present(*pud));
>  		pud_populate(&init_mm, pud, pmd);
>  		spin_unlock(&init_mm.page_table_lock);
>  	}
> @@ -657,6 +665,7 @@ phys_p4d_init(p4d_t *p4d_page, unsigned long paddr, unsigned long paddr_end,
>  					     E820_TYPE_RAM) &&
>  			    !e820__mapped_any(paddr & P4D_MASK, paddr_next,
>  					     E820_TYPE_RESERVED_KERN))
> +				WARN_ON_ONCE(p4d_present(*p4d));
>  				set_p4d(p4d, __p4d(0));
>  			continue;
>  		}
> @@ -674,6 +683,7 @@ phys_p4d_init(p4d_t *p4d_page, unsigned long paddr, unsigned long paddr_end,
>  					   page_size_mask);
>  
>  		spin_lock(&init_mm.page_table_lock);
> +		WARN_ON_ONCE(p4d_present(*p4d));
>  		p4d_populate(&init_mm, p4d, pud);
>  		spin_unlock(&init_mm.page_table_lock);
>  	}